linux-kernel - Re: [PATCHv3 3/8] drm/omap: add support for manually updated displays

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Date:   Fri, 20 Apr 2018 11:19:35 +0100
From:   Daniel Stone <daniel@...ishbar.org>
To:     Tomi Valkeinen <tomi.valkeinen@...com>
Cc:     Sebastian Reichel <sebastian.reichel@...labora.co.uk>,
        Sebastian Reichel <sre@...nel.org>,
        Tony Lindgren <tony@...mide.com>, Pavel Machek <pavel@....cz>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        devicetree@...r.kernel.org, linux-omap@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kernel@...labora.com
Subject: Re: [PATCHv3 3/8] drm/omap: add support for manually updated displays

Hi Tomi,

On 20 April 2018 at 08:09, Tomi Valkeinen <tomi.valkeinen@...com> wrote:
> It's actually not quite clear to me how manual update displays work with
> DRM...
>
> As far as I see, we have essentially two cases: 1) single buffering,
> where the userspace must set an area in the fb dirty, which then
> triggers the update, 2) multi buffering, which doesn't need fb dirty,
> but just a page flip which triggers the update.
>
> In the 2) case (which I think is the optimal case which all the modern
> apps should use), there's no need for delayed work or any work, and the
> code flow should be very similar to the auto-update model.

Correct. There's been talk (and I think patches?) of adding a
per-plane dirty property, so userspace can as an optimisation inform
the kernel of the area changed between frames. But short of that, a
pageflip needs to trigger a full-plane update, with no dirtyfb
required.

Cheers,
Daniel