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-
-VGA Arbiter
-===========
-
-Graphic devices are accessed through ranges in I/O or memory space. While most
-modern devices allow relocation of such ranges, some "Legacy" VGA devices
-implemented on PCI will typically have the same "hard-decoded" addresses as
-they did on ISA. For more details see "PCI Bus Binding to IEEE Std 1275-1994
-Standard for Boot (Initialization Configuration) Firmware Revision 2.1"
-Section 7, Legacy Devices.
-
-The Resource Access Control (RAC) module inside the X server [0] existed for
-the legacy VGA arbitration task (besides other bus management tasks) when more
-than one legacy device co-exists on the same machine. But the problem happens
-when these devices are trying to be accessed by different userspace clients
-(e.g. two server in parallel). Their address assignments conflict. Moreover,
-ideally, being a userspace application, it is not the role of the X server to
-control bus resources. Therefore an arbitration scheme outside of the X server
-is needed to control the sharing of these resources. This document introduces
-the operation of the VGA arbiter implemented for the Linux kernel.
-
-----------------------------------------------------------------------------
-
-I. Details and Theory of Operation
- I.1 vgaarb
- I.2 libpciaccess
- I.3 xf86VGAArbiter (X server implementation)
-II. Credits
-III.References
-
-
-I. Details and Theory of Operation
-==================================
-
-I.1 vgaarb
-----------
-
-The vgaarb is a module of the Linux Kernel. When it is initially loaded, it
-scans all PCI devices and adds the VGA ones inside the arbitration. The
-arbiter then enables/disables the decoding on different devices of the VGA
-legacy instructions. Devices which do not want/need to use the arbiter may
-explicitly tell it by calling vga_set_legacy_decoding().
-
-The kernel exports a char device interface (/dev/vga_arbiter) to the clients,
-which has the following semantics:
-
- open : open user instance of the arbiter. By default, it's attached to
- the default VGA device of the system.
-
- close : close user instance. Release locks made by the user
-
- read : return a string indicating the status of the target like:
-
- "<card_ID>,decodes=<io_state>,owns=<io_state>,locks=<io_state> (ic,mc)"
-
- An IO state string is of the form {io,mem,io+mem,none}, mc and
- ic are respectively mem and io lock counts (for debugging/
- diagnostic only). "decodes" indicate what the card currently
- decodes, "owns" indicates what is currently enabled on it, and
- "locks" indicates what is locked by this card. If the card is
- unplugged, we get "invalid" then for card_ID and an -ENODEV
- error is returned for any command until a new card is targeted.
-
-
- write : write a command to the arbiter. List of commands:
-
- target <card_ID> : switch target to card <card_ID> (see below)
- lock <io_state> : acquires locks on target ("none" is an invalid io_state)
- trylock <io_state> : non-blocking acquire locks on target (returns EBUSY if
- unsuccessful)
- unlock <io_state> : release locks on target
- unlock all : release all locks on target held by this user (not
- implemented yet)
- decodes <io_state> : set the legacy decoding attributes for the card
-
- poll : event if something changes on any card (not just the
- target)
-
- card_ID is of the form "PCI:domain:bus:dev.fn". It can be set to "default"
- to go back to the system default card (TODO: not implemented yet). Currently,
- only PCI is supported as a prefix, but the userland API may support other bus
- types in the future, even if the current kernel implementation doesn't.
-
-Note about locks:
-
-The driver keeps track of which user has which locks on which card. It
-supports stacking, like the kernel one. This complexifies the implementation
-a bit, but makes the arbiter more tolerant to user space problems and able
-to properly cleanup in all cases when a process dies.
-Currently, a max of 16 cards can have locks simultaneously issued from
-user space for a given user (file descriptor instance) of the arbiter.
-
-In the case of devices hot-{un,}plugged, there is a hook - pci_notify() - to
-notify them being added/removed in the system and automatically added/removed
-in the arbiter.
