Re: [RFC PATCH v2 00/21] Add DSI display support for Exynos based boards

[Tomasz Figa <t.figa@samsung.com>]

On 12.03.2014 11:08, Inki Dae wrote:
> 2014-03-07 19:00 GMT+09:00 Andrzej Hajda <a.hajda@samsung.com>:
>> On 03/05/2014 03:56 AM, Inki Dae wrote:
>>> Hi Andrzej,
>>>
>>> Thanks for your contributions.
>>>
>>> 2014-02-12 20:31 GMT+09:00 Andrzej Hajda <a.hajda@samsung.com>:
>>>> Hi,
>>>>
>>>> This patchset adds drivers and bindings to the following devices:
>>>> - Exynos DSI master,
>>>> - S6E8AA0 DSI panel,
>>>> - TC358764 DSI/LVDS bridge,
>>>> - HV070WSA-100 LVDS panel.
>>>>
>>>> It adds also display support in DTS files for the following boards:
>>>> - Exynos4210/Trats,
>>>> - Exynos4412/Trats2,
>>>> - Exynos5250/Arndale.
>>>>
>>>> Things worth mentioning:
>>>>
>>>> 1. I have implemented DSI/LVDS bridge using drm_panel framework, ie.
>>>> the driver exposes drm_panel interface on DSI side, and interact with
>>>> panels on LVDS side using drm_panel framework. This approach seems to
>>>> me simpler and more natural than using drm_bridge.
>>> Can you give me more details about why you think better to use panel
>>> framework than using drm_bridge?  "Simpler" and "more natural" are
>>> ambiguous to me.
>> In this particular case DSI master expects on the other end
>> any device having DSI slave interface, it could be panel or bridge.
>> So it seems natural that both types of slave devices should expose
>> the same interface also  on programming level.
>> Another problem with drm_bridge is that it is not scalable -
>> if some manufacturer will decide to add another block between the bridge
>> and the panel there is no drm component which can be used for it.
>> Using drm_panel the way I have used in toshiba bridge makes scalability
>> possible,
>> it will be only a matter of adding a driver for new block and making
>> proper links in device tree, I see no easy way of doing it with
>> drm_bridge approach.
>
> Now drm_bridge may not cover all hardware. However drm_bridge has
> already been merged to mainline so I think we need to use drm_bridge
> somehow instead of using other one, and also we could extend
> drm_bridge if needed. It would be definitely impossible for a new
> framework to cover all hardware because there may be other hardware
> not appeared yet. That is what we are doing for mainline until now.
>

Well, maybe drm_bridge has been merged, but so has been drm_panel. 
Moreover, merged code is not carved in stone, if there is a better 
option that could replace it, users of it can be converted to the new 
approach and the old one can be removed.

As I believe Andrzej has demonstrated, drm_panel framework is clearly 
superior over drm_bridge and I can't think of any good reason why it 
couldn't become more generic and replace drm_bridge. Of course it can be 
renamed then to something more generic appropriately.

>>
>>
>>>
>>> Using same drm_panel framework for LDVS bridge and real panel drivers
>>> isn't reasonable to me as now because drm_panel framework would be for
>>> real panel device even if the use of drm_panel framework looks like
>>> suitable to LVDS bridge driver. I thought Sean's way, ptn3460 driver
>>> using drm_bride stuff, is good enough, and that would be why
>>> drm_bridge exists and why drm_encoder has drm_bridge.
>>>
>>> And I'm finding more generic way, how to handle LVDS bridge using
>>> super node so that  LVDS bridge driver isn't embedded to connector
>>> drivers such as eDP and MIPI-DSI, and dt binding of LVDS bridge can be
>>> done at top level of Exynos drm. Once the binding is done, encoder of
>>> display bus driver will have drm_bridge object of LVDS bridge driver
>>> so that display bus driver can handle LVDS bridge driver.
>> Could you explain what you mean by "dt binding of LVDS bridge can be
>> done at top level of Exynos drm" ? How it will look like if there
>> will be more bridges, one for DSI, one for HDMI, etc... What if there
>> will be two
>> bridges in one chain. How it will cope with video pipeline bindings?
>
> it was just my idea so I have no implementation about it yet.
>
> My idea is that crtc and encoder are binded at top level of Exynos drm
> as is. And for bridge support, the only difference is, in case that
> encoder driver has bridge, the dt binding of the encoder driver is
> done once last one between encoder and bridge driver is binded. It
> would mean that bridge driver can use driver model and it doesn't need
> to concern about probe order issue.
>
> For this, encoder driver with bridge, MIPI-DSI or eDP, would need to
> use component interfaces specific to Exynos drm. As a result, once the
> dt bindings of crtc and encoder are completed at top level, encoder
> driver has its own drm_bridge for bridge, and dt binding you proposed
> could be used without any change, and drm_panel could also be used
> only for real lcd panel driver.
>
> And below is a block diagram I think,
>
>                                    DRM KMS
>                     /                      |                 \
>                /                           |                      \
>           crtc                      encoder              connector
>             |                           /     \                          |
>             |                       /             \                      |
>             |                      |           drm_bridge   drm_panel
>             |                      |                   |                 |
>             |                      |                   |                 |
>          FIMD         MIPI-DSI    LVDS bridge    Panel
>

Hmm, this doesn't seem to be complete. Several bridges can be chained 
together. Also I believe "Panel" and "drm_panel" on your diagram should 
be basically the same. This leads to obvious conclusion that drm_bridge 
and drm_panel should be merged and Andrzej has shown an example (and 
IMHO good) way to do it, as drm_panel already provides a significant 
amount of existing infrastructure.

Best regards,
Tomasz