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Date:   Thu, 6 Jul 2023 10:03:15 +0200
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From:   Neil Armstrong <neil.armstrong@linaro.org>
Reply-To: neil.armstrong@linaro.org
Subject: Re: RFC: DSI host capabilities (was: [PATCH RFC 03/10] drm/panel: Add
 LGD panel driver for Sony Xperia XZ3)
To:     Maxime Ripard <mripard@kernel.org>
Cc:     Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        AngeloGioacchino Del Regno 
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On 06/07/2023 09:59, Maxime Ripard wrote:
> On Thu, Jul 06, 2023 at 09:33:15AM +0200, Neil Armstrong wrote:
>> On 06/07/2023 09:24, Maxime Ripard wrote:
>>> On Wed, Jul 05, 2023 at 11:09:40PM +0300, Dmitry Baryshkov wrote:
>>>> On 05/07/2023 19:53, Maxime Ripard wrote:
>>>>> On Wed, Jul 05, 2023 at 06:20:13PM +0300, Dmitry Baryshkov wrote:
>>>>>> On Wed, 5 Jul 2023 at 17:24, Maxime Ripard <mripard@kernel.org> wrote:
>>>>>>>
>>>>>>> On Wed, Jul 05, 2023 at 04:37:57PM +0300, Dmitry Baryshkov wrote:
>>>>>>>>>>>
>>>>>>>>>>> Either way, I'm not really sure it's a good idea to multiply the
>>>>>>>>>>> capabilities flags of the DSI host, and we should just stick to the
>>>>>>>>>>> spec. If the spec says that we have to support DSC while video is
>>>>>>>>>>> output, then that's what the panels should expect.
>>>>>>>>>>
>>>>>>>>>> Except some panels supports DSC & non-DSC, Video and Command mode, and
>>>>>>>>>> all that is runtime configurable. How do you handle that ?
>>>>>>>>>
>>>>>>>>> In this case, most of the constraints are going to be on the encoder
>>>>>>>>> still so it should be the one driving it. The panel will only care about
>>>>>>>>> which mode has been selected, but it shouldn't be the one driving it,
>>>>>>>>> and thus we still don't really need to expose the host capabilities.
>>>>>>>>
>>>>>>>> This is an interesting perspective. This means that we can and actually have
>>>>>>>> to extend the drm_display_mode with the DSI data and compression
>>>>>>>> information.
>>>>>>>
>>>>>>> I wouldn't extend drm_display_mode, but extending one of the state
>>>>>>> structures definitely.
>>>>>>>
>>>>>>> We already have some extra variables in drm_connector_state for HDMI,
>>>>>>> I don't think it would be a big deal to add a few for MIPI-DSI.
>>>>>>>
>>>>>>> We also floated the idea for a while to create bus-specific states, with
>>>>>>> helpers to match. Maybe it would be a good occasion to start doing it?
>>>>>>>
>>>>>>>> For example, the panel that supports all four types for the 1080p should
>>>>>>>> export several modes:
>>>>>>>>
>>>>>>>> 1920x1080-command
>>>>>>>> 1920x1080-command-DSC
>>>>>>>> 1920x1080-video
>>>>>>>> 1920x1080-video-DSC
>>>>>>>>
>>>>>>>> where video/command and DSC are some kinds of flags and/or information in
>>>>>>>> the drm_display_mode? Ideally DSC also has several sub-flags, which denote
>>>>>>>> what kind of configuration is supported by the DSC sink (e.g. bpp, yuv,
>>>>>>>> etc).
>>>>>>>
>>>>>>> So we have two things to do, right? We need to expose what the panel can
>>>>>>> take (ie, EDID for HDMI), and then we need to tell it what we picked
>>>>>>> (infoframes).
>>>>>>>
>>>>>>> We already express the former in mipi_dsi_device, so we could extend the
>>>>>>> flags stored there.
>>>>>>>
>>>>>>> And then, we need to tie what the DSI host chose to a given atomic state
>>>>>>> so the panel knows what was picked and how it should set everything up.
>>>>>>
>>>>>> This is definitely something we need. Marijn has been stuck with the
>>>>>> panels that support different models ([1]).
>>>>>>
>>>>>> Would you prefer a separate API for this kind of information or
>>>>>> abusing atomic_enable() is fine from your point of view?
>>>>>>
>>>>>> My vote would be for going with existing operations, with the slight
>>>>>> fear of ending up with another DSI-specific hack (like
>>>>>> pre_enable_prev_first).
>>>>>
>>>>> I don't think we can get away without getting access to the atomic_state
>>>>> from the panel at least.
>>>>>
>>>>> Choosing one setup over another is likely going to depend on the mode,
>>>>> and that's only available in the state.
>>>>>
>>>>> We don't have to go the whole way though and create the sub-classes of
>>>>> drm_connector_state, but I think we should at least provide it to the
>>>>> panel.
>>>>>
>>>>> What do you think of creating a new set of atomic_* callbacks for
>>>>> panels, call that new set of functions from msm and start from there?
>>>>
>>>> We are (somewhat) bound by the panel_bridge, but yeah, it seems possible.
>>>
>>> Bridges have access to the atomic state already, so it's another place
>>> to plumb this through but I guess it would still be doable?
>>
>> It's definitely doable, but I fear we won't be able to test most of the
>> panel drivers, should we introduce a new atomic set of panel callbacks ?
> 
> That was my original intent yeah :)
> 
> Creating an atomic_enable/disable/ etc. and then switch
> drm_panel_enable() to take the state (and fixing up all the callers), or
> create a drm_panel_enable_atomic() function.
> 
> The latter is probably simpler, something like:
> 
> int drm_panel_enable_atomic(struct drm_panel *panel,
>      			    struct drm_atomic_state *state)
> {
> 	struct drm_panel_funcs *funcs = panel->funcs;
> 
> 	if (funcs->atomic_enable)
> 		return funcs->atomic_enable(panel, state);
> 
> 	return funcs->enable(panel);
> }
> 
> And we should probably mention that it supersedes/deprecates
> drm_panel_enable.
> 
> We've switched most of the other atomic hooks to take the full
> drm_atomic_state so I'd prefer to use it. However, for it to be somewhat
> useful we'd need to have access to the connector assigned to that panel.
> 
> drm_panel doesn't store the drm_connector it's connected to at all, and
> of_drm_find_panel() doesn't take it as an argument so we can't fill it
> when we retrieve it either.
> 
> So I guess we can go for:
> 
>   - Create a new set of atomic hooks
> 
>   - Create a new set of functions to call those hooks, that we would
>     document as deprecating the former functions. Those functions would
>     take a pointer to the drm_connector_state of the drm_connector it's
>     connected to.
> 
>   - We add a TODO item to add a pointer to the connector in drm_panel
> 
>   - We add a TODO item that depend on the first one to switch the new
>     functions and hooks to drm_atomic_state
> 
>   - We add a TODO item to convert callers of drm_panel_enable et al. to
>     our new functions.
> 
> It should work in all setups, paves a nice way forward and documents the
> trade-offs we had to take and eventually address. And without creating a
> dependency on 30+ patches series.
> 
> Does it sound like a plan?

Yep that looks a fine plan to start of

> 
>> Or shall be simply move the "new" panel driver supporting atomic to bridge
>> and only use panel_bridge for basic panels ?
> 
> I don't think we can expect panel_bridge to be used all the time any
> time soon, so I'd rather avoid to rely on it.

Ack

> 
> Maxime