-
-There is also an in-kernel API of the arbiter in case DRM, vgacon, or other
-drivers want to use it.
-
-
-I.2 libpciaccess
-----------------
-
-To use the vga arbiter char device it was implemented an API inside the
-libpciaccess library. One field was added to struct pci_device (each device
-on the system):
-
- /* the type of resource decoded by the device */
- int vgaarb_rsrc;
-
-Besides it, in pci_system were added:
-
- int vgaarb_fd;
- int vga_count;
- struct pci_device *vga_target;
- struct pci_device *vga_default_dev;
-
-
-The vga_count is used to track how many cards are being arbitrated, so for
-instance, if there is only one card, then it can completely escape arbitration.
-
-
-These functions below acquire VGA resources for the given card and mark those
-resources as locked. If the resources requested are "normal" (and not legacy)
-resources, the arbiter will first check whether the card is doing legacy
-decoding for that type of resource. If yes, the lock is "converted" into a
-legacy resource lock. The arbiter will first look for all VGA cards that
-might conflict and disable their IOs and/or Memory access, including VGA
-forwarding on P2P bridges if necessary, so that the requested resources can
-be used. Then, the card is marked as locking these resources and the IO and/or
-Memory access is enabled on the card (including VGA forwarding on parent
-P2P bridges if any). In the case of vga_arb_lock(), the function will block
-if some conflicting card is already locking one of the required resources (or
-any resource on a different bus segment, since P2P bridges don't differentiate
-VGA memory and IO afaik). If the card already owns the resources, the function
-succeeds. vga_arb_trylock() will return (-EBUSY) instead of blocking. Nested
-calls are supported (a per-resource counter is maintained).
-
-
-Set the target device of this client.
- int pci_device_vgaarb_set_target (struct pci_device *dev);
-
-
-For instance, in x86 if two devices on the same bus want to lock different
-resources, both will succeed (lock). If devices are in different buses and
-trying to lock different resources, only the first who tried succeeds.
- int pci_device_vgaarb_lock (void);
- int pci_device_vgaarb_trylock (void);
-
-Unlock resources of device.
- int pci_device_vgaarb_unlock (void);
-
-Indicates to the arbiter if the card decodes legacy VGA IOs, legacy VGA
-Memory, both, or none. All cards default to both, the card driver (fbdev for
-example) should tell the arbiter if it has disabled legacy decoding, so the
-card can be left out of the arbitration process (and can be safe to take
-interrupts at any time.
- int pci_device_vgaarb_decodes (int new_vgaarb_rsrc);
-
-Connects to the arbiter device, allocates the struct
- int pci_device_vgaarb_init (void);
-
-Close the connection
- void pci_device_vgaarb_fini (void);
-
-
-I.3 xf86VGAArbiter (X server implementation)
---------------------------------------------
-
-(TODO)
-
-X server basically wraps all the functions that touch VGA registers somehow.
-
-
-II. Credits
-===========
-
-Benjamin Herrenschmidt (IBM?) started this work when he discussed such design
-with the Xorg community in 2005 [1, 2]. In the end of 2007, Paulo Zanoni and
-Tiago Vignatti (both of C3SL/Federal University of ParanĂ¡) proceeded his work
-enhancing the kernel code to adapt as a kernel module and also did the
-implementation of the user space side [3]. Now (2009) Tiago Vignatti and Dave
-Airlie finally put this work in shape and queued to Jesse Barnes' PCI tree.
-
-
-III. References
-==============
-
-[0] http://cgit.freedesktop.org/xorg/xserver/commit/?id=4b42448a2388d40f257774fbffdccaea87bd0347
-[1] http://lists.freedesktop.org/archives/xorg/2005-March/006663.html
-[2] http://lists.freedesktop.org/archives/xorg/2005-March/006745.html
-[3] http://lists.freedesktop.org/archives/xorg/2007-October/029507.html