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