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::
  90
  91  modprobe udlfb fb_defio=0 console=1 shadow=1
  92
  93Or change options on the fly by editing
  94/sys/module/udlfb/parameters/PARAMETER_NAME ::
  95
  96  cd /sys/module/udlfb/parameters
  97  ls # to see a list of parameter names
  98  sudo nano PARAMETER_NAME
  99  # change the parameter in place, and save the file.
 100
 101Unplug/replug USB device to apply with new settings.
 102
 103Or to apply options permanently, create a modprobe configuration file
 104like /etc/modprobe.d/udlfb.conf with text::
 105
 106  options udlfb fb_defio=0 console=1 shadow=1
 107
 108Accepted boolean options:
 109
 110=============== ================================================================
 111fb_defio        Make use of the fb_defio (CONFIG_FB_DEFERRED_IO) kernel
 112                module to track changed areas of the framebuffer by page faults.
 113                Standard fbdev applications that use mmap but that do not
 114                report damage, should be able to work with this enabled.
 115                Disable when running with X server that supports reporting
 116                changed regions via ioctl, as this method is simpler,
 117                more stable, and higher performance.
 118                default: fb_defio=1
 119
 120console         Allow fbcon to attach to udlfb provided framebuffers.
 121                Can be disabled if fbcon and other clients
 122                (e.g. X with --shared-vt) are in conflict.
 123                default: console=1
 124
 125shadow          Allocate a 2nd framebuffer to shadow what's currently across
 126                the USB bus in device memory. If any pixels are unchanged,
 127                do not transmit. Spends host memory to save USB transfers.
 128                Enabled by default. Only disable on very low memory systems.
 129                default: shadow=1
 130=============== ================================================================
 131
 132Sysfs Attributes
 133================
 134
 135Udlfb creates several files in /sys/class/graphics/fb?
 136Where ? is the sequential framebuffer id of the particular DisplayLink device
 137
 138======================== ========================================================
 139edid                     If a valid EDID blob is written to this file (typically
 140                         by a udev rule), then udlfb will use this EDID as a
 141                         backup in case reading the actual EDID of the monitor
 142                         attached to the DisplayLink device fails. This is
 143                         especially useful for fixed panels, etc. that cannot
 144                         communicate their capabilities via EDID. Reading
 145                         this file returns the current EDID of the attached
 146                         monitor (or last backup value written). This is
 147                         useful to get the EDID of the attached monitor,
 148                         which can be passed to utilities like parse-edid.
 149
 150metrics_bytes_rendered   32-bit count of pixel bytes rendered
 151
 152metrics_bytes_identical  32-bit count of how many of those bytes were found to be
 153                         unchanged, based on a shadow framebuffer check
 154
 155metrics_bytes_sent       32-bit count of how many bytes were transferred over
 156                         USB to communicate the resulting changed pixels to the
 157                         hardware. Includes compression and protocol overhead
 158
 159metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
 160                         above pixels (in thousands of cycles).
 161
 162metrics_reset            Write-only. Any write to this file resets all metrics
 163                         above to zero.  Note that the 32-bit counters above
 164                         roll over very quickly. To get reliable results, design
 165                         performance tests to start and finish in a very short
 166                         period of time (one minute or less is safe).
 167======================== ========================================================
 168
 169Bernie Thompson <bernie@plugable.com>
 170