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From:   Jernej =?utf-8?B?xaBrcmFiZWM=?= <jernej.skrabec@gmail.com>
To:     linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-sunxi@lists.linux.dev,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Cc:     Yong Deng <yong.deng@magewell.com>,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Chen-Yu Tsai <wens@csie.org>,
        Samuel Holland <samuel@sholland.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Subject: Re: [PATCH v6 18/43] media: sun6i-csi: Add capture state using vsync for page flip
Date:   Mon, 31 Oct 2022 10:15:21 +0100
Message-ID: <4778394.31r3eYUQgx@kista>
In-Reply-To: <20220826183240.604834-19-paul.kocialkowski@bootlin.com>
References: <20220826183240.604834-1-paul.kocialkowski@bootlin.com> <20220826183240.604834-19-paul.kocialkowski@bootlin.com>
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Dne petek, 26. avgust 2022 ob 20:32:15 CET je Paul Kocialkowski napisal(a):
> The current implementation requires up to 3 buffers to properly
> implement page flipping without losing frames: one is configured
> before the video stream is started, one just after that and page
> flipping is synchronized to the frame done interrupt. The comment in
> the code mentions that "CSI will lookup the next dma buffer for next
> frame before the current frame done IRQ triggered".
> 
> Based on observations of the CSI unit behavior, it seems that the
> buffer DMA address is sampled when the frame scan begins (in addition
> to starting the stream), which corresponds to the vblank interrupt
> that hits just before the frame-done interrupt of the previous frame.
> 
> As a result, the address configured at the frame done interrupt is not
> actually used for the next frame but for the one after that.
> 
> This proposal changes the page flipping sync point to the vsync
> interrupt, which allows the DMA address to be sampled for the next
> frame instead and allows operating with only two buffers.
> 
> In addition to the change in the sync point, the code is refactored
> to introduce a notion of state that clarifies tracking of the buffers
> with tidy functions.
> 
> Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>

Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>

Best regards,
Jernej