[Mesa-dev] [PATCH 3/4] i965: Prevent infinite finish_batch recursion

[Kenneth Graunke]

On Friday, February 27, 2015 10:22:10 AM Ben Widawsky wrote:
> From the comments in the code:
> 
>    Called from intel_batchbuffer_flush before emitting MI_BATCHBUFFER_END and
>    sending it off.
> 
> This fixes a possible, unlikely infinite recursion in our batch flush path. More
> importantly it allows me to add some code here.
> 
> The relevant part of the call chain for flush
> intel_batchbuffer_flush()->brw_finish_batch(). The problem arises if anything in
> the time from intel_batchbuffer_flush, until brw_finish_batch ends up calling
> itself. This can happen as a result of a call to
> intel_batchbuffer_begin()->intel_batchbuffer_require_space()->intel_batchbuffer_flush().
> 
> There are two possible cases today which can spawn this recursion.
> 1. There is a ring switch occurring (impossible, see below)
> 2. The ring is out of space (fairly unlikely)

NAK.  brw_finish_batch is for putting any last-minute commands in the
batchbuffer before sending it off.

Such commands must fit in the BATCH_RESERVED space (which is probably
already large enough since SNB has a zillion workarounds).  This
eliminates reason #2.

#1 means that you're trying to emit commands for the wrong ring, i.e.
trying to do RCS workarounds in a BLT batch.  This isn't what you want.
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