On ma, 2015-06-08 at 18:08 +0100, Dave Gordon wrote: > On 08/06/15 17:28, Imre Deak wrote: > > By running igt/store_dword_loop_render on BXT we can hit a coherency > > problem where the seqno written at GPU command completion time is not > > seen by the CPU. This results in __i915_wait_request seeing the stale > > seqno and not completing the request (not considering the lost > > interrupt/GPU reset mechanism). I also verified that this isn't a case > > of a lost interrupt, or that the command didn't complete somehow: when > > the coherency issue occured I read the seqno via an uncached GTT mapping > > too. While the cached version of the seqno still showed the stale value > > the one read via the uncached mapping was the correct one. > > > > Work around this issue by clflushing the corresponding CPU cacheline > > following any store of the seqno and preceding any reading of it. When > > reading it do this only when the caller expects a coherent view. > > > > Testcase: igt/store_dword_loop_render > > Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx> > > Not necessarily a cure for this, but BSpec says of MI_STORE_DATA_IMM > (and MI_STORE_DATA_INDEX): > > This command simply initiates the write operation with > command execution proceeding normally. Although the write > operation is guaranteed to complete "eventually", there is > no mechanism to synchronize command execution with the > completion (or even initiation) of these operations. > > So shouldn't we use MI_FLUSH_DW or PIPE_CONTROL to update the seqno in > the HWSP instead? I have tried to use PIPE_CONTROL instead of MI_STORE_DATA_IMM, but that didn't solve this issue. With the new igt/store_dword_batches_loop/cached-mapping you can also verify that this isn't some ordering issue. By waiting enough after the command completed the CPU's view is still stale. Reading it after a clflush results in the correct value on the other hand. --Imre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/intel-gfx
