Commits
Chris Wilson committed cd078bf95df
drm/i915/execlists: Use rmb() to order CSB reads
We assume that the CSB is written using the normal ringbuffer
coherency protocols, as outlined in kernel/events/ring_buffer.c:
* (HW) (DRIVER)
*
* if (LOAD ->data_tail) { LOAD ->data_head
* (A) smp_rmb() (C)
* STORE $data LOAD $data
* smp_wmb() (B) smp_mb() (D)
* STORE ->data_head STORE ->data_tail
* }
So we assume that the HW fulfils its ordering requirements (B), and so
we should use a complimentary rmb (C) to ensure that our read of its
WRITE pointer is completed before we start accessing the data.
The final mb (D) is implied by the uncached mmio we perform to inform
the HW of our READ pointer.
References: https://bugs.freedesktop.org/show_bug.cgi?id=105064
References: https://bugs.freedesktop.org/show_bug.cgi?id=105888
References: https://bugs.freedesktop.org/show_bug.cgi?id=106185
Fixes: 767a983ab255 ("drm/i915/execlists: Read the context-status HEAD from the HWSP")
References: 61bf9719fa17 ("drm/i915/cnl: Use mmio access to context status buffer")
Suggested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Timo Aaltonen <tjaalton@ubuntu.com>
Tested-by: Timo Aaltonen <tjaalton@ubuntu.com>
Acked-by: Michel Thierry <michel.thierry@intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511121147.31915-1-chris@chris-wilson.co.uk
(cherry picked from commit 77dfedb5be03779f9a5d83e323a1b36e32090105)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>