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Date:   Tue, 6 Sep 2022 18:24:00 +0100
From:   Rui Miguel Silva <rmfrfs@gmail.com>
To:     Paul Elder <paul.elder@ideasonboard.com>
Cc:     Steve Longerbeam <slongerbeam@gmail.com>,
        linux-media@vger.kernel.org,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        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>,
        linux-staging@lists.linux.dev,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] media: imx7-media-csi: Add support for fast-tracking
 queued buffers
Message-ID: <20220906172400.4oxeefxhmesl2spi@arch-thunder>
References: <20220906104437.4095745-1-paul.elder@ideasonboard.com>
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In-Reply-To: <20220906104437.4095745-1-paul.elder@ideasonboard.com>
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Hi Paul,

On Tue, Sep 06, 2022 at 07:44:37PM +0900, Paul Elder wrote:
> The CSI hardware compatible with this driver handles buffers using a
> ping-pong mechanism with two sets of destination addresses. Normally,
> when an interrupt comes in to signal the completion of one buffer, say
> FB0, it assigns the next buffer in the queue to the next FB0, and the
> hardware starts to capture into FB1 in the meantime.
> 
> In a buffer underrun situation, in the above example without loss of
> generality, if a new buffer is queued before the interrupt for FB0 comes
> in, we can program the buffer into FB1 (which is programmed with a dummy
> buffer, as there is a buffer underrun).
> 
> This of course races with the interrupt that signals FB0 completion, as
> once that interrupt comes in, we are no longer guaranteed that the
> programming of FB1 was in time and must assume it was too late. This
> race is resolved by locking the programming of FB1. If it came after the
> interrupt for FB0, then the variable that is used to determine which FB
> to program would have been swapped by the interrupt handler, thus
> resolving the race.
> 
> Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>

Thanks a lot for this patch, and spending time commenting the issue in
the code, and the good changelog.

LGTM.

Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>

Cheers,
  Rui

> ---
>  drivers/staging/media/imx/imx7-media-csi.c | 49 ++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
> 
> diff --git a/drivers/staging/media/imx/imx7-media-csi.c b/drivers/staging/media/imx/imx7-media-csi.c
> index a0553c24cce4..06e50080ed31 100644
> --- a/drivers/staging/media/imx/imx7-media-csi.c
> +++ b/drivers/staging/media/imx/imx7-media-csi.c
> @@ -1296,11 +1296,60 @@ static int imx7_csi_video_buf_prepare(struct vb2_buffer *vb)
>  	return 0;
>  }
>  
> +static int imx7_csi_fast_track_buffer(struct imx7_csi *csi,
> +				      struct imx7_csi_vb2_buffer *buf)
> +{
> +	unsigned long flags;
> +	dma_addr_t phys;
> +	int buf_num;
> +	int ret = -EBUSY;
> +
> +	if (!csi->is_streaming)
> +		return ret;
> +
> +	phys = vb2_dma_contig_plane_dma_addr(&buf->vbuf.vb2_buf, 0);
> +
> +	/*
> +	 * buf_num holds the fb id of the most recently (*not* the next
> +	 * anticipated) triggered interrupt. Without loss of generality, if
> +	 * buf_num is 0 and we get to this section before the irq for fb1, the
> +	 * buffer that we are fast-tracking into fb0 should be programmed in
> +	 * time to be captured into. If the irq for fb1 already happened, then
> +	 * buf_num would be 1, and we would fast-track the buffer into fb1
> +	 * instead. This guarantees that we won't try to fast-track into fb0
> +	 * and race against the start-of-capture into fb0.
> +	 *
> +	 * We only fast-track the buffer if the currently programmed buffer is
> +	 * a dummy buffer. We can check the active_vb2_buf instead as it is
> +	 * always modified along with programming the fb[0,1] registers via the
> +	 * lock (besides setup and cleanup). If it is not a dummy buffer then
> +	 * we queue it normally, as fast-tracking is not an option.
> +	 */
> +
> +	spin_lock_irqsave(&csi->irqlock, flags);
> +
> +	buf_num = csi->buf_num;
> +	if (csi->active_vb2_buf[buf_num] == NULL) {
> +		csi->active_vb2_buf[buf_num] = buf;
> +		imx7_csi_update_buf(csi, phys, buf_num);
> +		ret = 0;
> +	}
> +
> +	spin_unlock_irqrestore(&csi->irqlock, flags);
> +
> +	return ret;
> +}
> +
>  static void imx7_csi_video_buf_queue(struct vb2_buffer *vb)
>  {
>  	struct imx7_csi *csi = vb2_get_drv_priv(vb->vb2_queue);
>  	struct imx7_csi_vb2_buffer *buf = to_imx7_csi_vb2_buffer(vb);
>  	unsigned long flags;
> +	int ret;
> +
> +	ret = imx7_csi_fast_track_buffer(csi, buf);
> +	if (!ret)
> +		return;
>  
>  	spin_lock_irqsave(&csi->q_lock, flags);
>  
> -- 
> 2.30.2
>