Re: [PATCH] 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 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 <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>
> ---
>
> 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 */

If the gpu does ordered writes (with write buffer in between), this is ok.

If the gpu does not order writes, we would still need the rmb()
to prevent cpu from loading an stale csb entry ahead of tail read?

Quoting memory-barries.txt:
" (*) loads may be done speculatively, leading to the result having been fetched
     at the wrong time in the expected sequence of events;
"

So I would change the comment to /* Enforce tail vs csb entry read order */

-Mika

>  		}
>  		GEM_TRACE("%s cs-irq head=%d [%d%s], tail=%d [%d%s]\n",
>  			  engine->name,
> -- 
> 2.17.0
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/intel-gfx