[RFC v3] drm/exynos: g2d: fix runtime PM

[Tobias Jakobi]

Hello Marek,


Marek Szyprowski wrote:
> Hi Tobias,
> 
> On 2016-09-26 16:15, Tobias Jakobi wrote:
>> Marek Szyprowski wrote:
>>> On 2016-09-24 20:58, Tobias Jakobi wrote:
>>>> The commit b05984e21a7e000bf5074ace00d7a574944b2c16 broke
>>>> operation of the G2D. After this commit the following
>>>> happens.
>>>> - exynos_g2d_exec_ioctl() prepares a runqueue node and
>>>>     calls g2d_exec_runqueue()
>>>> - g2d_exec_runqueue() calls g2d_dma_start() which gets
>>>>     runtime PM sync
>>>> - runtime PM calls g2d_runtime_resume()
>>>> - g2d_runtime_resume() calls g2d_exec_runqueue()
>>>>
>>>> Luckily for us this doesn't really loop, but creates a
>>>> mutex lockup, which the kernel even predicts.
>>>>
>>>> Anyway, we fix this by reintroducing dedicated sleep
>>>> operations again, and only letting runtime PM control
>>>> the gate clock.
>>>>
>>>> This is not enough to fix the issue though.
>>>> - We switch runtime PM to autosuspend. Currently clocks get
>>>>     disabled, and then re-enabled again in the runqueue worker
>>>>     when a node is completed and the next is started.
>>>>     With the upcoming introduction of IOMMU runtime PM this
>>>>     situations gets worse, since now also the IOMMU goes
>>>>     through a disable/enable cycle before the next node is
>>>>     started.
>>>> - We consolidate all runtime PM management to the runqueue
>>>>     worker.
>>>> - We introduce g2d_wait_finish() which waits until the currently
>>>>     processed runqueue node is finished.
>>>>     If this takes too long, the engine is forcibly reset. This
>>>>     is necessary to properly close the driver in case the engine
>>>>     should hang with read/write access to some area of memory.
>>>>     In this situation we can't properly unmap GEM/userptr
>>>>     objects, since this might create a pagefault.
>>>> - Sleep suspend issues a suspend of the runqueue and then calls
>>>>     g2d_wait_finish(), which returns the engine in the idle state.
>>>>     The current runqueue node gets completed, all other queued
>>>>     nodes stay in the queue. There is no hardware state that
>>>>     needs to be retained.
>>>> - Sleep resume just pokes the runqueue worker, which, should
>>>>     something be in queue, continues its work, or just exits.
>>> IMHO there is too much done in one patch. It would be much easier to
>>> understand it if the changes are split into 2 parts: restoring dedicated
>>> speep callbacks and their integration with runqueue worker and addition
>>> of autosuspend-based runtime pm.
>> so you mean first revert your patch, and then put more changes on top of
>> it in a separate patch? I'm not sure into which 'functional units' I
>> should break this one down.
>>
>> I can of course create a separate patch for autosuspend, that that would
>> only replace a put_sync() with mark_last_busy() + put_autosuspend(),
>> wouldn't it?
>>
>>
>> My current idea of functional units:
>> 1) revert Marek's patch
>> 2) move runpm management to g2d_runqueue_worker()
>> 3) introduce g2d_wait_finish() and use it sleep call
>> 4) also use it in g2d_close() and g2d_remove()
>> 5) put autosuspend on top
>>
>> Would this make sense?
> 
> Yes, definitely this approach makes much more sense in my opinion. I'm
> sorry
> for my broken initial patch and the additional work you had to do.
No problem! Thanks for looking over this!

With best wishes,
Tobias


> 
> Best regards