linux-kernel - Re: [PATCH v7 0/3] drm: add explict fencing

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Message-ID: <20161108103508.GH18604@nuc-i3427.alporthouse.com>
Date:   Tue, 8 Nov 2016 10:35:08 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Gustavo Padovan <gustavo@...ovan.org>
Cc:     dri-devel@...ts.freedesktop.org, marcheu@...gle.com,
        Daniel Stone <daniels@...labora.com>, seanpaul@...gle.com,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        linux-kernel@...r.kernel.org, laurent.pinchart@...asonboard.com,
        Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
        John Harrison <John.C.Harrison@...el.com>, m.chehab@...sung.com
Subject: Re: [PATCH v7 0/3] drm: add explict fencing

On Tue, Nov 08, 2016 at 03:54:47PM +0900, Gustavo Padovan wrote:
> From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> 
> Hi,
> 
> This is yet another version of the DRM fences patches. Please refer
> to the cover letter[1] in a previous version to check for more details.

Explicit fencing is not a superset of the implicit fences. The driver
may be using implicit fences (on a reservation object) to serialise
asynchronous operations wrt to each other (such as dispatching threads
to flush cpu caches to memory, manipulating page tables and the like
before the flip).  Since the user doesn't know about these operations,
they are not included in the explicit fence they provide, at which point
we can't trust their fence to the exclusion of the implicit fences...
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre