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By default, udlfb allocates a 2nd buffer to shadow what's across the bus on the USB device. It can operate without this shadow, but then it cannot tell which pixels have changed, and must send all. Saves host memory, but worsens the USB 2.0 bus bottleneck. This option allows users in very low memory situations (e.g. bifferboard) to optionally turn off this shadow framebuffer. Signed-off-by: Stuart Hopkins <stuart@linux-depot.com> Signed-off-by: Bernie Thompson <bernie@plugable.com> Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
150 lines
6.7 KiB
Plaintext
150 lines
6.7 KiB
Plaintext
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What is udlfb?
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===============
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This is a driver for DisplayLink USB 2.0 era graphics chips.
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DisplayLink chips provide simple hline/blit operations with some compression,
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pairing that with a hardware framebuffer (16MB) on the other end of the
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USB wire. That hardware framebuffer is able to drive the VGA, DVI, or HDMI
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monitor with no CPU involvement until a pixel has to change.
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The CPU or other local resource does all the rendering; optinally compares the
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result with a local shadow of the remote hardware framebuffer to identify
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the minimal set of pixels that have changed; and compresses and sends those
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pixels line-by-line via USB bulk transfers.
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Because of the efficiency of bulk transfers and a protocol on top that
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does not require any acks - the effect is very low latency that
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can support surprisingly high resolutions with good performance for
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non-gaming and non-video applications.
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Mode setting, EDID read, etc are other bulk or control transfers. Mode
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setting is very flexible - able to set nearly arbitrary modes from any timing.
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Advantages of USB graphics in general:
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* Ability to add a nearly arbitrary number of displays to any USB 2.0
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capable system. On Linux, number of displays is limited by fbdev interface
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(FB_MAX is currently 32). Of course, all USB devices on the same
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host controller share the same 480Mbs USB 2.0 interface.
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Advantages of supporting DisplayLink chips with kernel framebuffer interface:
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* The actual hardware functionality of DisplayLink chips matches nearly
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one-to-one with the fbdev interface, making the driver quite small and
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tight relative to the functionality it provides.
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* X servers and other applications can use the standard fbdev interface
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from user mode to talk to the device, without needing to know anything
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about USB or DisplayLink's protocol at all. A "displaylink" X driver
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and a slightly modified "fbdev" X driver are among those that already do.
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Disadvantages:
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* Fbdev's mmap interface assumes a real hardware framebuffer is mapped.
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In the case of USB graphics, it is just an allocated (virtual) buffer.
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Writes need to be detected and encoded into USB bulk transfers by the CPU.
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Accurate damage/changed area notifications work around this problem.
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In the future, hopefully fbdev will be enhanced with an small standard
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interface to allow mmap clients to report damage, for the benefit
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of virtual or remote framebuffers.
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* Fbdev does not arbitrate client ownership of the framebuffer well.
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* Fbcon assumes the first framebuffer it finds should be consumed for console.
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* It's not clear what the future of fbdev is, given the rise of KMS/DRM.
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How to use it?
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==============
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Udlfb, when loaded as a module, will match against all USB 2.0 generation
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DisplayLink chips (Alex and Ollie family). It will then attempt to read the EDID
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of the monitor, and set the best common mode between the DisplayLink device
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and the monitor's capabilities.
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If the DisplayLink device is successful, it will paint a "green screen" which
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means that from a hardware and fbdev software perspective, everything is good.
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At that point, a /dev/fb? interface will be present for user-mode applications
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to open and begin writing to the framebuffer of the DisplayLink device using
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standard fbdev calls. Note that if mmap() is used, by default the user mode
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application must send down damage notifcations to trigger repaints of the
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changed regions. Alternatively, udlfb can be recompiled with experimental
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defio support enabled, to support a page-fault based detection mechanism
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that can work without explicit notifcation.
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The most common client of udlfb is xf86-video-displaylink or a modified
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xf86-video-fbdev X server. These servers have no real DisplayLink specific
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code. They write to the standard framebuffer interface and rely on udlfb
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to do its thing. The one extra feature they have is the ability to report
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rectangles from the X DAMAGE protocol extension down to udlfb via udlfb's
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damage interface (which will hopefully be standardized for all virtual
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framebuffers that need damage info). These damage notifications allow
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udlfb to efficiently process the changed pixels.
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Module Options
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==============
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Special configuration for udlfb is usually unnecessary. There are a few
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options, however.
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From the command line, pass options to modprobe
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modprobe udlfb defio=1 console=1
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Or for permanent option, create file like /etc/modprobe.d/options with text
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options udlfb defio=1 console=1
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Accepted options:
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fb_defio Make use of the fb_defio (CONFIG_FB_DEFERRED_IO) kernel
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module to track changed areas of the framebuffer by page faults.
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Standard fbdev applications that use mmap but that do not
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report damage, may be able to work with this enabled.
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Disabled by default because of overhead and other issues.
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console Allow fbcon to attach to udlfb provided framebuffers. This
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is disabled by default because fbcon will aggressively consume
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the first framebuffer it finds, which isn't usually what the
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user wants in the case of USB displays.
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shadow Allocate a 2nd framebuffer to shadow what's currently across
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the USB bus in device memory. If any pixels are unchanged,
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do not transmit. Spends host memory to save USB transfers.
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Enabled by default. Only disable on very low memory systems.
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Sysfs Attributes
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================
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Udlfb creates several files in /sys/class/graphics/fb?
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Where ? is the sequential framebuffer id of the particular DisplayLink device
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edid If a valid EDID blob is written to this file (typically
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by a udev rule), then udlfb will use this EDID as a
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backup in case reading the actual EDID of the monitor
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attached to the DisplayLink device fails. This is
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especially useful for fixed panels, etc. that cannot
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communicate their capabilities via EDID. Reading
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this file returns the current EDID of the attached
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monitor (or last backup value written). This is
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useful to get the EDID of the attached monitor,
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which can be passed to utilities like parse-edid.
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metrics_bytes_rendered 32-bit count of pixel bytes rendered
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metrics_bytes_identical 32-bit count of how many of those bytes were found to be
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unchanged, based on a shadow framebuffer check
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metrics_bytes_sent 32-bit count of how many bytes were transferred over
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USB to communicate the resulting changed pixels to the
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hardware. Includes compression and protocol overhead
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metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the
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above pixels (in thousands of cycles).
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metrics_reset Write-only. Any write to this file resets all metrics
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above to zero. Note that the 32-bit counters above
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roll over very quickly. To get reliable results, design
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performance tests to start and finish in a very short
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period of time (one minute or less is safe).
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--
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Bernie Thompson <bernie@plugable.com>
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