Re: [PATCH v6 3/4] media: raspberrypi: Add support for RP1-CFE

From: Tomi Valkeinen
Date: Thu Oct 24 2024 - 07:09:22 EST


Hi Hans,

On 24/10/2024 11:20, Hans Verkuil wrote:
Hi Tomi,

I know this driver is already merged, but while checking for drivers that use
q->max_num_buffers I stumbled on this cfe code:

<snip>

+/*
+ * vb2 ops
+ */
+
+static int cfe_queue_setup(struct vb2_queue *vq, unsigned int *nbuffers,
+ unsigned int *nplanes, unsigned int sizes[],
+ struct device *alloc_devs[])
+{
+ struct cfe_node *node = vb2_get_drv_priv(vq);
+ struct cfe_device *cfe = node->cfe;
+ unsigned int size = is_image_node(node) ?
+ node->vid_fmt.fmt.pix.sizeimage :
+ node->meta_fmt.fmt.meta.buffersize;
+
+ cfe_dbg(cfe, "%s: [%s] type:%u\n", __func__, node_desc[node->id].name,
+ node->buffer_queue.type);
+
+ if (vq->max_num_buffers + *nbuffers < 3)
+ *nbuffers = 3 - vq->max_num_buffers;

This makes no sense: max_num_buffers is 32, unless explicitly set when vb2_queue_init
is called. So 32 + *nbuffers is never < 3.

If the idea is that at least 3 buffers should be allocated by REQBUFS, then set
q->min_reqbufs_allocation = 3; before calling vb2_queue_init and vb2 will handle this
for you.

Drivers shouldn't modify *nbuffers, except in very rare circumstances, especially
since the code is almost always wrong.

Indeed, the code doesn't make sense. I have to say I don't know what was the intent here, but I think "at least 3 buffers should be allocated by REQBUFS" is the likely explanation.

I think the hardware should work with even just a single buffer, so is it then fine to not set either q->min_queued_buffers nor q->min_reqbufs_allocation before calling vb2_queue_init()? This seems to result in REQBUFS giving at least two buffers.

Tomi


Regards,

Hans

+
+ if (*nplanes) {
+ if (sizes[0] < size) {
+ cfe_err(cfe, "sizes[0] %i < size %u\n", sizes[0], size);
+ return -EINVAL;
+ }
+ size = sizes[0];
+ }
+
+ *nplanes = 1;
+ sizes[0] = size;
+
+ return 0;
+}