[PATCH 07/74] drm/i915/execlists: Use rmb() to order CSB reads
Chris Wilson
chris at chris-wilson.co.uk
Thu May 10 11:31:47 UTC 2018
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 at linux.intel.com>
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: MichaĆ Winiarski <michal.winiarski at intel.com>
Cc: Rafael Antognolli <rafael.antognolli at intel.com>
Cc: Michel Thierry <michel.thierry at intel.com>
Cc: Timo Aaltonen <tjaalton at ubuntu.com>
Tested-by: Timo Aaltonen <tjaalton at ubuntu.com>
---
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 09fded9d409f..f9e226b5a80a 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1016,6 +1016,7 @@ 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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