linux-kernel - Re: [PATCH v2 32/66] media: sun6i-csi: Add capture state using vsync for page flip

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Message-ID: <20220211164503.5hftzazx56cwvk66@houat>
Date:   Fri, 11 Feb 2022 17:45:03 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Cc:     linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-phy@...ts.infradead.org,
        linux-clk@...r.kernel.org, linux-staging@...ts.linux.dev,
        Yong Deng <yong.deng@...ewell.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Helen Koike <helen.koike@...labora.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v2 32/66] media: sun6i-csi: Add capture state using vsync
 for page flip

On Fri, Feb 11, 2022 at 04:40:55PM +0100, Paul Kocialkowski wrote:
> Hi,
> 
> On Wed 09 Feb 22, 10:26, Maxime Ripard wrote:
> > On Sat, Feb 05, 2022 at 07:53:55PM +0100, Paul Kocialkowski wrote:
> > > This introduces a new state structure and associated helpers for
> > > capture, which handles the buffer queue and state for each submitted
> > > buffer.
> > > 
> > > Besides from the code refactoring, this changes the page flip point
> > > to vsync instead of frame done, which allows working with only
> > > two buffers without losing frames. This is apparently a more
> > > appropriate thing to do with this controller.
> > 
> > Why? What issues were you seeing before, how does using a separate
> > interrupt addresses it, and what makes you think it's more appropriate?
> 
> I'll try to update the commit log to address this, thanks.
> 
> One of the main issues that the rework is trying to address is the way
> that double buffering is handled, which currently requires up to 3 buffers
> to work, due to how buffer configuration is implemented. In particular,
> it's synchronizing to the frame done interrupt which seems to hit after
> scanout of the next frame begins, so page flips are always delayed by one
> frame.
> 
> This is currently solved by setting two buffers when starting the stream:
> the first one is set before vcap is enabled and seems to be sampled when the
> first frame scan begins and the second one is set after and stays until the
> second vsync hits, at which point it becomes the current active buffer.
> This way no frame is lost but 3 frames are needed to start.
> 
> This proposal changes the sync point to vsync which allows page flipping to
> happen for the next frame, thus only 2 buffers are required.

This should be in your commit log

Maxime

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