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 it's 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
References: 61bf9719fa17 ("drm/i915/cnl: Use mmio access to context status
buffer")
Suggested-by: Mika Kuoppala <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Michał Winiarski <[email protected]>
Cc: Rafael Antognolli <[email protected]>
Cc: Michel Thierry <[email protected]>
Cc: Timo Aaltonen <[email protected]>
Tested-by: Timo Aaltonen <[email protected]>
---
Just tweaked the commitmsg to cross reference the mb against the
diagram.
-Chris
---
drivers/gpu/drm/i915/intel_engine_cs.c | 3 ---
drivers/gpu/drm/i915/intel_lrc.c | 1 +
2 files changed, 1 insertion(+), 3 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c
b/drivers/gpu/drm/i915/intel_engine_cs.c
index 70325e0824e3..8303e05b0c7d 100644
--- a/drivers/gpu/drm/i915/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/intel_engine_cs.c
@@ -470,9 +470,6 @@ static bool csb_force_mmio(struct drm_i915_private *i915)
if (intel_vgpu_active(i915) && !intel_vgpu_has_hwsp_emulation(i915))
return true;
- if (IS_CANNONLAKE(i915))
- return true;
-
return false;
}
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 911f288f78aa..8977600f0d81 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -992,6 +992,7 @@ 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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