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- Dynamic DMA mapping using the generic device
- ============================================
-
- James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
-
-This document describes the DMA API. For a more gentle introduction
-of the API (and actual examples) see
-Documentation/DMA-API-HOWTO.txt.
-
-This API is split into two pieces. Part I describes the API. Part II
-describes the extensions to the API for supporting non-consistent
-memory machines. Unless you know that your driver absolutely has to
-support non-consistent platforms (this is usually only legacy
-platforms) you should only use the API described in part I.
-
-Part I - dma_ API
--------------------------------------
-
-To get the dma_ API, you must #include <linux/dma-mapping.h>
-
-
-Part Ia - Using large dma-coherent buffers
-------------------------------------------
-
-void *
-dma_alloc_coherent(struct device *dev, size_t size,
- dma_addr_t *dma_handle, gfp_t flag)
-
-Consistent memory is memory for which a write by either the device or
-the processor can immediately be read by the processor or device
-without having to worry about caching effects. (You may however need
-to make sure to flush the processor's write buffers before telling
-devices to read that memory.)
-
-This routine allocates a region of <size> bytes of consistent memory.
-It also returns a <dma_handle> which may be cast to an unsigned
-integer the same width as the bus and used as the physical address
-base of the region.
-
-Returns: a pointer to the allocated region (in the processor's virtual
-address space) or NULL if the allocation failed.
-
-Note: consistent memory can be expensive on some platforms, and the
-minimum allocation length may be as big as a page, so you should
-consolidate your requests for consistent memory as much as possible.
-The simplest way to do that is to use the dma_pool calls (see below).
-
-The flag parameter (dma_alloc_coherent only) allows the caller to
-specify the GFP_ flags (see kmalloc) for the allocation (the
-implementation may choose to ignore flags that affect the location of
-the returned memory, like GFP_DMA).
-
-void *
-dma_zalloc_coherent(struct device *dev, size_t size,
- dma_addr_t *dma_handle, gfp_t flag)
-
-Wraps dma_alloc_coherent() and also zeroes the returned memory if the
-allocation attempt succeeded.
-
-void
-dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
- dma_addr_t dma_handle)
-
-Free the region of consistent memory you previously allocated. dev,
-size and dma_handle must all be the same as those passed into the
-consistent allocate. cpu_addr must be the virtual address returned by
-the consistent allocate.
-
-Note that unlike their sibling allocation calls, these routines
-may only be called with IRQs enabled.
-
-
-Part Ib - Using small dma-coherent buffers
-------------------------------------------
-
-To get this part of the dma_ API, you must #include <linux/dmapool.h>
-
-Many drivers need lots of small dma-coherent memory regions for DMA
-descriptors or I/O buffers. Rather than allocating in units of a page
-or more using dma_alloc_coherent(), you can use DMA pools. These work
-much like a struct kmem_cache, except that they use the dma-coherent allocator,
-not __get_free_pages(). Also, they understand common hardware constraints
-for alignment, like queue heads needing to be aligned on N-byte boundaries.
-
-
- struct dma_pool *
- dma_pool_create(const char *name, struct device *dev,
- size_t size, size_t align, size_t alloc);
-
-The pool create() routines initialize a pool of dma-coherent buffers
-for use with a given device. It must be called in a context which
-can sleep.
-
-The "name" is for diagnostics (like a struct kmem_cache name); dev and size
-are like what you'd pass to dma_alloc_coherent(). The device's hardware
-alignment requirement for this type of data is "align" (which is expressed
-in bytes, and must be a power of two). If your device has no boundary
-crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
-from this pool must not cross 4KByte boundaries.
-
-
- void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
- dma_addr_t *dma_handle);
-
-This allocates memory from the pool; the returned memory will meet the size
-and alignment requirements specified at creation time. Pass GFP_ATOMIC to
-prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks),
-pass GFP_KERNEL to allow blocking. Like dma_alloc_coherent(), this returns
-two values: an address usable by the cpu, and the dma address usable by the
-pool's device.
