[SRU][OEM-B][PATCH] UBUNTU: SAUCE: drm/i915/execlists: Use rmb() to order CSB reads

[Timo Aaltonen]

From: Chris Wilson <chris at chris-wilson.co.uk>

BugLink: http://bugs.launchpad.net/bugs/1769843

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, and so we
should use a complimentary rmb() to ensure that our read of its WRITE
pointer is completed before we start accessing the data.

The final mb() 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 <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: Michał 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>
Signed-off-by: Timo Aaltonen <timo.aaltonen at canonical.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 e8af93e9235a..c4bcaa0cf677 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -838,6 +838,7 @@ static void intel_lrc_irq_handler(unsigned long data)
 
 			head = execlists->csb_head;
 			tail = READ_ONCE(buf[write_idx]);
+			rmb(); /* Hopefully paired with a wmb() in HW */
 		}
 
 		while (head != tail) {
-- 
2.17.0