Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
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 fb_defio Make use of the fb_defio (CONFIG_FB_DEFERRED_IO) kernel 104 module to track changed areas of the framebuffer by page faults. 105 Standard fbdev applications that use mmap but that do not 106 report damage, should be able to work with this enabled. 107 Disable when running with X server that supports reporting 108 changed regions via ioctl, as this method is simpler, 109 more stable, and higher performance. 110 default: fb_defio=1 111 112 console Allow fbcon to attach to udlfb provided framebuffers. 113 Can be disabled if fbcon and other clients 114 (e.g. X with --shared-vt) are in conflict. 115 default: console=1 116 117 shadow Allocate a 2nd framebuffer to shadow what's currently across 118 the USB bus in device memory. If any pixels are unchanged, 119 do not transmit. Spends host memory to save USB transfers. 120 Enabled by default. Only disable on very low memory systems. 121 default: shadow=1 122 123 Sysfs Attributes 124 ================ 125 126 Udlfb creates several files in /sys/class/graphics/fb? 127 Where ? is the sequential framebuffer id of the particular DisplayLink device 128 129 edid If a valid EDID blob is written to this file (typically 130 by a udev rule), then udlfb will use this EDID as a 131 backup in case reading the actual EDID of the monitor 132 attached to the DisplayLink device fails. This is 133 especially useful for fixed panels, etc. that cannot 134 communicate their capabilities via EDID. Reading 135 this file returns the current EDID of the attached 136 monitor (or last backup value written). This is 137 useful to get the EDID of the attached monitor, 138 which can be passed to utilities like parse-edid. 139 140 metrics_bytes_rendered 32-bit count of pixel bytes rendered 141 142 metrics_bytes_identical 32-bit count of how many of those bytes were found to be 143 unchanged, based on a shadow framebuffer check 144 145 metrics_bytes_sent 32-bit count of how many bytes were transferred over 146 USB to communicate the resulting changed pixels to the 147 hardware. Includes compression and protocol overhead 148 149 metrics_cpu_kcycles_used 32-bit count of CPU cycles used in processing the 150 above pixels (in thousands of cycles). 151 152 metrics_reset Write-only. Any write to this file resets all metrics 153 above to zero. Note that the 32-bit counters above 154 roll over very quickly. To get reliable results, design 155 performance tests to start and finish in a very short 156 period of time (one minute or less is safe). 157 158 -- 159 Bernie Thompson <email@example.com>