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@xxxxxxxxxxxxxxx>
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Michał Winiarski <michal.winiarski@xxxxxxxxx>
Cc: Rafael Antognolli <rafael.antognolli@xxxxxxxxx>
Cc: Michel Thierry <michel.thierry@xxxxxxxxx>
Cc: Timo Aaltonen <tjaalton@xxxxxxxxxx>
Tested-by: Timo Aaltonen <tjaalton@xxxxxxxxxx>
Acked-by: Michel Thierry <michel.thierry@xxxxxxxxx>
Acked-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
---
drivers/gpu/drm/i915/intel_lrc.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 7f98dda3c929..11170bc36456 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1008,6 +1008,7 @@ static void execlists_submission_tasklet(unsigned long data)
head = execlists->csb_head;
tail = READ_ONCE(buf[write_idx]);
+ rmb(); /* Hopefully paired with a wmb() in HW */
}
GEM_TRACE("%s cs-irq head=%d [%d%s], tail=%d [%d%s]\n",
engine->name,
--
2.17.0
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