Re: [PATCH v3] media: imx: imx7-media-csi: Add support for fast-tracking queued buffers

[Paul Elder <paul.elder@ideasonboard.com>]

Gentle ping


Paul

On Thu, Sep 08, 2022 at 11:37:39AM +0300, Laurent Pinchart wrote:
> From: Paul Elder <paul.elder@ideasonboard.com>
> 
> 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
> FB1, it assigns the next buffer in the queue to the next FB1, and the
> hardware starts to capture into FB2 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 FB1 comes
> in, we can program the buffer into FB2 (which is programmed with a dummy
> buffer, as there is a buffer underrun).
> 
> This of course races with the interrupt that signals FB1 completion, as
> once that interrupt comes in, we are no longer guaranteed that the
> programming of FB2 was in time and must assume it was too late. This
> race is resolved partly by locking the programming of FB2. If it came
> after the interrupt for FB1, then the variable that is used to determine
> which FB to program would have been swapped by the interrupt handler.
> 
> This alone isn't sufficient, however, because the interrupt could still
> be generated (thus the hardware starts capturing into the other fb)
> while the fast-tracking routine has the irq lock. Thus, after
> programming the fb register to fast-track the buffer, the isr also must
> be checked to confirm that an interrupt didn't come in the meantime. If
> it has, we must assume that programming the register for the
> fast-tracked buffer was not in time, and queue the buffer normally.
> 
> Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
> Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> ---
> Changes since v2:
> 
> - Update comments
> 
> Changes since v1:
> 
> - Fix the potential race condition where the interrupt comes in while
>   the fast tracking routine has the irqlock
> - Change return value from int to bool
> ---
>  drivers/staging/media/imx/imx7-media-csi.c | 76 ++++++++++++++++++++++
>  1 file changed, 76 insertions(+)
> 
> diff --git a/drivers/staging/media/imx/imx7-media-csi.c b/drivers/staging/media/imx/imx7-media-csi.c
> index a0553c24cce4..ac35cbc16e73 100644
> --- a/drivers/staging/media/imx/imx7-media-csi.c
> +++ b/drivers/staging/media/imx/imx7-media-csi.c
> @@ -1296,12 +1296,88 @@ static int imx7_csi_video_buf_prepare(struct vb2_buffer *vb)
>  	return 0;
>  }
>  
> +static bool 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;
> +	u32 isr;
> +
> +	if (!csi->is_streaming)
> +		return false;
> +
> +	phys = vb2_dma_contig_plane_dma_addr(&buf->vbuf.vb2_buf, 0);
> +
> +	/*
> +	 * buf_num holds the framebuffer ID of the most recently (*not* the
> +	 * next anticipated) triggered interrupt. Without loss of generality,
> +	 * if buf_num is 0, the hardware is capturing to FB2. If FB1 has been
> +	 * programmed with a dummy buffer (as indicated by active_vb2_buf[0]
> +	 * being NULL), then we can fast-track the new buffer by programming
> +	 * its address in FB1 before the hardware completes FB2, instead of
> +	 * adding it to the buffer queue and incurring a delay of one
> +	 * additional frame.
> +	 *
> +	 * The irqlock prevents races with the interrupt handler that updates
> +	 * buf_num when it programs the next buffer, but we can still race with
> +	 * the hardware if we program the buffer in FB1 just after the hardware
> +	 * completes FB2 and switches to FB1 and before buf_num can be updated
> +	 * by the interrupt handler for FB2.  The fast-tracked buffer would
> +	 * then be ignored by the hardware while the driver would think it has
> +	 * successfully been processed.
> +	 *
> +	 * To avoid this problem, if we can't avoid the race, we can detect
> +	 * that we have lost it by checking, after programming the buffer in
> +	 * FB1, if the interrupt flag indicating completion of FB2 has been
> +	 * raised. If that is not the case, fast-tracking succeeded, and we can
> +	 * update active_vb2_buf[0]. Otherwise, we may or may not have lost the
> +	 * race (as the interrupt flag may have been raised just after
> +	 * programming FB1 and before we read the interrupt status register),
> +	 * and we need to assume the worst case of a race loss and queue the
> +	 * buffer through the slow path.
> +	 */
> +
> +	spin_lock_irqsave(&csi->irqlock, flags);
> +
> +	buf_num = csi->buf_num;
> +	if (csi->active_vb2_buf[buf_num]) {
> +		spin_unlock_irqrestore(&csi->irqlock, flags);
> +		return false;
> +	}
> +
> +	imx7_csi_update_buf(csi, phys, buf_num);
> +
> +	isr = imx7_csi_reg_read(csi, CSI_CSISR);
> +	if (isr & (buf_num ? BIT_DMA_TSF_DONE_FB1 : BIT_DMA_TSF_DONE_FB2)) {
> +		/*
> +		 * The interrupt for the /other/ FB just came (the isr hasn't
> +		 * run yet though, because we have the lock here); we can't be
> +		 * sure we've programmed buf_num FB in time, so queue the buffer
> +		 * to the buffer queue normally. No need to undo writing the FB
> +		 * register, since we won't return it as active_vb2_buf is NULL,
> +		 * so it's okay to potentially write it to both FB1 and FB2;
> +		 * only the one where it was queued normally will be returned.
> +		 */
> +		spin_unlock_irqrestore(&csi->irqlock, flags);
> +		return false;
> +	}
> +
> +	csi->active_vb2_buf[buf_num] = buf;
> +
> +	spin_unlock_irqrestore(&csi->irqlock, flags);
> +	return true;
> +}
> +
>  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;
>  
> +	if (imx7_csi_fast_track_buffer(csi, buf))
> +		return;
> +
>  	spin_lock_irqsave(&csi->q_lock, flags);
>  
>  	list_add_tail(&buf->list, &csi->ready_q);
> -- 
> Regards,
> 
> Laurent Pinchart
>