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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 18:49:45 +0000
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+=========================
+OMAP2/3 Display Subsystem
+=========================
+
+This is an almost total rewrite of the OMAP FB driver in drivers/video/omap
+(let's call it DSS1). The main differences between DSS1 and DSS2 are DSI,
+TV-out and multiple display support, but there are lots of small improvements
+also.
+
+The DSS2 driver (omapdss module) is in arch/arm/plat-omap/dss/, and the FB,
+panel and controller drivers are in drivers/video/omap2/. DSS1 and DSS2 live
+currently side by side, you can choose which one to use.
+
+Features
+--------
+
+Working and tested features include:
+
+- MIPI DPI (parallel) output
+- MIPI DSI output in command mode
+- MIPI DBI (RFBI) output
+- SDI output
+- TV output
+- All pieces can be compiled as a module or inside kernel
+- Use DISPC to update any of the outputs
+- Use CPU to update RFBI or DSI output
+- OMAP DISPC planes
+- RGB16, RGB24 packed, RGB24 unpacked
+- YUV2, UYVY
+- Scaling
+- Adjusting DSS FCK to find a good pixel clock
+- Use DSI DPLL to create DSS FCK
+
+Tested boards include:
+- OMAP3 SDP board
+- Beagle board
+- N810
+
+omapdss driver
+--------------
+
+The DSS driver does not itself have any support for Linux framebuffer, V4L or
+such like the current ones, but it has an internal kernel API that upper level
+drivers can use.
+
+The DSS driver models OMAP's overlays, overlay managers and displays in a
+flexible way to enable non-common multi-display configuration. In addition to
+modelling the hardware overlays, omapdss supports virtual overlays and overlay
+managers. These can be used when updating a display with CPU or system DMA.
+
+omapdss driver support for audio
+--------------------------------
+There exist several display technologies and standards that support audio as
+well. Hence, it is relevant to update the DSS device driver to provide an audio
+interface that may be used by an audio driver or any other driver interested in
+the functionality.
+
+The audio_enable function is intended to prepare the relevant
+IP for playback (e.g., enabling an audio FIFO, taking in/out of reset
+some IP, enabling companion chips, etc). It is intended to be called before
+audio_start. The audio_disable function performs the reverse operation and is
+intended to be called after audio_stop.
+
+While a given DSS device driver may support audio, it is possible that for
+certain configurations audio is not supported (e.g., an HDMI display using a
+VESA video timing). The audio_supported function is intended to query whether
+the current configuration of the display supports audio.
+
+The audio_config function is intended to configure all the relevant audio
+parameters of the display. In order to make the function independent of any
+specific DSS device driver, a struct omap_dss_audio is defined. Its purpose
+is to contain all the required parameters for audio configuration. At the
+moment, such structure contains pointers to IEC-60958 channel status word
+and CEA-861 audio infoframe structures. This should be enough to support
+HDMI and DisplayPort, as both are based on CEA-861 and IEC-60958.
+
+The audio_enable/disable, audio_config and audio_supported functions could be
+implemented as functions that may sleep. Hence, they should not be called
+while holding a spinlock or a readlock.
+
+The audio_start/audio_stop function is intended to effectively start/stop audio
+playback after the configuration has taken place. These functions are designed
+to be used in an atomic context. Hence, audio_start should return quickly and be
+called only after all the needed resources for audio playback (audio FIFOs,
+DMA channels, companion chips, etc) have been enabled to begin data transfers.
+audio_stop is designed to only stop the audio transfers. The resources used
+for playback are released using audio_disable.
+
+The enum omap_dss_audio_state may be used to help the implementations of
+the interface to keep track of the audio state. The initial state is _DISABLED;
+then, the state transitions to _CONFIGURED, and then, when it is ready to
+play audio, to _ENABLED. The state _PLAYING is used when the audio is being
+rendered.
+
+
+Panel and controller drivers
+----------------------------
+
+The drivers implement panel or controller specific functionality and are not
+usually visible to users except through omapfb driver. They register
+themselves to the DSS driver.
+
+omapfb driver
+-------------
+
+The omapfb driver implements arbitrary number of standard linux framebuffers.
+These framebuffers can be routed flexibly to any overlays, thus allowing very
+dynamic display architecture.
+
+The driver exports some omapfb specific ioctls, which are compatible with the
+ioctls in the old driver.
+
+The rest of the non standard features are exported via sysfs. Whether the final
+implementation will use sysfs, or ioctls, is still open.
+
+V4L2 drivers
+------------
+
+V4L2 is being implemented in TI.
+
+From omapdss point of view the V4L2 drivers should be similar to framebuffer
+driver.
