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From: Chris Wilson <chris@chris-wilson.co.uk>
To: "Michał Winiarski" <michal.winiarski@intel.com>,
	intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH v2 4/5] drm/i915/guc: Make adding GuC work items lockless
Date: Tue, 12 Sep 2017 22:22:12 +0100	[thread overview]
Message-ID: <150525133228.18386.5080488191524013276@mail.alporthouse.com> (raw)
In-Reply-To: <20170912174056.20139-4-michal.winiarski@intel.com>

Quoting Michał Winiarski (2017-09-12 18:40:55)
> @@ -434,11 +430,16 @@ static void guc_wq_item_append(struct i915_guc_client *client,
>          */
>         BUILD_BUG_ON(wqi_size != 16);
>  
> -       /* postincrement WQ tail for next time */
> -       wq_off = client->wq_tail;
> +       /* Find our offset and postincrement WQ tail for next time, free space
> +        * is guaranteed.
> +        */
> +       do {
> +               wq_off = READ_ONCE(desc->tail);
> +               wq_next = (wq_off + wqi_size) & (GUC_WQ_SIZE - 1);
> +               GEM_BUG_ON(CIRC_SPACE(wq_off, READ_ONCE(desc->head),
> +                                     GUC_WQ_SIZE) < wqi_size);
> +       } while (cmpxchg(&desc->tail, wq_off, wq_next) != wq_off);
>         GEM_BUG_ON(wq_off & (wqi_size - 1));

I forgot the trick in cmpxchg to avoid the READ_ONCE in every loop:
	wq_next = READ_ONCE(desc->tail);
	do {
		wq_off = wq_next;
		wq_next = (wq_off + wqi_size) & (GUC_WQ_SIZE - 1);
		GEM_BUG_ON(...);
	} while ((wq_next = cmpxchg(&desc->tail, wq_off, wq_next) != wq_off);
-Chris
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  reply	other threads:[~2017-09-12 21:22 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-09-12 17:40 [PATCH 1/5] drm/i915/guc: Remove obsolete comments and remove unused variable Michał Winiarski
2017-09-12 17:40 ` [PATCH 2/5] drm/i915/guc: Submit GuC workitems containing coalesced requests Michał Winiarski
2017-09-12 17:40 ` [PATCH 3/5] drm/i915/guc: Simplify GuC doorbell logic Michał Winiarski
2017-09-12 20:24   ` Chris Wilson
2017-09-12 17:40 ` [PATCH v2 4/5] drm/i915/guc: Make adding GuC work items lockless Michał Winiarski
2017-09-12 21:22   ` Chris Wilson [this message]
2017-09-12 17:40 ` [PATCH 5/5] HAX Enable GuC Submission for CI Michał Winiarski
2017-09-12 18:36 ` ✗ Fi.CI.BAT: failure for series starting with [1/5] drm/i915/guc: Remove obsolete comments and remove unused variable Patchwork

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