linux/Documentation/fb/udlfb.rst
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   1==============
   2What is udlfb?
   3==============
   4
   5This is a driver for DisplayLink USB 2.0 era graphics chips.
   6
   7DisplayLink chips provide simple hline/blit operations with some compression,
   8pairing that with a hardware framebuffer (16MB) on the other end of the
   9USB wire.  That hardware framebuffer is able to drive the VGA, DVI, or HDMI
  10monitor with no CPU involvement until a pixel has to change.
  11
  12The CPU or other local resource does all the rendering; optionally compares the
  13result with a local shadow of the remote hardware framebuffer to identify
  14the minimal set of pixels that have changed; and compresses and sends those
  15pixels line-by-line via USB bulk transfers.
  16
  17Because of the efficiency of bulk transfers and a protocol on top that
  18does not require any acks - the effect is very low latency that
  19can support surprisingly high resolutions with good performance for
  20non-gaming and non-video applications.
  21
  22Mode setting, EDID read, etc are other bulk or control transfers. Mode
  23setting is very flexible - able to set nearly arbitrary modes from any timing.
  24
  25Advantages of USB graphics in general:
  26
  27 * Ability to add a nearly arbitrary number of displays to any USB 2.0
  28   capable system. On Linux, number of displays is limited by fbdev interface
  29   (FB_MAX is currently 32). Of course, all USB devices on the same
  30   host controller share the same 480Mbs USB 2.0 interface.
  31
  32Advantages of supporting DisplayLink chips with kernel framebuffer interface:
  33
  34 * The actual hardware functionality of DisplayLink chips matches nearly
  35   one-to-one with the fbdev interface, making the driver quite small and
  36   tight relative to the functionality it provides.
  37 * X servers and other applications can use the standard fbdev interface
  38   from user mode to talk to the device, without needing to know anything
  39   about USB or DisplayLink's protocol at all. A "displaylink" X driver
  40   and a slightly modified "fbdev" X driver are among those that already do.
  41
  42Disadvantages:
  43
  44 * Fbdev's mmap interface assumes a real hardware framebuffer is mapped.
  45   In the case of USB graphics, it is just an allocated (virtual) buffer.
  46   Writes need to be detected and encoded into USB bulk transfers by the CPU.
  47   Accurate damage/changed area notifications work around this problem.
  48   In the future, hopefully fbdev will be enhanced with an small standard
  49   interface to allow mmap clients to report damage, for the benefit
  50   of virtual or remote framebuffers.
  51 * Fbdev does not arbitrate client ownership of the framebuffer well.
  52 * Fbcon assumes the first framebuffer it finds should be consumed for console.
  53 * It's not clear what the future of fbdev is, given the rise of KMS/DRM.
  54
  55How to use it?
  56==============
  57
  58Udlfb, when loaded as a module, will match against all USB 2.0 generation
  59DisplayLink chips (Alex and Ollie family). It will then attempt to read the EDID
  60of the monitor, and set the best common mode between the DisplayLink device
  61and the monitor's capabilities.
  62
  63If the DisplayLink device is successful, it will paint a "green screen" which
  64means that from a hardware and fbdev software perspective, everything is good.
  65
  66At that point, a /dev/fb? interface will be present for user-mode applications
  67to open and begin writing to the framebuffer of the DisplayLink device using
  68standard fbdev calls.  Note that if mmap() is used, by default the user mode
  69application must send down damage notifications to trigger repaints of the
  70changed regions.  Alternatively, udlfb can be recompiled with experimental
  71defio support enabled, to support a page-fault based detection mechanism
  72that can work without explicit notification.
  73
  74The most common client of udlfb is xf86-video-displaylink or a modified
  75xf86-video-fbdev X server. These servers have no real DisplayLink specific
  76code. They write to the standard framebuffer interface and rely on udlfb
  77to do its thing.  The one extra feature they have is the ability to report
  78rectangles from the X DAMAGE protocol extension down to udlfb via udlfb's
  79damage interface (which will hopefully be standardized for all virtual
  80framebuffers that need damage info). These damage notifications allow
  81udlfb to efficiently process the changed pixels.
  82
  83Module Options
  84==============
  85
  86Special configuration for udlfb is usually unnecessary. There are a few
  87options, however.
  88
  89From the command line, pass options to modprobe
  90modprobe udlfb fb_defio=0 console=1 shadow=1
  91
  92Or modify options on the fly at /sys/module/udlfb/parameters directory via
  93sudo nano fb_defio
  94change the parameter in place, and save the file.
  95
  96Unplug/replug USB device to apply with new settings
  97
  98Or for permanent option, create file like /etc/modprobe.d/udlfb.conf with text
  99options udlfb fb_defio=0 console=1 shadow=1
 100
 101Accepted boolean options:
 102
 103=============== ================================================================
 104fb_defio        Make use of the fb_defio (CONFIG_FB_DEFERRED_IO) kernel
 105                module to track changed areas of the framebuffer by page faults.
 106                Standard fbdev applications that use mmap but that do not
 107                report damage, should be able to work with this enabled.
 108                Disable when running with X server that supports reporting
 109                changed regions via ioctl, as this method is simpler,
 110                more stable, and higher performance.
 111                default: fb_defio=1
 112
 113console         Allow fbcon to attach to udlfb provided framebuffers.
 114                Can be disabled if fbcon and other clients
 115                (e.g. X with --shared-vt) are in conflict.
 116                default: console=1
 117
 118shadow          Allocate a 2nd framebuffer to shadow what's currently across
 119                the USB bus in device memory. If any pixels are unchanged,
 120                do not transmit. Spends host memory to save USB transfers.
 121                Enabled by default. Only disable on very low memory systems.
 122                default: shadow=1
 123=============== ================================================================
 124
 125Sysfs Attributes
 126================
 127
 128Udlfb creates several files in /sys/class/graphics/fb?
 129Where ? is the sequential framebuffer id of the particular DisplayLink device
 130
 131======================== ========================================================
 132edid                     If a valid EDID blob is written to this file (typically
 133                         by a udev rule), then udlfb will use this EDID as a
 134                         backup in case reading the actual EDID of the monitor
 135                         attached to the DisplayLink device fails. This is
 136                         especially useful for fixed panels, etc. that cannot
 137                         communicate their capabilities via EDID. Reading
 138                         this file returns the current EDID of the attached
 139                         monitor (or last backup value written). This is
 140                         useful to get the EDID of the attached monitor,
 141                         which can be passed to utilities like parse-edid.
 142
 143metrics_bytes_rendered   32-bit count of pixel bytes rendered
 144
 145metrics_bytes_identical  32-bit count of how many of those bytes were found to be
 146                         unchanged, based on a shadow framebuffer check
 147
 148metrics_bytes_sent       32-bit count of how many bytes were transferred over
 149                         USB to communicate the resulting changed pixels to the
 150                         hardware. Includes compression and protocol overhead
 151
 152metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
 153                         above pixels (in thousands of cycles).
 154
 155metrics_reset            Write-only. Any write to this file resets all metrics
 156                         above to zero.  Note that the 32-bit counters above
 157                         roll over very quickly. To get reliable results, design
 158                         performance tests to start and finish in a very short
 159                         period of time (one minute or less is safe).
 160======================== ========================================================
 161
 162Bernie Thompson <bernie@plugable.com>
 163