Date: Thu, 30 May 2024 15:13:42 +0530
From: Aradhya Bhatia <a-bhatia1@...com>
To: Maxime Ripard <mripard@...nel.org>
CC: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
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Subject: Re: [PATCH 6/7] drm/bridge: Introduce early_enable and late disable

Hi Maxime,

On 28/05/24 17:13, Maxime Ripard wrote:
> On Fri, May 24, 2024 at 04:38:13PM GMT, Aradhya Bhatia wrote:
>> Hi Maxime,
>>
>> On 21/05/24 18:45, Maxime Ripard wrote:
>>> Hi,
>>>
>>> On Thu, May 16, 2024 at 03:10:15PM GMT, Aradhya Bhatia wrote:
>>>>>>  	/**
>>>>>>  	 * @pre_enable:
>>>>>>  	 *
>>>>>> @@ -285,6 +319,26 @@ struct drm_bridge_funcs {
>>>>>>  	 */
>>>>>>  	void (*enable)(struct drm_bridge *bridge);
>>>>>>  
>>>>>> +	/**
>>>>>> +	 * @atomic_early_enable:
>>>>>> +	 *
>>>>>> +	 * This callback should enable the bridge. It is called right before
>>>>>> +	 * the preceding element in the display pipe is enabled. If the
>>>>>> +	 * preceding element is a bridge this means it's called before that
>>>>>> +	 * bridge's @atomic_early_enable. If the preceding element is a
>>>>>> +	 * &drm_crtc it's called right before the crtc's
>>>>>> +	 * &drm_crtc_helper_funcs.atomic_enable hook.
>>>>>> +	 *
>>>>>> +	 * The display pipe (i.e. clocks and timing signals) feeding this bridge
>>>>>> +	 * will not yet be running when this callback is called. The bridge can
>>>>>> +	 * enable the display link feeding the next bridge in the chain (if
>>>>>> +	 * there is one) when this callback is called.
>>>>>> +	 *
>>>>>> +	 * The @early_enable callback is optional.
>>>>>> +	 */
>>>>>> +	void (*atomic_early_enable)(struct drm_bridge *bridge,
>>>>>> +				    struct drm_bridge_state *old_bridge_state);
>>>>>> +
>>>>>>  	/**
>>>>>>  	 * @atomic_pre_enable:
>>>>>>  	 *
>>>>>> @@ -361,6 +415,21 @@ struct drm_bridge_funcs {
>>>>>>  	void (*atomic_post_disable)(struct drm_bridge *bridge,
>>>>>>  				    struct drm_bridge_state *old_bridge_state);
>>>>>>  
>>>>>> +	/**
>>>>>> +	 * @atomic_late_disable:
>>>>>> +	 *
>>>>>> +	 * This callback should disable the bridge. It is called right after the
>>>>>> +	 * preceding element in the display pipe is disabled. If the preceding
>>>>>> +	 * element is a bridge this means it's called after that bridge's
>>>>>> +	 * @atomic_late_disable. If the preceding element is a &drm_crtc it's
>>>>>> +	 * called right after the crtc's &drm_crtc_helper_funcs.atomic_disable
>>>>>> +	 * hook.
>>>>>> +	 *
>>>>>> +	 * The @atomic_late_disable callback is optional.
>>>>>> +	 */
>>>>>> +	void (*atomic_late_disable)(struct drm_bridge *bridge,
>>>>>> +				    struct drm_bridge_state *old_bridge_state);
>>>>>> +
>>>>>
>>>>> But more importantly, I don't quite get the use case you're trying to
>>>>> solve here.
>>>>>
>>>>> If I got the rest of your series, the Cadence DSI bridge needs to be
>>>>> powered up before its source is started. You can't use atomic_enable or
>>>>> atomic_pre_enable because it would start the source before the DSI
>>>>> bridge. Is that correct?
>>>>>
>>>>
>>>> That's right. I cannot use bridge_atomic_pre_enable /
>>>> bridge_atomic_enable here. But that's because my source is CRTC, which
>>>> gets enabled via crtc_atomic_enable.
>>>>
>>>>
>>>>> If it is, then how is it different from what
>>>>> drm_atomic_bridge_chain_pre_enable is doing? The assumption there is
