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diff --git a/Documentation/fb/udlfb.txt b/Documentation/fb/udlfb.txt new file mode 100644 index 00000000000..7fdde2a02a2 --- /dev/null +++ b/Documentation/fb/udlfb.txt @@ -0,0 +1,144 @@ + +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> |