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