-
-
- void dma_pool_free(struct dma_pool *pool, void *vaddr,
- dma_addr_t addr);
-
-This puts memory back into the pool. The pool is what was passed to
-the pool allocation routine; the cpu (vaddr) and dma addresses are what
-were returned when that routine allocated the memory being freed.
-
-
- void dma_pool_destroy(struct dma_pool *pool);
-
-The pool destroy() routines free the resources of the pool. They must be
-called in a context which can sleep. Make sure you've freed all allocated
-memory back to the pool before you destroy it.
-
-
-Part Ic - DMA addressing limitations
-------------------------------------
-
-int
-dma_supported(struct device *dev, u64 mask)
-
-Checks to see if the device can support DMA to the memory described by
-mask.
-
-Returns: 1 if it can and 0 if it can't.
-
-Notes: This routine merely tests to see if the mask is possible. It
-won't change the current mask settings. It is more intended as an
-internal API for use by the platform than an external API for use by
-driver writers.
-
-int
-dma_set_mask(struct device *dev, u64 mask)
-
-Checks to see if the mask is possible and updates the device
-parameters if it is.
-
-Returns: 0 if successful and a negative error if not.
-
-int
-dma_set_coherent_mask(struct device *dev, u64 mask)
-
-Checks to see if the mask is possible and updates the device
-parameters if it is.
-
-Returns: 0 if successful and a negative error if not.
-
-u64
-dma_get_required_mask(struct device *dev)
-
-This API returns the mask that the platform requires to
-operate efficiently. Usually this means the returned mask
-is the minimum required to cover all of memory. Examining the
-required mask gives drivers with variable descriptor sizes the
-opportunity to use smaller descriptors as necessary.
-
-Requesting the required mask does not alter the current mask. If you
-wish to take advantage of it, you should issue a dma_set_mask()
-call to set the mask to the value returned.
-
-
-Part Id - Streaming DMA mappings
---------------------------------
-
-dma_addr_t
-dma_map_single(struct device *dev, void *cpu_addr, size_t size,
- enum dma_data_direction direction)
-
-Maps a piece of processor virtual memory so it can be accessed by the
-device and returns the physical handle of the memory.
-
-The direction for both api's may be converted freely by casting.
-However the dma_ API uses a strongly typed enumerator for its
-direction:
-
-DMA_NONE no direction (used for debugging)
-DMA_TO_DEVICE data is going from the memory to the device
-DMA_FROM_DEVICE data is coming from the device to the memory
-DMA_BIDIRECTIONAL direction isn't known
-
-Notes: Not all memory regions in a machine can be mapped by this
-API. Further, regions that appear to be physically contiguous in
-kernel virtual space may not be contiguous as physical memory. Since
-this API does not provide any scatter/gather capability, it will fail
-if the user tries to map a non-physically contiguous piece of memory.
-For this reason, it is recommended that memory mapped by this API be
-obtained only from sources which guarantee it to be physically contiguous
-(like kmalloc).
-
-Further, the physical address of the memory must be within the
-dma_mask of the device (the dma_mask represents a bit mask of the
-addressable region for the device. I.e., if the physical address of
-the memory anded with the dma_mask is still equal to the physical
-address, then the device can perform DMA to the memory). In order to
-ensure that the memory allocated by kmalloc is within the dma_mask,
-the driver may specify various platform-dependent flags to restrict
-the physical memory range of the allocation (e.g. on x86, GFP_DMA
-guarantees to be within the first 16Mb of available physical memory,
-as required by ISA devices).
-
-Note also that the above constraints on physical contiguity and
-dma_mask may not apply if the platform has an IOMMU (a device which
-supplies a physical to virtual mapping between the I/O memory bus and
-the device). However, to be portable, device driver writers may *not*
-assume that such an IOMMU exists.
