[PATCH 1/1] drm/bridge: Fix handling of bridges with pre_enable_prev_first flag

[Frieder Schrempf]

On 14.07.23 19:16, Dave Stevenson wrote:
> Hi Frieder
> 
> On Mon, 10 Jul 2023 at 08:46, Frieder Schrempf
> <frieder.schrempf at kontron.de> wrote:
>>
>> On 07.07.23 21:00, Vladimir Lypak wrote:
>>> [Sie erhalten nicht häufig E-Mails von vladimir.lypak at gmail.com. Weitere Informationen, warum dies wichtig ist, finden Sie unter https://aka.ms/LearnAboutSenderIdentification ]
>>>
>>> In function drm_atomic_bridge_chain_post_disable handling of
>>> pre_enable_prev_first flag is broken because "next" variable will always
>>> end up set to value of "bridge". This breaks loop which should disable
>>> bridges in reverse:
>>>
>>>  next = list_next_entry(bridge, chain_node);
>>>
>>>  if (next->pre_enable_prev_first) {
>>>         /* next bridge had requested that prev
>>>          * was enabled first, so disabled last
>>>          */
>>>         limit = next;
>>>
>>>         /* Find the next bridge that has NOT requested
>>>          * prev to be enabled first / disabled last
>>>          */
>>>         list_for_each_entry_from(next, &encoder->bridge_chain,
>>>                                  chain_node) {
>>> // Next condition is always true. It is likely meant to be inversed
>>> // according to comment above. But doing this uncovers another problem:
>>> // it won't work if there are few bridges with this flag set at the end.
>>>                 if (next->pre_enable_prev_first) {
>>>                         next = list_prev_entry(next, chain_node);
>>>                         limit = next;
>>> // Here we always set next = limit = branch at first iteration.
>>>                         break;
>>>                 }
>>>         }
>>>
>>>         /* Call these bridges in reverse order */
>>>         list_for_each_entry_from_reverse(next, &encoder->bridge_chain,
>>>                                          chain_node) {
>>> // This loop never executes past this branch.
>>>                 if (next == bridge)
>>>                         break;
>>>
>>>                 drm_atomic_bridge_call_post_disable(next, old_state);
>>>
>>> In this patch logic for handling the flag is simplified. Temporary
>>> "iter" variable is introduced instead of "next" which is used only
>>> inside inner loops.
>>>
>>> Fixes: 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order")
>>> Signed-off-by: Vladimir Lypak <vladimir.lypak at gmail.com>
>>
>> I haven't had a chance to look at this, but I still want to reference
>> another patch by Jagan that intends to fix some bug in this area:
>>
>> https://patchwork.kernel.org/project/dri-devel/patch/20230328170752.1102347-1-jagan@amarulasolutions.com/
>>
>> +Cc: Jagan
>>
>> Dave, as you introduced this feature, did you have a chance to look at
>> Jagan's and Vladimir's patches?
> 
> Sorry, they'd fallen off my radar.
> I'm having a look at the moment, but will probably need to carry it
> over to Monday.

Sure, take your time. I just wanted to make sure that you are aware of
these patches and the existence of a bug in the code.

Thanks!