[Intel-gfx] [PATCH] drm/i915/guc: Stop using mutex while sending CTB messages

[Matthew Brost]

On Fri, Jan 31, 2020 at 03:33:55PM +0000, Chris Wilson wrote:
>Quoting Michal Wajdeczko (2020-01-31 14:58:34)
>> While we are always using CT "send" buffer to send request messages
>> to GuC, we usually don't ask GuC to use CT "receive" buffer to send
>> back response messages, since almost all returned data can fit into
>> reserved bits in status dword inside CT descriptor. However, relying
>> on data modifications inside CT descriptor requires use of mutex to
>> allow only single CT request in flight, until we read back that status
>> dword from the CT descriptor.
>
>Q. do we need the same lock for ct_read() and ct_write()?
>
>Could ct_read() use a lock-free ringbuffer, and then if I've read it
>right, you wouldn't have any overlapping spinlock between the interrupt
>handler and the rest (thus avoiding the interrupt-off).
>-Chris

Agree with Chris, it would nice if ct_read() didn't need a lock. At a minimum I
think it needs a different lock than the write path.

Matt