-
-Warnings: Memory coherency operates at a granularity called the cache
-line width. In order for memory mapped by this API to operate
-correctly, the mapped region must begin exactly on a cache line
-boundary and end exactly on one (to prevent two separately mapped
-regions from sharing a single cache line). Since the cache line size
-may not be known at compile time, the API will not enforce this
-requirement. Therefore, it is recommended that driver writers who
-don't take special care to determine the cache line size at run time
-only map virtual regions that begin and end on page boundaries (which
-are guaranteed also to be cache line boundaries).
-
-DMA_TO_DEVICE synchronisation must be done after the last modification
-of the memory region by the software and before it is handed off to
-the driver. Once this primitive is used, memory covered by this
-primitive should be treated as read-only by the device. If the device
-may write to it at any point, it should be DMA_BIDIRECTIONAL (see
-below).
-
-DMA_FROM_DEVICE synchronisation must be done before the driver
-accesses data that may be changed by the device. This memory should
-be treated as read-only by the driver. If the driver needs to write
-to it at any point, it should be DMA_BIDIRECTIONAL (see below).
-
-DMA_BIDIRECTIONAL requires special handling: it means that the driver
-isn't sure if the memory was modified before being handed off to the
-device and also isn't sure if the device will also modify it. Thus,
-you must always sync bidirectional memory twice: once before the
-memory is handed off to the device (to make sure all memory changes
-are flushed from the processor) and once before the data may be
-accessed after being used by the device (to make sure any processor
-cache lines are updated with data that the device may have changed).
-
-void
-dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
- enum dma_data_direction direction)
-
-Unmaps the region previously mapped. All the parameters passed in
-must be identical to those passed in (and returned) by the mapping
-API.
-
-dma_addr_t
-dma_map_page(struct device *dev, struct page *page,
- unsigned long offset, size_t size,
- enum dma_data_direction direction)
-void
-dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
- enum dma_data_direction direction)
-
-API for mapping and unmapping for pages. All the notes and warnings
-for the other mapping APIs apply here. Also, although the <offset>
-and <size> parameters are provided to do partial page mapping, it is
-recommended that you never use these unless you really know what the
-cache width is.
-
-int
-dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
-
-In some circumstances dma_map_single and dma_map_page will fail to create
-a mapping. A driver can check for these errors by testing the returned
-dma address with dma_mapping_error(). A non-zero return value means the mapping
-could not be created and the driver should take appropriate action (e.g.
-reduce current DMA mapping usage or delay and try again later).
-
- int
- dma_map_sg(struct device *dev, struct scatterlist *sg,
- int nents, enum dma_data_direction direction)
-
-Returns: the number of physical segments mapped (this may be shorter
-than <nents> passed in if some elements of the scatter/gather list are
-physically or virtually adjacent and an IOMMU maps them with a single
-entry).
-
-Please note that the sg cannot be mapped again if it has been mapped once.
-The mapping process is allowed to destroy information in the sg.
-
-As with the other mapping interfaces, dma_map_sg can fail. When it
-does, 0 is returned and a driver must take appropriate action. It is
-critical that the driver do something, in the case of a block driver
-aborting the request or even oopsing is better than doing nothing and
-corrupting the filesystem.
-
-With scatterlists, you use the resulting mapping like this:
-
- int i, count = dma_map_sg(dev, sglist, nents, direction);
- struct scatterlist *sg;
-
- for_each_sg(sglist, sg, count, i) {
- hw_address[i] = sg_dma_address(sg);
- hw_len[i] = sg_dma_len(sg);
- }
-
-where nents is the number of entries in the sglist.
-
-The implementation is free to merge several consecutive sglist entries
-into one (e.g. with an IOMMU, or if several pages just happen to be
-physically contiguous) and returns the actual number of sg entries it
-mapped them to. On failure 0, is returned.
