Root/Documentation/fb/udlfb.txt

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; optinally 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 notifcations 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 notifcation.
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 defio=1 console=1
91
92Or for permanent option, create file like /etc/modprobe.d/options with text
93options udlfb defio=1 console=1
94
95Accepted options:
96
97fb_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
103console 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
108Sysfs Attributes
109================
110
111Udlfb creates several files in /sys/class/graphics/fb?
112Where ? is the sequential framebuffer id of the particular DisplayLink device
113
114edid 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
125metrics_bytes_rendered 32-bit count of pixel bytes rendered
126
127metrics_bytes_identical 32-bit count of how many of those bytes were found to be
128            unchanged, based on a shadow framebuffer check
129
130metrics_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
134metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
135            above pixels (in thousands of cycles).
136
137metrics_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--
144Bernie Thompson <bernie@plugable.com>
145

Archive Download this file



interactive