How to support various hardware blocks in drm driver

[Andrzej Hajda]

Hi,

I have already proposed to reuse drm_panel infrastructure to
implement bridges in my RFC [1] and I have implemented DSI/LVDS bridge
in this way [2]. I guess this discussion is a result of my discussion
with Inki in thread [1]. More comments below.

[1]: http://permalink.gmane.org/gmane.linux.kernel.samsung-soc/27044
[2]: http://permalink.gmane.org/gmane.linux.drivers.devicetree/61559

On 03/18/2014 06:37 PM, Daniel Vetter wrote:
> On Tue, Mar 18, 2014 at 09:58:25PM +0900, Inki Dae wrote:
>> 2014-03-18 21:47 GMT+09:00 Daniel Vetter <daniel at ffwll.ch>:
>>> On Tue, Mar 18, 2014 at 1:42 PM, Inki Dae <inki.dae at samsung.com> wrote:
>>>> I think now drm_bridge couldn't do what we want for embedded systems
>>>> as long as drm_encoder has drm_bridge.
>>>> See the blow hardware pipeline,
>>>> Display Controller-----Image Enhancement chip-----MIP DSI-----MIPI TO
>>>> LVDS Bridge-----LCD Panel
>>>>
>>>> In above hardware pipeline, Display controller is controlled by crtc,
>>>> and Image Enhancement chip receives output from display controller.
>>>> So the use of existing drm_bridge would be suitable to only bridge
>>>> devices between MIPI DSI and LCD Panel, but not to Image Enhancement
>>>> chip.
>>>>
>>>> For such hardware, drm_panel infrastructure is more reasonable to me,
>>>> and that is why I try to integrate drm_panel and drm_bridge to one
>>>> integrated framework which has infrastructure same as existing
>>>> drm_panel.
>>>> The important thing is to free this integrated framework from
>>>> drm_encoder so that crtc device can also use this framework.
>>>
>>> Hm, what is this image enhancement chip? Is that some IP block on the
>>> SoC? Is it optional? Can it be attached to different crtcs?
>>
>> In case of Exynos, this chip is in SoC, and can be only attached to
>> one crtc, display controller. But I'm not sure other SoC have similar
>> chip.
>>
>>>
>>> I think we have similar things on intel hardware, but without details
>>> on what it does and how it works I can't really say how to best expose
>>> it to userspace and how to best handle it internally in the driver.
>>> -Daniel
>>
>> Simply saying, the image enhancement chip can enhance image data from
>> display controller, i.e. saturation enhancement, color reproduction,
>> dithering, and so on.
>> And this chip receives image data through hardware wired lines
>> connected internally between display controller and this chip.
> 
> To me this sounds like you simply need to expose all these capabilities to
> userspace as crtc properties. Which addresses one part of this issue.
> 
> The other side is how you are going to track this in the driver, and there
> you can do whatever you want - just add a pointer/structure to the exynos
> crtc structure for the display enhancement block.
> 
> The MIPI DSI block would then be treated as a drm_encoder, and all the
> later stages as drm_bridges up to the very last (the actual lvds panel)
> which would be a simple drm_panel.
> 
> I don't really see what additional complexity you need here. Especially
> since this image enhancer is on your SoC (and I guess a samgsung IP block
> not shared with any other SoC manufacture) you can easily keep the driver
> code for it in the exynos driver. So really no need to have a generic
> interface here.
> -Daniel
> 

But what you proposes is complex. Blocks will be implemented as:
1. drm_encoder,
2. drm_bridge,
3. drm_panel,
4. another 'framework' to implement image enhancers, which are after crtc.

But these blocks are just 'video consumers' which can be implemented
using drm_panel framework and 'video producers' (except real panels)
which can be implemented as drm_panel clients. Of course drm_panel
should be renamed to sth like drm_video_input and its ops should be
extended.

Anyway instead of 4 or more different frameworks we will have only one
framework.

In general I think it would be better to model just device interfaces
instead of modeling whole devices. Btw this approach will allow to model
such monsters as TC358710XBG hub [3].

[3]: http://www.toshiba-components.com/prpdf/5992e.pdf

Regards
Andrzej