+
+Architecture
+--------------------
+
+Some clarification what the different components do:
+
+ - Framebuffer is a memory area inside OMAP's SRAM/SDRAM that contains the
+ pixel data for the image. Framebuffer has width and height and color
+ depth.
+ - Overlay defines where the pixels are read from and where they go on the
+ screen. The overlay may be smaller than framebuffer, thus displaying only
+ part of the framebuffer. The position of the overlay may be changed if
+ the overlay is smaller than the display.
+ - Overlay manager combines the overlays in to one image and feeds them to
+ display.
+ - Display is the actual physical display device.
+
+A framebuffer can be connected to multiple overlays to show the same pixel data
+on all of the overlays. Note that in this case the overlay input sizes must be
+the same, but, in case of video overlays, the output size can be different. Any
+framebuffer can be connected to any overlay.
+
+An overlay can be connected to one overlay manager. Also DISPC overlays can be
+connected only to DISPC overlay managers, and virtual overlays can be only
+connected to virtual overlays.
+
+An overlay manager can be connected to one display. There are certain
+restrictions which kinds of displays an overlay manager can be connected:
+
+ - DISPC TV overlay manager can be only connected to TV display.
+ - Virtual overlay managers can only be connected to DBI or DSI displays.
+ - DISPC LCD overlay manager can be connected to all displays, except TV
+ display.
+
+Sysfs
+-----
+The sysfs interface is mainly used for testing. I don't think sysfs
+interface is the best for this in the final version, but I don't quite know
+what would be the best interfaces for these things.
+
+The sysfs interface is divided to two parts: DSS and FB.
+
+/sys/class/graphics/fb? directory:
+mirror 0=off, 1=on
+rotate Rotation 0-3 for 0, 90, 180, 270 degrees
+rotate_type 0 = DMA rotation, 1 = VRFB rotation
+overlays List of overlay numbers to which framebuffer pixels go
+phys_addr Physical address of the framebuffer
+virt_addr Virtual address of the framebuffer
+size Size of the framebuffer
+
+/sys/devices/platform/omapdss/overlay? directory:
+enabled 0=off, 1=on
+input_size width,height (ie. the framebuffer size)
+manager Destination overlay manager name
+name
+output_size width,height
+position x,y
+screen_width width
+global_alpha global alpha 0-255 0=transparent 255=opaque
+
+/sys/devices/platform/omapdss/manager? directory:
+display Destination display
+name
+alpha_blending_enabled 0=off, 1=on
+trans_key_enabled 0=off, 1=on
+trans_key_type gfx-destination, video-source
+trans_key_value transparency color key (RGB24)
+default_color default background color (RGB24)
+
+/sys/devices/platform/omapdss/display? directory:
+
+=============== =============================================================
+ctrl_name Controller name
+mirror 0=off, 1=on
+update_mode 0=off, 1=auto, 2=manual
+enabled 0=off, 1=on
+name
+rotate Rotation 0-3 for 0, 90, 180, 270 degrees
+timings Display timings (pixclock,xres/hfp/hbp/hsw,yres/vfp/vbp/vsw)
+ When writing, two special timings are accepted for tv-out:
+ "pal" and "ntsc"
+panel_name
+tear_elim Tearing elimination 0=off, 1=on
+output_type Output type (video encoder only): "composite" or "svideo"
+=============== =============================================================
+
+There are also some debugfs files at <debugfs>/omapdss/ which show information
+about clocks and registers.
