staging: udlfb: Add intro udlfb documentation

Add documentation for the udlfb framebuffer driver for DisplayLink devices.

Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

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+
+What is udlfb?
+===============
+
+This is a driver for DisplayLink USB 2.0 era graphics chips.
+
+DisplayLink chips provide simple hline/blit operations with some compression,
+pairing that with a hardware framebuffer (16MB) on the other end of the
+USB wire.  That hardware framebuffer is able to drive the VGA, DVI, or HDMI
+monitor with no CPU involvement until a pixel has to change.
+
+The CPU or other local resource does all the rendering; optinally compares the
+result with a local shadow of the remote hardware framebuffer to identify
+the minimal set of pixels that have changed; and compresses and sends those
+pixels line-by-line via USB bulk transfers.
+
+Because of the efficiency of bulk transfers and a protocol on top that
+does not require any acks - the effect is very low latency that
+can support surprisingly high resolutions with good performance for
+non-gaming and non-video applications.
+
+Mode setting, EDID read, etc are other bulk or control transfers. Mode
+setting is very flexible - able to set nearly arbitrary modes from any timing.
+
+Advantages of USB graphics in general:
+
+ * Ability to add a nearly arbitrary number of displays to any USB 2.0
+   capable system. On Linux, number of displays is limited by fbdev interface
+   (FB_MAX is currently 32). Of course, all USB devices on the same
+   host controller share the same 480Mbs USB 2.0 interface.
+
+Advantages of supporting DisplayLink chips with kernel framebuffer interface:
+
+ * The actual hardware functionality of DisplayLink chips matches nearly
+   one-to-one with the fbdev interface, making the driver quite small and
+   tight relative to the functionality it provides.
+ * X servers and other applications can use the standard fbdev interface
+   from user mode to talk to the device, without needing to know anything
+   about USB or DisplayLink's protocol at all. A "displaylink" X driver
+   and a slightly modified "fbdev" X driver are among those that already do.
+
+Disadvantages:
+
+ * Fbdev's mmap interface assumes a real hardware framebuffer is mapped.
+   In the case of USB graphics, it is just an allocated (virtual) buffer.
+   Writes need to be detected and encoded into USB bulk transfers by the CPU.
+   Accurate damage/changed area notifications work around this problem.
+   In the future, hopefully fbdev will be enhanced with an small standard
+   interface to allow mmap clients to report damage, for the benefit
+   of virtual or remote framebuffers.
+ * Fbdev does not arbitrate client ownership of the framebuffer well.
+ * Fbcon assumes the first framebuffer it finds should be consumed for console.
+ * It's not clear what the future of fbdev is, given the rise of KMS/DRM.
+
+How to use it?
+==============
+
+Udlfb, when loaded as a module, will match against all USB 2.0 generation
+DisplayLink chips (Alex and Ollie family). It will then attempt to read the EDID
+of the monitor, and set the best common mode between the DisplayLink device
+and the monitor's capabilities.
+
+If the DisplayLink device is successful, it will paint a "green screen" which
+means that from a hardware and fbdev software perspective, everything is good.
+
+At that point, a /dev/fb? interface will be present for user-mode applications
+to open and begin writing to the framebuffer of the DisplayLink device using
+standard fbdev calls.  Note that if mmap() is used, by default the user mode
+application must send down damage notifcations to trigger repaints of the
+changed regions.  Alternatively, udlfb can be recompiled with experimental
+defio support enabled, to support a page-fault based detection mechanism
+that can work without explicit notifcation.
+
+The most common client of udlfb is xf86-video-displaylink or a modified
+xf86-video-fbdev X server. These servers have no real DisplayLink specific
+code. They write to the standard framebuffer interface and rely on udlfb
+to do its thing.  The one extra feature they have is the ability to report
+rectangles from the X DAMAGE protocol extension down to udlfb via udlfb's
+damage interface (which will hopefully be standardized for all virtual
+framebuffers that need damage info). These damage notifications allow
+udlfb to efficiently process the changed pixels.
+
+Module Options
+==============
+
+Special configuration for udlfb is usually unnecessary. There are a few
+options, however.
+
+From the command line, pass options to modprobe
+modprobe udlfb defio=1 console=1
+
+Or for permanent option, create file like /etc/modprobe.d/options with text
+options udlfb defio=1 console=1
+
+Accepted options:
+
+fb_defio       Make use of the fb_defio (CONFIG_FB_DEFERRED_IO) kernel
+               module to track changed areas of the framebuffer by page faults.
+               Standard fbdev applications that use mmap but that do not
+               report damage, may be able to work with this enabled.
+               Disabled by default because of overhead and other issues.
+
+console                Allow fbcon to attach to udlfb provided framebuffers. This
+               is disabled by default because fbcon will aggressively consume
+               the first framebuffer it finds, which isn't usually what the
+               user wants in the case of USB displays.
+
+Sysfs Attributes
+================
+
+Udlfb creates several files in /sys/class/graphics/fb?
+Where ? is the sequential framebuffer id of the particular DisplayLink device
+
+edid                   If a valid EDID blob is written to this file (typically
+                       by a udev rule), then udlfb will use this EDID as a
+                       backup in case reading the actual EDID of the monitor
+                       attached to the DisplayLink device fails. This is
+                       especially useful for fixed panels, etc. that cannot
+                       communicate their capabilities via EDID. Reading
+                       this file returns the current EDID of the attached
+                       monitor (or last backup value written). This is
+                       useful to get the EDID of the attached monitor,
+                       which can be passed to utilities like parse-edid.
+
+metrics_bytes_rendered 32-bit count of pixel bytes rendered
+
+metrics_bytes_identical 32-bit count of how many of those bytes were found to be
+                       unchanged, based on a shadow framebuffer check
+
+metrics_bytes_sent     32-bit count of how many bytes were transferred over
+                       USB to communicate the resulting changed pixels to the
+                       hardware. Includes compression and protocol overhead
+
+metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
+                       above pixels (in thousands of cycles).
+
+metrics_reset          Write-only. Any write to this file resets all metrics
+                       above to zero.  Note that the 32-bit counters above
+                       roll over very quickly. To get reliable results, design
+                       performance tests to start and finish in a very short
+                       period of time (one minute or less is safe).
+
+--
+Bernie Thompson <bernie@plugable.com>