Hi,
This is an RFC for OMAP Display DT support. The patches work fine, at least
for me, but they are not perfect. I mostly don't have any clear questions
about specific issues, but I would like to get feedback on the selected
approaches in general, and also ideas how to proceed with the series.
This series contains the following:
DT support for the following OMAP's display subsystem devices:
- DSS
- DISPC
- DPI
- HDMI
- VENC
- DSI
- (DBI/RFBI DT is not yet implemented)
DT support for the following external display devices:
- panel-dsi-cm (Generic DSI command mode panel)
- encoder-tfp410 (DPI-to-DVI encoder)
- connector-dvi
- encoder-tpd12s015 (HDMI level-shifter & ESD protection)
- hdmi-connector
- panel-dpi (Generic DPI panel)
- connector-analog-tv (S-Video & Composite)
DT support for the following boards:
- OMAP4 PandaBoard
- OMAP4 SDP
- OMAP3 BeagleBoard
- OMAP3 Overo with Palo43 LCD expansion-board
The patches are not final, and many contain quite brief descriptions.
Binding descriptions are also still missing. The code and bindings in the
patches should be pretty straightforward, though.
The series is based on v3.11-rc2 + a couple of non-DSS fixes. The series
can also be found from:
git://git.kernel.org/pub/scm/linux/kernel/git/tomba/linux.git work/dss-dev-model-dt
Vocabulary
==========
Display Entity - a hardware entity that takes one or more video streams as
input and outputs one or more video streams.
Upstream Entity - A display entity in the "upstream" side of the video
stream, i.e. closer to the original video source.
Downstream Entity - A display entity in the "downstream" side of the video
stream, i.e. closer to the panel or monitor.
Video pipeline - A chain of multiple display entities, starting from the
SoC, going to the display device the user sees.
Display or Panel - A display entity showing the pixels to the user
Encoder - A display entity that takes video as an input and (usually)
outputs it in some other format.
Connector - HDMI/DVI/etc Connector on the board, to which an external
monitor is connected.
About Stable DT Bindings
========================
Generally speaking, the DT bindings should be stable. This brings the
following problems:
We already have DT bindings for some OMAP4 and OMAP3 boards, and OMAP4
boards do not even have board files anymore. There are no display bindings
for those OMAP4 boards, but the display support is currently enabled as a
hack, by calling board-file-like code to add the display devices for the
selected boards. So, when we add the display bindings, we should still
support the current DT files which do not have the display bindings. Which
would mean that we'd need to keep the hacky code forever. Considering the
fact that the hacky code does not work quite correct in all cases, I don't
see keeping it as a very good option.
CDF (Common Display Framework) is in the works, and will most likely have
different or more detailed bindings. Moving to CDF would mean we'd somehow
need to still support the old OMAP bindings. In theory the display DT
bindings should stay the same, as they represent the HW, not any SW
constructs, but in practice I find it hard to believe the OMAP display
bindings would be so perfect that there would be no need for changes.
We most likely should somehow represent DSS clock tree in DT. That is not a
simple task, and when we manage to do it, it again means supporting the DT
files without clock tree data.
All in all, I'm a bit scared to push the display bindings, as it's already
clear there are changes coming. Then again, supporting the current hack for
OMAP4 based boards is not nice at all, and has issues, so it would be really
nice to get OMAP4 boards use proper display bindings.
General description of the DT bindings
======================================
All the display entities are represented as DT nodes of their own, and have
a matching Linux driver. The display entities are organized by their control
bus; that is, if a display entity is not controlled via any bus, it's a
platform device, and if, say, it's controlled via i2c device, it's an i2c
device.
The exception to the above are DSI and DBI. DSI and DBI are combined control
and video busses, but the use of the busses for control purposes is not
independent of the video stream. Also, the the busses are, in practice,
one-to-one links. And last, DSI and DBI display entities are often also
controllable via, say, i2c. For these reasons there is no real Linux bus for
DSI and DBI and thus the DSI and DBI devices are either platform devices or
i2c devices.
The display entities are connected via "video-source" property. The
video-source points to the upstream display entity where the video data
comes from, and a chain of display entities thus form a full video pipeline.
All video pipelines end with either a panel or a connector.
All the data related to a display entity, and how it is connected on the
given board, is defined in the DT node of the display entity. This means
that the DT node of the upstream entity does not have to be modified when
adding support for new boards.
As an example, consider OMAP3's DPI and two boards using it for panels.
omap3.dtsi contains a node for the DPI, and the board dts files contain
nodes for their panels. The board dts files do not change anything in the
included DPI node. So:
omap3.dtsi:
dpi: encoder@0 {
compatible = "ti,omap3-dpi";
};
omap3-board1.dts:
lcd0: display@0 {
compatible = "samsung,lte430wq-f0c", "panel-dpi";
video-source = <&dpi>;
data-lines = <24>;
};
omap3-board2.dts:
lcd0: display@0 {
compatible = "samsung,lte430wq-f0c", "panel-dpi";
video-source = <&dpi>;
data-lines = <16>;
};
The logic here is that the boards may have multiple panels that are
connected to the same source, even if the panels can only be used one at a
time. Each panel may thus have different properties for the bus, like the
number of data-lines.