-
-Then you should loop count times (note: this can be less than nents times)
-and use sg_dma_address() and sg_dma_len() macros where you previously
-accessed sg->address and sg->length as shown above.
-
- void
- dma_unmap_sg(struct device *dev, struct scatterlist *sg,
- int nhwentries, enum dma_data_direction direction)
-
-Unmap the previously mapped scatter/gather list. All the parameters
-must be the same as those and passed in to the scatter/gather mapping
-API.
-
-Note: <nents> must be the number you passed in, *not* the number of
-physical entries returned.
-
-void
-dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
- enum dma_data_direction direction)
-void
-dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
- enum dma_data_direction direction)
-void
-dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems,
- enum dma_data_direction direction)
-void
-dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems,
- enum dma_data_direction direction)
-
-Synchronise a single contiguous or scatter/gather mapping for the cpu
-and device. With the sync_sg API, all the parameters must be the same
-as those passed into the single mapping API. With the sync_single API,
-you can use dma_handle and size parameters that aren't identical to
-those passed into the single mapping API to do a partial sync.
-
-Notes: You must do this:
-
-- Before reading values that have been written by DMA from the device
- (use the DMA_FROM_DEVICE direction)
-- After writing values that will be written to the device using DMA
- (use the DMA_TO_DEVICE) direction
-- before *and* after handing memory to the device if the memory is
- DMA_BIDIRECTIONAL
-
-See also dma_map_single().
-
-dma_addr_t
-dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
- enum dma_data_direction dir,
- struct dma_attrs *attrs)
-
-void
-dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
- size_t size, enum dma_data_direction dir,
- struct dma_attrs *attrs)
-
-int
-dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
- int nents, enum dma_data_direction dir,
- struct dma_attrs *attrs)
-
-void
-dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
- int nents, enum dma_data_direction dir,
- struct dma_attrs *attrs)
-
-The four functions above are just like the counterpart functions
-without the _attrs suffixes, except that they pass an optional
-struct dma_attrs*.
-
-struct dma_attrs encapsulates a set of "dma attributes". For the
-definition of struct dma_attrs see linux/dma-attrs.h.
-
-The interpretation of dma attributes is architecture-specific, and
-each attribute should be documented in Documentation/DMA-attributes.txt.
-
-If struct dma_attrs* is NULL, the semantics of each of these
-functions is identical to those of the corresponding function
-without the _attrs suffix. As a result dma_map_single_attrs()
-can generally replace dma_map_single(), etc.
-
-As an example of the use of the *_attrs functions, here's how
-you could pass an attribute DMA_ATTR_FOO when mapping memory
-for DMA:
-
-#include <linux/dma-attrs.h>
-/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
- * documented in Documentation/DMA-attributes.txt */
-...
-
- DEFINE_DMA_ATTRS(attrs);
- dma_set_attr(DMA_ATTR_FOO, &attrs);
- ....
- n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
- ....
-
-Architectures that care about DMA_ATTR_FOO would check for its
-presence in their implementations of the mapping and unmapping
-routines, e.g.:
-
-void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
- size_t size, enum dma_data_direction dir,
- struct dma_attrs *attrs)
-{
- ....
- int foo = dma_get_attr(DMA_ATTR_FOO, attrs);
- ....
- if (foo)
- /* twizzle the frobnozzle */
- ....
-
-
-Part II - Advanced dma_ usage
------------------------------
-
-Warning: These pieces of the DMA API should not be used in the
-majority of cases, since they cater for unlikely corner cases that
-don't belong in usual drivers.
-
-If you don't understand how cache line coherency works between a
-processor and an I/O device, you should not be using this part of the
-API at all.
-
-void *
-dma_alloc_noncoherent(struct device *dev, size_t size,
- dma_addr_t *dma_handle, gfp_t flag)
-
-Identical to dma_alloc_coherent() except that the platform will
-choose to return either consistent or non-consistent memory as it sees
-fit. By using this API, you are guaranteeing to the platform that you
-have all the correct and necessary sync points for this memory in the
-driver should it choose to return non-consistent memory.
