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Message-ID: <fc0baa11373888c3dc09e76fbbbb6d382028d738.camel@pengutronix.de>
Subject: Re: [PATCH v2 30/36] media: imx: Fix misuse of min_buffers_needed
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From:   Philipp Zabel <p.zabel@pengutronix.de>
To:     Benjamin Gaignard <benjamin.gaignard@collabora.com>,
        hverkuil@xs4all.nl, mchehab@kernel.org, tfiga@chromium.org,
        m.szyprowski@samsung.com, matt.ranostay@konsulko.com
Cc:     linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
        linux-staging@lists.linux.dev, kernel@collabora.com,
        Steve Longerbeam <slongerbeam@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        Fabio Estevam <festevam@gmail.com>,
        NXP Linux Team <linux-imx@nxp.com>
Date:   Mon, 04 Dec 2023 14:52:32 +0100
In-Reply-To: <20231204132323.22811-31-benjamin.gaignard@collabora.com>
References: <20231204132323.22811-1-benjamin.gaignard@collabora.com>
         <20231204132323.22811-31-benjamin.gaignard@collabora.com>
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Hi Benjamin,

On Mo, 2023-12-04 at 14:23 +0100, Benjamin Gaignard wrote:
> 'min_buffers_needed' is suppose to be used to indicate the number
> of buffers needed by DMA engine to start streaming.
> imx-media-capture driver doesn't use DMA engine and just want to specify
> the minimum number of buffers to allocate when calling VIDIOC_REQBUFS.

This is used by imx-media-csi and imx-ic-prpencvf. Both expect two
buffers to setup the IDMAC DMA engine double buffer (see
csi_idmac_setup_vb2_buf()/prp_setup_vb2_buf()).

These functions can cope with less than two buffers queued, by using a
special "underrun" buffer as a workaround, but the hardware does
require two buffer addresses, so I'm not sure whether this really
should be called a misuse of min_buffers_needed.

regards
Philipp