Re: [PATCH v7 11/12] drm/atomic-helper: Re-order bridge chain pre-enable and post-disable

[Aradhya Bhatia <aradhya.bhatia@linux.dev>]

Hi Dmitry,

On 14/01/25 16:54, Dmitry Baryshkov wrote:
> On Tue, Jan 14, 2025 at 11:26:25AM +0530, Aradhya Bhatia wrote:
>> Move the bridge pre_enable call before crtc enable, and the bridge
>> post_disable call after the crtc disable.
>>
>> The sequence of enable after this patch will look like:
>>
>> 	bridge[n]_pre_enable
>> 	...
>> 	bridge[1]_pre_enable
>>
>> 	crtc_enable
>> 	encoder_enable
>>
>> 	bridge[1]_enable
>> 	...
>> 	bridge[n]_enable
>>
>> And, the disable sequence for the display pipeline will look like:
>>
>> 	bridge[n]_disable
>> 	...
>> 	bridge[1]_disable
>>
>> 	encoder_disable
>> 	crtc_disable
>>
>> 	bridge[1]_post_disable
>> 	...
>> 	bridge[n]_post_disable
>>
>> The definition of bridge pre_enable hook says that,
>> "The display pipe (i.e. clocks and timing signals) feeding this bridge
>> will not yet be running when this callback is called".
>>
>> Since CRTC is also a source feeding the bridge, it should not be enabled
>> before the bridges in the pipeline are pre_enabled. Fix that by
>> re-ordering the sequence of bridge pre_enable and bridge post_disable.
> 
> The patch contains both refactoring of the corresponding functions and
> changing of the order. Please split it into two separate commits.
> 

I assume that you already understand this, so this is just for the
record -

There is no trivial way to do this. Initially, this is what I had in my
mind - about what the split patches would look like.

#1
 * refactoring of entire loops into separate functions.
 * Separate out the pre_enable and enable operations inside the
   encoder_bridge_enable().
 * call them in their (seemingly) original sequences
	- crtc_enable
	- encoder_bridge_enable(pre_enable)
	- encoder_bridge_enable(!pre_enable)

#2
 * rearrange the calls to re-order the operation
	- encoder_bridge_enable(pre_enable)
	- crtc_enable
	- encoder_bridge_enable(!pre_enable)

This would have made the patch#2 seem quite trivial, as patch#1 would
take the brunt of changes, while keeping the functionality intact.


What I have now realized is that, the above isn't possible,
unfortunately. The moment we separate out pre_enable and enable into 2
different calls, the overall sequence gets even changed when there are
multiple pipelines in the system.

So to implement the split properly, the first patch would look like this

#1
 * refactoring of entire loops into separate functions.
 * The calling sequences will be as follows -
 	- crtc_enable()
	- encoder_bridge_enable()
		- this will do both pre_enable and enable
		  unconditionally.

The pre_enable and enable operations wouldn't be separated in patch 1,
just that the crtc enable and encoder bridge pre_enable/enable loops
would be put into individual functions.

The next patch will then introduce booleans, and separate out pre_enable
and enable, and implement the new order -

#2
 * pre_enable and enable operations will be conditionally segregated
   inside encoder_bridge_enable(), based on the pre_enable boolean.
 * The calling sequences will then be updated to -
	- encoder_bridge_enable(pre_enable)
	- crtc_enable()
	- encoder_bridge_enable(!pre_enable)

This wouldn't be all that much trivial as I had imagined it to be earlier.

It would also *kind of* like these patches in a previous revision,
v5:11/13 [0], and v5:12/13 [1]. The only differences being,

1) these older patches separated out only the bridge/encoder operations
into a function, and not the crtc operations, and

2) An enum is being used instead of the booleans.


I believe this is what you are looking for? If I have misunderstood
something, do let me know.


Regards
Aradhya


[0]: v5:11/13
drm/atomic-helper: Separate out Encoder-Bridge enable and disable
https://lore.kernel.org/all/20241019200530.270738-4-aradhya.bhatia@linux.dev/

[1]: v5:12/13
drm/atomic-helper: Re-order bridge chain pre-enable and post-disable
https://lore.kernel.org/all/20241019200530.270738-5-aradhya.bhatia@linux.dev/