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

[Ajay kumar]

Hi Andrej,

On Thu, May 8, 2014 at 12:11 PM, Andrzej Hajda <a.hajda at 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.
>
> 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.
This discussion should have ideally happened when Sean added bridge
into drm-core!
And, we do need (pre|post)_(enable|disable) calls for bridges, but the drm_panel
framework supports only enable/disable.

> 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
>>
>