Video bus properties
====================
One question I've been pondering for a long time is related to the bus
between two display entities. As an example, DPI (parallel RGB) requires
configuring the number of datalines used. As described above, the properties
of the video bus are represented in the downstream entity.
This approach has one drawback: how to represent features specific to the
upstream entity? Say, if OMAP's DSI has a bus-related foo-feature, which can
be used in some scenarios, the only place where this foo-feature can be
specified is the OMAP DSI's properties. Not in the DSI Panel's properties,
which in the current model contains properties related to the bus.
So Laurent has proposed to use V4L2-like ports, as described in
Documentation/devicetree/bindings/media/video-interfaces.txt. I have not
implemented such feature for OMAP DSS for the following reasons:
- The current supported displays we use work fine with the current method
- If I were to implement such system, it'd most certainly be different than
what CDF will have.
That said, the port based approach does sound good, and it would also remove
the design issue with OMAP DPI and SDI modules as described later. So maybe
I should just go forward with it and hope that CDF will do it in similar
manner.
DSI Module ID
=============
On OMAP4 we have two DSI modules. To configure the clock routing and pin
muxing, we need to know the hardware module ID for the DSI device, i.e. is
this Linux platform device DSI1 or DSI2. The same issue exists with other
SoCs with multiple outputs of the same kind.
With non-DT case, we used the platform device's ID directly. With DT, that
doesn't work. I don't currently have a good solution for this, so as a
temporary solution the DSI driver contains a table, from which it can find
the HW module ID by using the device's base address.
I believe, but I am not totally sure, that we can remove the concept of DSI
module ID if we have a good representation of the DSS clock tree and DSI pin
muxing in DT. The clock tree is probably a huge undertaking, but the pin
muxing should be much easier. However, pinmuxing also is some complications,
like the pins requiring a regulator to be enabled.
Display names and aliases
=========================
With the board-file based model each display was given a name in the board
file. Additionally, each display was given an alias in the style "displayX",
where X is in incrementing number.
The name could be used by the user to see which display device is what, i.e.
on Pandaboard there are displays names "dvi" and "hdmi".
The DT bindings do not have such a name. It would be simple to add a
"nickname" property to each display node, but it just looked rather ugly so
I have left it out.
Additionally, as there's no clear order in which the displays are created,
and thus the number in "displayX" alias could change semi-randomly, I added
the displays to "aliases" node. This keeps the display number the same over
boots, and also gives us some way to define a default display, i.e. which
display to use initially if the user has not specified it.
omapdss virtual device
======================
In addition to the DSS devices matching to DSS hardware modules, we have a
"virtual" omapdss device which does not match to any actual HW module. The
device is there mostly for legacy reasons, but it has also allowed us to
easily pass platform callbacks. The same device is also present in DT
solution. It is created in code, and not present in DT bindings.
Obviously, this omapdss virtual device is on the hack side, and nobody would
mind if it would disappear.
The following data is passed via omapdss device's platform data:
- OMAP DSS version. In theory, the DSS revision registers should tell us
which features the HW supports. In practice, the HW people have not
bothered to change the revision number each time they've made changes. So
we pass a DSS version from the platform code, based on OMAP revision
number.
- omap_dss_set_min_bus_tput() and omap_pm_get_dev_context_loss_count() to
manage PM
- DSI pin muxing functions.
I have some ideas how to deduce the DSS version by poking to certain DSS
registers, but it is not yet tested so I don't know if it will work.
We could do altogether without omap_pm_get_dev_context_loss_count(), so that
should be removable with some work. I am not sure if
omap_dss_set_min_bus_tput() is supported via standard PM calls or not.
However, the use of set_min_bus_tput() is actually a hack, as we're not
really setting min bus tput. What we want to do is prevent OPP50. DSS clocks
have different maximums on OPP100 and OPP50. So if DSS uses a clock over
OPP50 limit, OPP50 cannot be entered. We prevent OPP50 by setting an
arbitrarily high min bus tput.
The DSI pin muxing should also be solvable with DT based solution, but is
not the most trivial task and needs some work.
So, I presume that at some point we can remove the omapdss device, but in
the current solution it exists for the above reasons.
DSS submodules in DT bindings
=============================
The OMAP DSS modules are accessed via L4 or L3, and in that sense they
should be on the same level in the DT bindings. However, we do not have them
in the same level, but there is a main "dss" node, under which all the other
DSS modules reside. The main reason for this is that the main DSS device
needs to be enabled for the other modules to work properly, and having this
structure makes runtime PM handle enabling DSS automatically.
If I recall right, somebody (Paul?) mentioned that in the hardware there is
actually some DSS internal bus, and thus the DT structure is good in that
sense also.
We also have nodes (and matching Linux devices) for DPI and SDI. Neither of
them are actuall a separate IP block in the hardware, but are really more
parts of DSS and maybe DISPC. They are still represented in the same way as
the other DSS modules, to have similar architecture for all DSS outputs. But
as they do not have an address-space of their own, the DT nodes use 0 and 1
as "addresses", i.e. "dpi: encoder@0".