+
+Examples
+--------
+
+The following definitions have been made for the examples below::
+
+ ovl0=/sys/devices/platform/omapdss/overlay0
+ ovl1=/sys/devices/platform/omapdss/overlay1
+ ovl2=/sys/devices/platform/omapdss/overlay2
+
+ mgr0=/sys/devices/platform/omapdss/manager0
+ mgr1=/sys/devices/platform/omapdss/manager1
+
+ lcd=/sys/devices/platform/omapdss/display0
+ dvi=/sys/devices/platform/omapdss/display1
+ tv=/sys/devices/platform/omapdss/display2
+
+ fb0=/sys/class/graphics/fb0
+ fb1=/sys/class/graphics/fb1
+ fb2=/sys/class/graphics/fb2
+
+Default setup on OMAP3 SDP
+--------------------------
+
+Here's the default setup on OMAP3 SDP board. All planes go to LCD. DVI
+and TV-out are not in use. The columns from left to right are:
+framebuffers, overlays, overlay managers, displays. Framebuffers are
+handled by omapfb, and the rest by the DSS::
+
+ FB0 --- GFX -\ DVI
+ FB1 --- VID1 --+- LCD ---- LCD
+ FB2 --- VID2 -/ TV ----- TV
+
+Example: Switch from LCD to DVI
+-------------------------------
+
+::
+
+ w=`cat $dvi/timings | cut -d "," -f 2 | cut -d "/" -f 1`
+ h=`cat $dvi/timings | cut -d "," -f 3 | cut -d "/" -f 1`
+
+ echo "0" > $lcd/enabled
+ echo "" > $mgr0/display
+ fbset -fb /dev/fb0 -xres $w -yres $h -vxres $w -vyres $h
+ # at this point you have to switch the dvi/lcd dip-switch from the omap board
+ echo "dvi" > $mgr0/display
+ echo "1" > $dvi/enabled
+
+After this the configuration looks like:::
+
+ FB0 --- GFX -\ -- DVI
+ FB1 --- VID1 --+- LCD -/ LCD
+ FB2 --- VID2 -/ TV ----- TV
+
+Example: Clone GFX overlay to LCD and TV
+----------------------------------------
+
+::
+
+ w=`cat $tv/timings | cut -d "," -f 2 | cut -d "/" -f 1`
+ h=`cat $tv/timings | cut -d "," -f 3 | cut -d "/" -f 1`
+
+ echo "0" > $ovl0/enabled
+ echo "0" > $ovl1/enabled
+
+ echo "" > $fb1/overlays
+ echo "0,1" > $fb0/overlays
+
+ echo "$w,$h" > $ovl1/output_size
+ echo "tv" > $ovl1/manager
+
+ echo "1" > $ovl0/enabled
+ echo "1" > $ovl1/enabled
+
+ echo "1" > $tv/enabled
+
+After this the configuration looks like (only relevant parts shown)::
+
+ FB0 +-- GFX ---- LCD ---- LCD
+ \- VID1 ---- TV ---- TV
+
+Misc notes
+----------
+
+OMAP FB allocates the framebuffer memory using the standard dma allocator. You
+can enable Contiguous Memory Allocator (CONFIG_CMA) to improve the dma
+allocator, and if CMA is enabled, you use "cma=" kernel parameter to increase
+the global memory area for CMA.
+
+Using DSI DPLL to generate pixel clock it is possible produce the pixel clock
+of 86.5MHz (max possible), and with that you get 1280x1024@57 output from DVI.
+
+Rotation and mirroring currently only supports RGB565 and RGB8888 modes. VRFB
+does not support mirroring.
+
+VRFB rotation requires much more memory than non-rotated framebuffer, so you
+probably need to increase your vram setting before using VRFB rotation. Also,
+many applications may not work with VRFB if they do not pay attention to all
+framebuffer parameters.
+
+Kernel boot arguments
+---------------------
+
+omapfb.mode=<display>:<mode>[,...]
+ - Default video mode for specified displays. For example,
+ "dvi:800x400MR-24@60". See drivers/video/modedb.c.
+ There are also two special modes: "pal" and "ntsc" that
+ can be used to tv out.
+
+omapfb.vram=<fbnum>:<size>[@<physaddr>][,...]
+ - VRAM allocated for a framebuffer. Normally omapfb allocates vram
+ depending on the display size. With this you can manually allocate
+ more or define the physical address of each framebuffer. For example,
+ "1:4M" to allocate 4M for fb1.
+
+omapfb.debug=<y|n>
+ - Enable debug printing. You have to have OMAPFB debug support enabled
+ in kernel config.
+
+omapfb.test=<y|n>
+ - Draw test pattern to framebuffer whenever framebuffer settings change.
+ You need to have OMAPFB debug support enabled in kernel config.
+
+omapfb.vrfb=<y|n>
+ - Use VRFB rotation for all framebuffers.
+
+omapfb.rotate=<angle>
+ - Default rotation applied to all framebuffers.
+ 0 - 0 degree rotation
+ 1 - 90 degree rotation
+ 2 - 180 degree rotation
+ 3 - 270 degree rotation
+
+omapfb.mirror=<y|n>
+ - Default mirror for all framebuffers. Only works with DMA rotation.
+
+omapdss.def_disp=<display>
+ - Name of default display, to which all overlays will be connected.
+ Common examples are "lcd" or "tv".
+
+omapdss.debug=<y|n>
+ - Enable debug printing. You have to have DSS debug support enabled in
+ kernel config.
+
+TODO
+----
+
+DSS locking
+
+Error checking
+
+- Lots of checks are missing or implemented just as BUG()
+
+System DMA update for DSI
+
+- Can be used for RGB16 and RGB24P modes. Probably not for RGB24U (how
+ to skip the empty byte?)
+
+OMAP1 support
+
+- Not sure if needed