-
-Note: where the platform can return consistent memory, it will
-guarantee that the sync points become nops.
-
-Warning: Handling non-consistent memory is a real pain. You should
-only ever use this API if you positively know your driver will be
-required to work on one of the rare (usually non-PCI) architectures
-that simply cannot make consistent memory.
-
-void
-dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
- dma_addr_t dma_handle)
-
-Free memory allocated by the nonconsistent API. All parameters must
-be identical to those passed in (and returned by
-dma_alloc_noncoherent()).
-
-int
-dma_get_cache_alignment(void)
-
-Returns the processor cache alignment. This is the absolute minimum
-alignment *and* width that you must observe when either mapping
-memory or doing partial flushes.
-
-Notes: This API may return a number *larger* than the actual cache
-line, but it will guarantee that one or more cache lines fit exactly
-into the width returned by this call. It will also always be a power
-of two for easy alignment.
-
-void
-dma_cache_sync(struct device *dev, void *vaddr, size_t size,
- enum dma_data_direction direction)
-
-Do a partial sync of memory that was allocated by
-dma_alloc_noncoherent(), starting at virtual address vaddr and
-continuing on for size. Again, you *must* observe the cache line
-boundaries when doing this.
-
-int
-dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr,
- dma_addr_t device_addr, size_t size, int
- flags)
-
-Declare region of memory to be handed out by dma_alloc_coherent when
-it's asked for coherent memory for this device.
-
-bus_addr is the physical address to which the memory is currently
-assigned in the bus responding region (this will be used by the
-platform to perform the mapping).
-
-device_addr is the physical address the device needs to be programmed
-with actually to address this memory (this will be handed out as the
-dma_addr_t in dma_alloc_coherent()).
-
-size is the size of the area (must be multiples of PAGE_SIZE).
-
-flags can be or'd together and are:
-
-DMA_MEMORY_MAP - request that the memory returned from
-dma_alloc_coherent() be directly writable.
-
-DMA_MEMORY_IO - request that the memory returned from
-dma_alloc_coherent() be addressable using read/write/memcpy_toio etc.
-
-One or both of these flags must be present.
-
-DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
-dma_alloc_coherent of any child devices of this one (for memory residing
-on a bridge).
-
-DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions.
-Do not allow dma_alloc_coherent() to fall back to system memory when
-it's out of memory in the declared region.
-
-The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
-must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
-if only DMA_MEMORY_MAP were passed in) for success or zero for
-failure.
-
-Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
-dma_alloc_coherent() may no longer be accessed directly, but instead
-must be accessed using the correct bus functions. If your driver
-isn't prepared to handle this contingency, it should not specify
-DMA_MEMORY_IO in the input flags.
-
-As a simplification for the platforms, only *one* such region of
-memory may be declared per device.
-
-For reasons of efficiency, most platforms choose to track the declared
-region only at the granularity of a page. For smaller allocations,
-you should use the dma_pool() API.
-
-void
-dma_release_declared_memory(struct device *dev)
-
-Remove the memory region previously declared from the system. This
-API performs *no* in-use checking for this region and will return
-unconditionally having removed all the required structures. It is the
-driver's job to ensure that no parts of this memory region are
-currently in use.
-
-void *
-dma_mark_declared_memory_occupied(struct device *dev,
- dma_addr_t device_addr, size_t size)
-
-This is used to occupy specific regions of the declared space
-(dma_alloc_coherent() will hand out the first free region it finds).
-
-device_addr is the *device* address of the region requested.
-
-size is the size (and should be a page-sized multiple).
-
-The return value will be either a pointer to the processor virtual
-address of the memory, or an error (via PTR_ERR()) if any part of the
-region is occupied.
