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

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Dear Tobias,

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.

Changes in v2:
- disable autosuspend mode again in g2d_remove()
- only get sync in g2d_runqueue_worker() if there is node
   in the queue left

Changes in v3:
- actually delete node in g2d_remove_runqueue_nodes()

Signed-off-by: Tobias Jakobi <tjakobi@xxxxxxxxxxxxxxxxxxxxx>

[...]

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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