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drm/i915/execlists: Use rmb() to order CSB reads

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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: 767a983ab2 ("drm/i915/execlists: Read the context-status HEAD from the HWSP")
References: 61bf9719fa ("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 77dfedb5be)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
This commit is contained in:
Chris Wilson 2018-05-11 13:11:45 +01:00 committed by Joonas Lahtinen
parent 20943f9849
commit cd078bf95d
1 changed files with 1 additions and 0 deletions
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drivers/gpu/drm/i915/intel_lrc.c
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@ -884,6 +884,7 @@ 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,