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

[Hans Verkuil]

On 24/10/2024 13:08, Tomi Valkeinen wrote:
> 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.

min_queued_buffers is really HW dependent. If not set, then start_streaming can be called even if there are no buffers queued.

If your hardware can handle that, then it's fine to not set it.

Regards,

	Hans

> 
>  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;
>>> +}
>>
>