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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; optionally 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 notifications 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 notification.
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 fb_defio=0 console=1 shadow=1
91
92 Or modify options on the fly at /sys/module/udlfb/parameters directory via
93 sudo nano fb_defio
94 change the parameter in place, and save the file.
95
96 Unplug/replug USB device to apply with new settings
97
98 Or for permanent option, create file like /etc/modprobe.d/udlfb.conf with text
99 options udlfb fb_defio=0 console=1 shadow=1
100
101 Accepted boolean options:
102
103 =============== ================================================================
104 fb_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
113 console 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
118 shadow 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
125 Sysfs Attributes
126 ================
127
128 Udlfb creates several files in /sys/class/graphics/fb?
129 Where ? is the sequential framebuffer id of the particular DisplayLink device
130
131 ======================== ========================================================
132 edid 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
143 metrics_bytes_rendered 32-bit count of pixel bytes rendered
144
145 metrics_bytes_identical 32-bit count of how many of those bytes were found to be
146 unchanged, based on a shadow framebuffer check
147
148 metrics_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
152 metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
153 above pixels (in thousands of cycles).
154
155 metrics_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
162 Bernie Thompson <bernie@plugable.com>