Re: [RFC V2 0/3] drm/bridge: panel and chaining

[Andrzej Hajda <a.hajda@samsung.com>]

On 05/08/2014 08:24 PM, Rob Clark wrote:
> On Thu, May 8, 2014 at 2:41 AM, Andrzej Hajda <a.hajda@samsung.com> wrote:
>> On 05/05/2014 09:52 PM, Ajay Kumar wrote:
>>> This patchset is based on exynos-drm-next-todo branch of Inki Dae's tree at:
>>> git://git.kernel.org/pub/scm/linux/kernel/git/daeinki/drm-exynos.git
>>>
>>> I have just put up Rob's and Sean's idea of chaining up the bridges
>>> in code, and have implemented basic panel controls as a chained bridge.
>>> This works well with ptn3460 bridge chip on exynos5250-snow board.
>>>
>>> Still need to make use of standard list calls and figure out proper way
>>> of deleting the bridge chain. So, this is just a rough version.
>>
>> As I understand this patchset tries to solve two things:
>> 1. Implement panel as drm_bridge, to ease support for hardware chains:
>> Crtc -> Encoder -> Bridge -> Panel
>> 2. Add support to drm_bridge chaining, to allow software chains:
>> drm_crtc -> drm_encoder -> drm_bridge -> drm_bridge,panel
>>
>> It is done using Russian doll schema, ops from the bridge calls the same
>> ops from the next bridge and the next bridge ops can do the same.
>>
>> This schema means that all the bridges including the last one are seen
>> from the drm core point of view as a one big drm_bridge. Additionally in
>> this particular case, the first bridge (ptn3460) implements connector
>> so it is hard to guess what is the location of the 2nd bridge in video
>> stream chain, sometimes it is after the connector, sometimes before.
>> All this is quite confusing.
>>
>> But if you look at the bridge from upstream video interface point of
>> view it is just a panel, edp panel in case of ptn3460, ie ptn3460 on its
>> video input side acts as a panel. On the output side it expects a panel,
>> lvds panel in this case.
> 
> tbh, this is mostly about what we call it.  Perhaps "bridge" isn't the
> best name.. I wouldn't object to changing it.
> 
> But my thinking was to leave in drm_panel_funcs things that are just
> needed by the connector (get_modes().. and maybe some day we need
> detect/etc).  Then leave everything else in drm_bridge_funcs.  A panel
> could (if needed) implement both interfaces.
> 
> That is basically the same as what you are proposing, but without
> renaming bridge to panel ;-)

Good to hear that. However there are points which are not clear for me.
But first lets clarify names, I will use panel and bridge words
to describe the hardware, and drm_panel, drm_bridge to describe the
software interfaces.

What bothers me:
1. You want to leave connector related callbacks in drm_panel and
the rest in drm_bridge. In case of ptn3460 it does not work, ptn3460
must implement connector internally because of this limitation. I guess
it is quite typical bridge. This problem does not exists in case
of using drm_panel for ptn3460.

2. drm_bridge is attached to the encoder, this and the callback order
suggests the video data flow should be as below:
drm_crtc -> drm_encoder [-> drm_bridge] -> drm_connector [-> drm_panel]

ptn3460 implements drm_bridge and drm_connector so it suggests its
drm_bridge should be the last one, so there should be no place to add
lvds panel implemented as a drm_bridge after it, as it is done in this
patchset.

Additionally it clearly shows that there should be two categories of
drm_bridges - non-terminal and terminal.

3. drm_dev uses all-or-nothing approach, ie. it will start only when all
its components are up. It simplifies synchronization but is quite
fragile - the whole drm will be down due to error in some of its components.
For this reason I prefer drm_panel as it is not real drm component
it can be attached/detached to/from drm_connector anytime. I am not
really sure but drm_bridge does not allow for that.

Real life example to show importance of it: I have a phone with MIPI-DSI
panel and HDMI. Due to initialization issues HDMI bridge driver
sometimes fails during probe and the drmdev do not start. Of course this
is development stage so I have serial console I can diagnose the
problem, disable HDMI, fix the problem, etc...
But what happens in case of end-user. He will see black screen - bricked
phone. In case the bridge will be implemented using drm_panel
he will have working phone with broken HDMI, much better.

4. And the last thing, it is more about the concept/design. drm_bridge,
drm_hw_block suggests that those interfaces describes the whole device:
bridge, panel, whatever. In my approach I have an interface
to describe only one video input port of the device. And drm_panel is
in fact misleading name, drm_sink may be better. So real panel
would implement drm_sink interface. Bridge would implement drm_sink
interface and it will request other drm_sink interface, to interact with
the panel which is after it.
This approach seems to me more flexible. Beside things I have described
above it will allow to implement also more complicated devices, dsi
hubs, video mixers, etc.


Regards
Andrzej

> 
> BR,
> -R
> 
>> So why not implement ptn3460 bridge as drm_panel which internally uses
>> another drm_panel. With this approach everything fits much better.
>> You do not need those (pre|post)_(enable|disable) calls, you do not need
>> to implement connector in the bridge and you have a driver following
>> linux driver model. And no single bit changed in drm core.
>>
>> I have implemented this way DSI/LVDS bridge, it was sent as RFC [1][2].
>> It was not accepted as Inki preferred drm_bridge but as I see the
>> problems with drm_bridges I have decide to attract attention to much
>> more cleaner solution.
>>
>> [1]: http://permalink.gmane.org/gmane.linux.drivers.devicetree/61559
>> [2]: http://permalink.gmane.org/gmane.linux.kernel.samsung-soc/27044
>>
>> Regards
>> Andrzej
>>
>>
>>>
>>> Ajay Kumar (3):
>>>   [RFC V2 1/3] drm: implement chaining of drm bridges
>>>   [RFC V2 2/3] drm/bridge: add a dummy panel driver to support lvds bridges
>>>   [RFC V2 3/3] drm/bridge: ptn3460: support bridge chaining
>>>
>>>  .../bindings/drm/bridge/bridge_panel.txt           |  45 ++++
>>>  drivers/gpu/drm/bridge/Kconfig                     |   6 +
>>>  drivers/gpu/drm/bridge/Makefile                    |   1 +
>>>  drivers/gpu/drm/bridge/bridge_panel.c              | 240 +++++++++++++++++++++
>>>  drivers/gpu/drm/bridge/ptn3460.c                   |  21 +-
>>>  drivers/gpu/drm/drm_crtc.c                         |  13 +-
>>>  include/drm/bridge/bridge_panel.h                  |  37 ++++
>>>  include/drm/drm_crtc.h                             |   2 +
>>>  8 files changed, 360 insertions(+), 5 deletions(-)
>>>  create mode 100644 Documentation/devicetree/bindings/drm/bridge/bridge_panel.txt
>>>  create mode 100644 drivers/gpu/drm/bridge/bridge_panel.c
>>>  create mode 100644 include/drm/bridge/bridge_panel.h
>>>
>>