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-
-What is udlfb?
-===============
-
-This is a driver for DisplayLink USB 2.0 era graphics chips.
-
-DisplayLink chips provide simple hline/blit operations with some compression,
-pairing that with a hardware framebuffer (16MB) on the other end of the
-USB wire. That hardware framebuffer is able to drive the VGA, DVI, or HDMI
-monitor with no CPU involvement until a pixel has to change.
-
-The CPU or other local resource does all the rendering; optinally compares the
-result with a local shadow of the remote hardware framebuffer to identify
-the minimal set of pixels that have changed; and compresses and sends those
-pixels line-by-line via USB bulk transfers.
-
-Because of the efficiency of bulk transfers and a protocol on top that
-does not require any acks - the effect is very low latency that
-can support surprisingly high resolutions with good performance for
-non-gaming and non-video applications.
-
-Mode setting, EDID read, etc are other bulk or control transfers. Mode
-setting is very flexible - able to set nearly arbitrary modes from any timing.
-
-Advantages of USB graphics in general:
-
- * Ability to add a nearly arbitrary number of displays to any USB 2.0
- capable system. On Linux, number of displays is limited by fbdev interface
- (FB_MAX is currently 32). Of course, all USB devices on the same
- host controller share the same 480Mbs USB 2.0 interface.
-
-Advantages of supporting DisplayLink chips with kernel framebuffer interface:
-
- * The actual hardware functionality of DisplayLink chips matches nearly
- one-to-one with the fbdev interface, making the driver quite small and
- tight relative to the functionality it provides.
- * X servers and other applications can use the standard fbdev interface
- from user mode to talk to the device, without needing to know anything
- about USB or DisplayLink's protocol at all. A "displaylink" X driver
- and a slightly modified "fbdev" X driver are among those that already do.
-
-Disadvantages:
-
- * Fbdev's mmap interface assumes a real hardware framebuffer is mapped.
- In the case of USB graphics, it is just an allocated (virtual) buffer.
- Writes need to be detected and encoded into USB bulk transfers by the CPU.
- Accurate damage/changed area notifications work around this problem.
- In the future, hopefully fbdev will be enhanced with an small standard
- interface to allow mmap clients to report damage, for the benefit
- of virtual or remote framebuffers.
- * Fbdev does not arbitrate client ownership of the framebuffer well.
- * Fbcon assumes the first framebuffer it finds should be consumed for console.
- * It's not clear what the future of fbdev is, given the rise of KMS/DRM.
-
-How to use it?
-==============
-
-Udlfb, when loaded as a module, will match against all USB 2.0 generation
-DisplayLink chips (Alex and Ollie family). It will then attempt to read the EDID
-of the monitor, and set the best common mode between the DisplayLink device
-and the monitor's capabilities.
-
-If the DisplayLink device is successful, it will paint a "green screen" which
-means that from a hardware and fbdev software perspective, everything is good.
-
-At that point, a /dev/fb? interface will be present for user-mode applications
-to open and begin writing to the framebuffer of the DisplayLink device using
-standard fbdev calls. Note that if mmap() is used, by default the user mode
-application must send down damage notifcations to trigger repaints of the
-changed regions. Alternatively, udlfb can be recompiled with experimental
-defio support enabled, to support a page-fault based detection mechanism
-that can work without explicit notifcation.
-
-The most common client of udlfb is xf86-video-displaylink or a modified
-xf86-video-fbdev X server. These servers have no real DisplayLink specific
-code. They write to the standard framebuffer interface and rely on udlfb
-to do its thing. The one extra feature they have is the ability to report
-rectangles from the X DAMAGE protocol extension down to udlfb via udlfb's
-damage interface (which will hopefully be standardized for all virtual
-framebuffers that need damage info). These damage notifications allow
-udlfb to efficiently process the changed pixels.
-
-Module Options
-==============
-
-Special configuration for udlfb is usually unnecessary. There are a few
-options, however.
-
-From the command line, pass options to modprobe
-modprobe udlfb fb_defio=0 console=1 shadow=1
-
-Or modify options on the fly at /sys/module/udlfb/parameters directory via
-sudo nano fb_defio
-change the parameter in place, and save the file.
-
-Unplug/replug USB device to apply with new settings
-
-Or for permanent option, create file like /etc/modprobe.d/udlfb.conf with text
-options udlfb fb_defio=0 console=1 shadow=1
-
-Accepted boolean options:
-
-fb_defio Make use of the fb_defio (CONFIG_FB_DEFERRED_IO) kernel
- module to track changed areas of the framebuffer by page faults.
- Standard fbdev applications that use mmap but that do not
- report damage, should be able to work with this enabled.
- Disable when running with X server that supports reporting
- changed regions via ioctl, as this method is simpler,
- more stable, and higher performance.
- default: fb_defio=1
-
-console Allow fbcon to attach to udlfb provided framebuffers.
- Can be disabled if fbcon and other clients
- (e.g. X with --shared-vt) are in conflict.
- default: console=1
-
-shadow Allocate a 2nd framebuffer to shadow what's currently across
- the USB bus in device memory. If any pixels are unchanged,
- do not transmit. Spends host memory to save USB transfers.
- Enabled by default. Only disable on very low memory systems.
- default: shadow=1
-
-Sysfs Attributes
-================
-
-Udlfb creates several files in /sys/class/graphics/fb?
-Where ? is the sequential framebuffer id of the particular DisplayLink device
-
-edid If a valid EDID blob is written to this file (typically
- by a udev rule), then udlfb will use this EDID as a
- backup in case reading the actual EDID of the monitor
- attached to the DisplayLink device fails. This is
- especially useful for fixed panels, etc. that cannot
- communicate their capabilities via EDID. Reading
- this file returns the current EDID of the attached
- monitor (or last backup value written). This is
- useful to get the EDID of the attached monitor,
- which can be passed to utilities like parse-edid.
-
-metrics_bytes_rendered 32-bit count of pixel bytes rendered
-
-metrics_bytes_identical 32-bit count of how many of those bytes were found to be
- unchanged, based on a shadow framebuffer check
-
-metrics_bytes_sent 32-bit count of how many bytes were transferred over
- USB to communicate the resulting changed pixels to the
- hardware. Includes compression and protocol overhead
-
-metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
- above pixels (in thousands of cycles).
-
-metrics_reset Write-only. Any write to this file resets all metrics
- above to zero. Note that the 32-bit counters above
- roll over very quickly. To get reliable results, design
- performance tests to start and finish in a very short
- period of time (one minute or less is safe).
-
---
-Bernie Thompson <bernie@plugable.com>