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@xxxxxxxxxxxxxxxx>
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@xxxxxxxxx>
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
>
