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

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

On 05/08/2014 12:26 PM, Ajay kumar wrote:
> Hi Andrej,
>
> On Thu, May 8, 2014 at 12:11 PM, 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.
>>
>> 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!

Yes, I agree with this :)

> And, we do need (pre|post)_(enable|disable) calls for bridges, but the drm_panel
> framework supports only enable/disable.

For true bridges pre|post can be just implemented as a part of the call,
for example:

bridge_enable()
{
    /* pre-enable stuff */
    panel_enable();
    /* post-enable stuff */
}

And for your particular problem you have written:
> The LVDS datasheet says at least 200ms delay is needed from "Valid
> data" to "BL on".

I am not sure what exactly means 'valid data' in this case, if it is
after lcd_en gpio
why not just use schedule_delayed_work. If it should be called later I
guess it should
be possible to find a proper callback to drm_panel.

Regards
Andrzej

>> 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
>>>