3.18-stable review patch. If anyone has any objections, please let me know.
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From: Tomi Valkeinen tomi.valkeinen@ti.com
[ Upstream commit 538f66ba204944470a653a4cccc5f8befdf97c22 ]
A DMM timeout "timed out waiting for done" has been observed on DRA7 devices. The timeout happens rarely, and only when the system is under heavy load.
Debugging showed that the timeout can be made to happen much more frequently by optimizing the DMM driver, so that there's almost no code between writing the last DMM descriptors to RAM, and writing to DMM register which starts the DMM transaction.
The current theory is that a wmb() does not properly ensure that the data written to RAM is observable by all the components in the system.
This DMM timeout has caused interesting (and rare) bugs as the error handling was not functioning properly (the error handling has been fixed in previous commits):
* If a DMM timeout happened when a GEM buffer was being pinned for display on the screen, a timeout error would be shown, but the driver would continue programming DSS HW with broken buffer, leading to SYNCLOST floods and possible crashes.
* If a DMM timeout happened when other user (say, video decoder) was pinning a GEM buffer, a timeout would be shown but if the user handled the error properly, no other issues followed.
* If a DMM timeout happened when a GEM buffer was being released, the driver does not even notice the error, leading to crashes or hang later.
This patch adds wmb() and readl() calls after the last bit is written to RAM, which should ensure that the execution proceeds only after the data is actually in RAM, and thus observable by DMM.
The read-back should not be needed. Further study is required to understand if DMM is somehow special case and read-back is ok, or if DRA7's memory barriers do not work correctly.
Signed-off-by: Tomi Valkeinen tomi.valkeinen@ti.com Signed-off-by: Peter Ujfalusi peter.ujfalusi@ti.com Signed-off-by: Sasha Levin sashal@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/gpu/drm/omapdrm/omap_dmm_tiler.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
--- a/drivers/gpu/drm/omapdrm/omap_dmm_tiler.c +++ b/drivers/gpu/drm/omapdrm/omap_dmm_tiler.c @@ -256,6 +256,17 @@ static int dmm_txn_commit(struct dmm_txn }
txn->last_pat->next_pa = 0; + /* ensure that the written descriptors are visible to DMM */ + wmb(); + + /* + * NOTE: the wmb() above should be enough, but there seems to be a bug + * in OMAP's memory barrier implementation, which in some rare cases may + * cause the writes not to be observable after wmb(). + */ + + /* read back to ensure the data is in RAM */ + readl(&txn->last_pat->next_pa);
/* write to PAT_DESCR to clear out any pending transaction */ writel(0x0, dmm->base + reg[PAT_DESCR][engine->id]);