That is rather ugly, and maybe could use some cleaning up. A V4L2 port style
approach would probably allow us to add DPI and SDI ports as part of DSS.
Some of the DSS modules are actually a combination of multiple smaller
modules. For example, the DSI module contains DSI protocol, DSI PHY and DSI
PLL modules, each in their own address space. These could perhaps be
presented as separate DT nodes and Linux devices, but I am not sure if that
is a good approach or not.
DT Node Names
=============
I have used the following naming conventions with DT nodes:
- Panels are named "display"
- Connectors are named "connector"
- Encoders are named "encoder"
- DSS and DISPC are "dss" and "dispc", as they don't really match any of the
above
When appropriate, the address part of the node is the base address, as in
"dsi1: encoder@58004000". For platform devices, I have used an increasing
number for the address, as in "tpd12s015: encoder@1".
Final words
===========
So as is evident, I have things in my mind that should be improved. Maybe
the most important question for short term future is:
Can we add DSS DT bindings for OMAP4 as unstable bindings? It would give us
some proper testing of the related code, and would also allow us to remove
the related hacks (which don't even work quite right). However, I have no
idea yet when the unstable DSS bindings would turn stable.
If we shouldn't add the bindings as unstable, when should the bindings be
added? Wait until CDF is in the mainline, and use that?
I have not explained every piece of DSS in detail, as that would result in a
book instead of this email, so feel free to ask for more details about any
part.
And last but not least (for me, at least =), I'm on vacation for the next
two weeks, so answers may be delayed.
Tomi
Tomi Valkeinen (22):
ARM: OMAP: remove DSS DT hack
OMAPDSS: remove DT hacks for regulators
ARM: OMAP2+: add omapdss_init_of()
OMAPDSS: if dssdev->name==NULL, use alias
OMAPDSS: get dssdev->alias from DT alias
OMAPFB: clean up default display search
OMAPFB: search for default display with DT alias
OMAPDSS: Add DT support to DSS, DISPC, DPI, HDMI, VENC
OMAPDSS: Add DT support to DSI
ARM: omap3.dtsi: add omapdss information
ARM: omap4.dtsi: add omapdss information
ARM: omap4-panda.dts: add display information
ARM: omap4-sdp.dts: add display information
ARM: omap3-tobi.dts: add lcd (TEST)
ARM: omap3-beagle.dts: add display information
OMAPDSS: panel-dsi-cm: Add DT support
OMAPDSS: encoder-tfp410: Add DT support
OMAPDSS: connector-dvi: Add DT support
OMAPDSS: encoder-tpd12s015: Add DT support
OMAPDSS: hdmi-connector: Add DT support
OMAPDSS: panel-dpi: Add DT support
OMAPDSS: connector-analog-tv: Add DT support
arch/arm/boot/dts/omap3-beagle.dts | 29 ++++++++
arch/arm/boot/dts/omap3-tobi.dts | 33 ++++++++
arch/arm/boot/dts/omap3.dtsi | 43 +++++++++++
arch/arm/boot/dts/omap4-panda-common.dtsi | 48 ++++++++++++
arch/arm/boot/dts/omap4-sdp.dts | 70 +++++++++++++++++
arch/arm/boot/dts/omap4.dtsi | 59 +++++++++++++++
arch/arm/mach-omap2/board-generic.c | 13 +---
arch/arm/mach-omap2/common.h | 2 +
arch/arm/mach-omap2/display.c | 34 +++++++++
arch/arm/mach-omap2/dss-common.c | 23 ------
arch/arm/mach-omap2/dss-common.h | 2 -
.../video/omap2/displays-new/connector-analog-tv.c | 70 +++++++++++++++++
drivers/video/omap2/displays-new/connector-dvi.c | 49 ++++++++++++
drivers/video/omap2/displays-new/connector-hdmi.c | 36 +++++++++
drivers/video/omap2/displays-new/encoder-tfp410.c | 54 ++++++++++++++
.../video/omap2/displays-new/encoder-tpd12s015.c | 62 +++++++++++++++
drivers/video/omap2/displays-new/panel-dpi.c | 75 +++++++++++++++++++
drivers/video/omap2/displays-new/panel-dsi-cm.c | 87 ++++++++++++++++++++++
drivers/video/omap2/dss/dispc.c | 7 ++
drivers/video/omap2/dss/display.c | 23 +++++-
drivers/video/omap2/dss/dpi.c | 8 ++
drivers/video/omap2/dss/dsi.c | 58 +++++++++++++--
drivers/video/omap2/dss/dss.c | 10 +++
drivers/video/omap2/dss/hdmi.c | 11 +--
drivers/video/omap2/dss/venc.c | 7 ++
drivers/video/omap2/omapfb/omapfb-main.c | 67 ++++++++++++-----
26 files changed, 915 insertions(+), 65 deletions(-)
--
1.8.1.2
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