Documentation/fb/udlfb.txt

   1
   2 What is udlfb?
   3 ===============
   4
   5 This is a driver for DisplayLink USB 2.0 era graphics chips.
   6
   7 DisplayLink chips provide simple hline/blit operations with some compression,
   8 pairing that with a hardware framebuffer (16MB) on the other end of the
   9 USB wire.  That hardware framebuffer is able to drive the VGA, DVI, or HDMI
  10 monitor with no CPU involvement until a pixel has to change.
  11
  12 The CPU or other local resource does all the rendering; optinally compares the
  13 result with a local shadow of the remote hardware framebuffer to identify
  14 the minimal set of pixels that have changed; and compresses and sends those
  15 pixels line-by-line via USB bulk transfers.
  16
  17 Because of the efficiency of bulk transfers and a protocol on top that
  18 does not require any acks - the effect is very low latency that
  19 can support surprisingly high resolutions with good performance for
  20 non-gaming and non-video applications.
  21
  22 Mode setting, EDID read, etc are other bulk or control transfers. Mode
  23 setting is very flexible - able to set nearly arbitrary modes from any timing.
  24
  25 Advantages 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
  32 Advantages 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
  42 Disadvantages:
  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
  55 How to use it?
  56 ==============
  57
  58 Udlfb, when loaded as a module, will match against all USB 2.0 generation
  59 DisplayLink chips (Alex and Ollie family). It will then attempt to read the EDID
  60 of the monitor, and set the best common mode between the DisplayLink device
  61 and the monitor's capabilities.
  62
  63 If the DisplayLink device is successful, it will paint a "green screen" which
  64 means that from a hardware and fbdev software perspective, everything is good.
  65
  66 At that point, a /dev/fb? interface will be present for user-mode applications
  67 to open and begin writing to the framebuffer of the DisplayLink device using
  68 standard fbdev calls.  Note that if mmap() is used, by default the user mode
  69 application must send down damage notifcations to trigger repaints of the
  70 changed regions.  Alternatively, udlfb can be recompiled with experimental
  71 defio support enabled, to support a page-fault based detection mechanism
  72 that can work without explicit notifcation.
  73
  74 The most common client of udlfb is xf86-video-displaylink or a modified
  75 xf86-video-fbdev X server. These servers have no real DisplayLink specific
  76 code. They write to the standard framebuffer interface and rely on udlfb
  77 to do its thing.  The one extra feature they have is the ability to report
  78 rectangles from the X DAMAGE protocol extension down to udlfb via udlfb's
  79 damage interface (which will hopefully be standardized for all virtual
  80 framebuffers that need damage info). These damage notifications allow
  81 udlfb to efficiently process the changed pixels.
  82
  83 Module Options
  84 ==============
  85
  86 Special configuration for udlfb is usually unnecessary. There are a few
  87 options, however.
  88
  89 From the command line, pass options to modprobe
  90 modprobe udlfb defio=1 console=1
  91
  92 Or for permanent option, create file like /etc/modprobe.d/options with text
  93 options udlfb defio=1 console=1
  94
  95 Accepted options:
  96
  97 fb_defio        Make use of the fb_defio (CONFIG_FB_DEFERRED_IO) kernel
  98                 module to track changed areas of the framebuffer by page faults.
  99                 Standard fbdev applications that use mmap but that do not
 100                 report damage, may be able to work with this enabled.
 101                 Disabled by default because of overhead and other issues.
 102
 103 console         Allow fbcon to attach to udlfb provided framebuffers. This
 104                 is disabled by default because fbcon will aggressively consume
 105                 the first framebuffer it finds, which isn't usually what the
 106                 user wants in the case of USB displays.
 107
 108 Sysfs Attributes
 109 ================
 110
 111 Udlfb creates several files in /sys/class/graphics/fb?
 112 Where ? is the sequential framebuffer id of the particular DisplayLink device
 113
 114 edid                    If a valid EDID blob is written to this file (typically
 115                         by a udev rule), then udlfb will use this EDID as a
 116                         backup in case reading the actual EDID of the monitor
 117                         attached to the DisplayLink device fails. This is
 118                         especially useful for fixed panels, etc. that cannot
 119                         communicate their capabilities via EDID. Reading
 120                         this file returns the current EDID of the attached
 121                         monitor (or last backup value written). This is
 122                         useful to get the EDID of the attached monitor,
 123                         which can be passed to utilities like parse-edid.
 124
 125 metrics_bytes_rendered  32-bit count of pixel bytes rendered
 126
 127 metrics_bytes_identical 32-bit count of how many of those bytes were found to be
 128                         unchanged, based on a shadow framebuffer check
 129
 130 metrics_bytes_sent      32-bit count of how many bytes were transferred over
 131                         USB to communicate the resulting changed pixels to the
 132                         hardware. Includes compression and protocol overhead
 133
 134 metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
 135                         above pixels (in thousands of cycles).
 136
 137 metrics_reset           Write-only. Any write to this file resets all metrics
 138                         above to zero.  Note that the 32-bit counters above
 139                         roll over very quickly. To get reliable results, design
 140                         performance tests to start and finish in a very short
 141                         period of time (one minute or less is safe).
 142
 143 --
 144 Bernie Thompson <bernie@plugable.com>