>>>>> that it starts enabling bridges last to first, to it should be enabled
>>>>> before anything starts.
>>>>>
>>>>> The whole bridge enabling order code starts to be a bit of a mess, so it
>>>>> would be great if you could list all the order variations we have
>>>>> currently, and why none work for cdns-dsi.
>>>>>
>>>>
>>>> Of course! I can elaborate on the order.
>>>>
>>>> Without my patches (and given there isn't any bridge setting the
>>>> "pre_enable_prev_first" flag) the order of enable for any single display
>>>> chain, looks like this -
>>>>
>>>> 	crtc_enable
>>>> 	
>>>> 	bridge[n]_pre_enable
>>>> 	---
>>>> 	bridge[1]_pre_enable
>>>>
>>>> 	encoder_enable
>>>>
>>>> 	bridge[1]_enable
>>>> 	---
>>>> 	bridge[n]_enable
>>>>
>>>> The tidss enables at the crtc_enable level, and hence is the first
>>>> entity with stream on. cdns-dsi doesn't stand a chance with
>>>> bridge_atmoic_pre_enable / bridge_atmoic_enable hooks. And there is no
>>>> bridge call happening before crtc currently.
>>>
>>> Thanks for filling the blanks :)
>>>
>>> I assume that since cdns-dsi is a bridge, and it only has a simple
>>> encoder implementation, for it to receive some video signal we need to
>>> enable the CRTC before the bridge.
>>>
>>> If so, I think that's the original intent between the bridge pre_enable.
>>> The original documentation had:
>>>
>>>   pre_enable: this contains things needed to be done for the bridge
>>>   before this contains things needed to be done for the bridge before
>>>   this contains things needed to be done for the bridge before.
>>>
>>> and the current one has:
>>>
>>>   The display pipe (i.e. clocks and timing signals) feeding this bridge
>>>   will not yet be running when this callback is called. The bridge must
>>>   not enable the display link feeding the next bridge in the chain (if
>>>   there is one) when this callback is called.
>>>
>>> I would say the CRTC is such a source, even more so now that the encoder
>>> is usually transparent, so I think we should instead move the crtc
>>> enable call after the bridge pre_enable.
>>
>> Hmm, if I understand you right, the newer sequence of calls will look
>> like this,
>>
>> 	bridge[n]_pre_enable
>> 	---
>> 	bridge[1]_pre_enable
>>
>> 	crtc_enable
>> 	encoder_enable
>>
>> 	bridge[1]_enable
>> 	---
>> 	bridge[n]_enable
> 
> Yes :)
> 
>> I do agree with this. This makes sense. CRTC is indeed such a source,
>> and should ideally be enabled after the bridges are pre_enabled.
>>
>>>
>>> Would that work?
>>>
>>
>> So, this could potentially work, yes. The cdns-dsi would get pre_enabled
>> after all bridges after cdns-dsi are pre_enabled. But over a quick test
>> with BBAI64 + RPi Panel, I don't see any issue.
>>
>> However, the one concern that I have right now, is about breaking any
>> existing (albeit faulty) implementation which relies on CRTC being
>> enabled before the bridges are pre_enabled. =)
> 
> I don't think it'll be a big deal. If there was a proper encoder driver,
> it was probably gating the signal until it's enabled. If there isn't,
> then yeah it might disrupt things, but it mostly means that the driver
> wasn't properly split between pre_enable and enable.
> 
> So I think it's worth trying, and we'll see the outcome.
> 

Alright! =)

Have made the changes as per your suggestions in v2. Thanks!

Regards
Aradhya

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