From: Sharat Masetty <smasetty@xxxxxxxxxxxxxx>
There is a race condition issue between the IRQ context trying to
trigger preemption and the user context trying to submit commands to
the GPU. The check in a5xx_flush() API only updates the wptr if the GPU is
not in preemption. In the cases where we move from PREEMPT_START to
PREEMPT_NONE there is a small window where the preempt state is still
in START but the CPU context switches to the user thread which is in
the a5xx_flush() call to update the wptr, but fails to update the wptr to
the GPU since the preempt state is not PREEMPT_NONE. This leads to a
GPU stall.
Introduce a new intermediate state PREEMPT_ABORT and
change preempt_trigger() to use gpu's current ring instead of the
ring retrieved from get_next_ring() while in this state.
Signed-off-by: Sharat Masetty <smasetty@xxxxxxxxxxxxxx>
Signed-off-by: Jordan Crouse <jcrouse@xxxxxxxxxxxxxx>
---
drivers/gpu/drm/msm/adreno/a5xx_gpu.h | 8 +++++++-
drivers/gpu/drm/msm/adreno/a5xx_preempt.c | 17 ++++++++++++++---
2 files changed, 21 insertions(+), 4 deletions(-)
diff --git a/drivers/gpu/drm/msm/adreno/a5xx_gpu.h b/drivers/gpu/drm/msm/adreno/a5xx_gpu.h
index f062a90..6fb8c2f 100644
--- a/drivers/gpu/drm/msm/adreno/a5xx_gpu.h
+++ b/drivers/gpu/drm/msm/adreno/a5xx_gpu.h
@@ -56,6 +56,8 @@ struct a5xx_gpu {
* PREEMPT_NONE - no preemption in progress. Next state START.
* PREEMPT_START - The trigger is evaulating if preemption is possible. Next
* states: TRIGGERED, NONE
+ * PREEMPT_ABORT - An intermediate state before moving back to NONE. Next
+ * state: NONE.
* PREEMPT_TRIGGERED: A preemption has been executed on the hardware. Next
* states: FAULTED, PENDING
* PREEMPT_FAULTED: A preemption timed out (never completed). This will trigger
@@ -67,6 +69,7 @@ struct a5xx_gpu {
enum preempt_state {
PREEMPT_NONE = 0,
PREEMPT_START,
+ PREEMPT_ABORT,
PREEMPT_TRIGGERED,
PREEMPT_FAULTED,
PREEMPT_PENDING,
@@ -154,7 +157,10 @@ static inline int spin_usecs(struct msm_gpu *gpu, uint32_t usecs,
/* Return true if we are in a preempt state */
static inline bool a5xx_in_preempt(struct a5xx_gpu *a5xx_gpu)
{
- return !(atomic_read(&a5xx_gpu->preempt_state) == PREEMPT_NONE);
+ int preempt_state = atomic_read(&a5xx_gpu->preempt_state);
+
+ return !(preempt_state == PREEMPT_NONE ||
+ preempt_state == PREEMPT_ABORT);
}
#endif /* __A5XX_GPU_H__ */
diff --git a/drivers/gpu/drm/msm/adreno/a5xx_preempt.c b/drivers/gpu/drm/msm/adreno/a5xx_preempt.c
index 6a3767d..40f4840 100644
--- a/drivers/gpu/drm/msm/adreno/a5xx_preempt.c
+++ b/drivers/gpu/drm/msm/adreno/a5xx_preempt.c
@@ -122,9 +122,20 @@ void a5xx_preempt_trigger(struct msm_gpu *gpu)
* one do nothing except to update the wptr to the latest and greatest
*/
if (!ring || (a5xx_gpu->cur_ring == ring)) {
- update_wptr(gpu, ring);
-
- /* Set the state back to NONE */
+ /*
+ * Its possible that while a preemption request is in progress
+ * from an irq context, a user context trying to submit might
+ * fail to update the write pointer, because it determines
+ * that the preempt state is not PREEMPT_NONE.
+ *
+ * Close the race by introducing an intermediate
+ * state PREEMPT_ABORT to let the submit path
+ * know that the ringbuffer is not going to change
+ * and can safely update the write pointer.
+ */
+
+ set_preempt_state(a5xx_gpu, PREEMPT_ABORT);
+ update_wptr(gpu, a5xx_gpu->cur_ring);
set_preempt_state(a5xx_gpu, PREEMPT_NONE);
return;
}
--
1.9.1
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