-
-Part III - Debug drivers use of the DMA-API
--------------------------------------------
-
-The DMA-API as described above as some constraints. DMA addresses must be
-released with the corresponding function with the same size for example. With
-the advent of hardware IOMMUs it becomes more and more important that drivers
-do not violate those constraints. In the worst case such a violation can
-result in data corruption up to destroyed filesystems.
-
-To debug drivers and find bugs in the usage of the DMA-API checking code can
-be compiled into the kernel which will tell the developer about those
-violations. If your architecture supports it you can select the "Enable
-debugging of DMA-API usage" option in your kernel configuration. Enabling this
-option has a performance impact. Do not enable it in production kernels.
-
-If you boot the resulting kernel will contain code which does some bookkeeping
-about what DMA memory was allocated for which device. If this code detects an
-error it prints a warning message with some details into your kernel log. An
-example warning message may look like this:
-
-------------[ cut here ]------------
-WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
- check_unmap+0x203/0x490()
-Hardware name:
-forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
- function [device address=0x00000000640444be] [size=66 bytes] [mapped as
-single] [unmapped as page]
-Modules linked in: nfsd exportfs bridge stp llc r8169
-Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1
-Call Trace:
- <IRQ> [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
- [<ffffffff80647b70>] _spin_unlock+0x10/0x30
- [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
- [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
- [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
- [<ffffffff80252f96>] queue_work+0x56/0x60
- [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
- [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
- [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
- [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
- [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
- [<ffffffff803c7ea3>] check_unmap+0x203/0x490
- [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
- [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
- [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
- [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
- [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
- [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
- [<ffffffff8020c093>] ret_from_intr+0x0/0xa
- <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
-
-The driver developer can find the driver and the device including a stacktrace
-of the DMA-API call which caused this warning.
-
-Per default only the first error will result in a warning message. All other
-errors will only silently counted. This limitation exist to prevent the code
-from flooding your kernel log. To support debugging a device driver this can
-be disabled via debugfs. See the debugfs interface documentation below for
-details.
-
-The debugfs directory for the DMA-API debugging code is called dma-api/. In
-this directory the following files can currently be found:
-
- dma-api/all_errors This file contains a numeric value. If this
- value is not equal to zero the debugging code
- will print a warning for every error it finds
- into the kernel log. Be careful with this
- option, as it can easily flood your logs.
-
- dma-api/disabled This read-only file contains the character 'Y'
- if the debugging code is disabled. This can
- happen when it runs out of memory or if it was
- disabled at boot time
-
- dma-api/error_count This file is read-only and shows the total
- numbers of errors found.
-
- dma-api/num_errors The number in this file shows how many
- warnings will be printed to the kernel log
- before it stops. This number is initialized to
- one at system boot and be set by writing into
- this file
-
- dma-api/min_free_entries
- This read-only file can be read to get the
- minimum number of free dma_debug_entries the
- allocator has ever seen. If this value goes
- down to zero the code will disable itself
- because it is not longer reliable.
-
- dma-api/num_free_entries
- The current number of free dma_debug_entries
- in the allocator.
-
- dma-api/driver-filter
- You can write a name of a driver into this file
- to limit the debug output to requests from that
- particular driver. Write an empty string to
- that file to disable the filter and see
- all errors again.
-
-If you have this code compiled into your kernel it will be enabled by default.
-If you want to boot without the bookkeeping anyway you can provide
-'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
-Notice that you can not enable it again at runtime. You have to reboot to do
-so.
-
-If you want to see debug messages only for a special device driver you can
-specify the dma_debug_driver=<drivername> parameter. This will enable the
-driver filter at boot time. The debug code will only print errors for that
-driver afterwards. This filter can be disabled or changed later using debugfs.
-
-When the code disables itself at runtime this is most likely because it ran
-out of dma_debug_entries. These entries are preallocated at boot. The number
-of preallocated entries is defined per architecture. If it is too low for you
-boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
-architectural default.