Re: [Intel-gfx] [PATCH] drm/i915/guc/slpc: remove unneeded clflush calls

Lucas De Marchi
2021-09-21T05:47:19Z

On Wed, Sep 15, 2021 at 12:29:12PM -0700, John Harrison wrote:
>On 9/15/2021 12:24, Belgaumkar, Vinay wrote:
>>On 9/14/2021 12:51 PM, Lucas De Marchi wrote:
>>>The clflush calls here aren't doing anything since we are not writting
>>>something and flushing the cache lines to be visible to GuC. Here the
>>>intention seems to be to make sure whatever GuC has written is visible
>>>to the CPU before we read them. However a clflush from the CPU side is
>>>the wrong instruction to use.
>Is there a right instruction to use? Either we need to verify that no

how can there be a right instruction? If the GuC needs to flush, then
the GuC needs to do it, nothing to be done by the CPU.

Flushing the CPU cache line here is doing nothing to guarantee that what
was written by GuC hit the memory and we are reading it. Not sure why it
was actually added, but since it was added by Vinay and he reviewed this
patch, I'm assuming he also agrees

Lucas De Marchi