Re: [PATCH 1/2] drm/i915/execlists: Use rmb() to order CSB reads

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> 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@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: Mika Kuoppala <mika.kuoppala@intel.com>

This also makes the hwsp access work fine with iommu on (kbl).

-Mika

> ---
>  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 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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