Re: [PATCH v2 1/2] media: uvcvideo: Support partial control reads

[Hans de Goede <hdegoede@redhat.com>]

Hi Ricardo,

Thank you for your patch.

On 8-Oct-24 5:00 PM, Ricardo Ribalda wrote:
> Some cameras, like the ELMO MX-P3, do not return all the bytes
> requested from a control if it can fit in less bytes.
> Eg: Returning 0xab instead of 0x00ab.
> usb 3-9: Failed to query (GET_DEF) UVC control 3 on unit 2: 1 (exp. 2).
> 
> Extend the returned value from the camera and return it.
> 
> Cc: stable@vger.kernel.org
> Fixes: a763b9fb58be ("media: uvcvideo: Do not return positive errors in uvc_query_ctrl()")
> Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> ---
>  drivers/media/usb/uvc/uvc_video.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index cd9c29532fb0..f125b3ba50f2 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -76,14 +76,29 @@ int uvc_query_ctrl(struct uvc_device *dev, u8 query, u8 unit,
>  
>  	ret = __uvc_query_ctrl(dev, query, unit, intfnum, cs, data, size,
>  				UVC_CTRL_CONTROL_TIMEOUT);
> -	if (likely(ret == size))
> +	if (ret > 0) {
> +		if (size == ret)
> +			return 0;
> +
> +		/*
> +		 * In UVC the data is represented in little-endian by default.
> +		 * Some devices return shorter control packages that expected
> +		 * for GET_DEF/MAX/MIN if the return value can fit in less
> +		 * bytes.

What about GET_CUR/GET_RES ? are those not affected?

And if it is not affected should we limit this special handling to
GET_DEF/MAX/MIN ?


> +		 * Zero all the bytes that the device have not written.
> +		 */
> +		memset(data + ret, 0, size - ret);

So your new work around automatically applies to all UVC devices which
gives us a short return. I think that is both good and bad at the same
time. Good because it avoids the need to add quirks. Bad because what
if we get a short return for another reason.

You do warn on the short return. So if we get bugs due to hitting the short
return for another reason the warning will be i the logs.

So all in all think the good outways the bad.

So yes this seems like a good solution.

> +		dev_warn(&dev->udev->dev,
> +			 "UVC non compliance: %s control %u on unit %u returned %d bytes when we expected %u.\n",
> +			 uvc_query_name(query), cs, unit, ret, size);

I do wonder if we need to use dev_warn_ratelimited()
or dev_warn_once() here though.

If this only impacts GET_DEF/MAX/MIN we will only hit this
once per ctrl, after which the cache will be populated.

But if GET_CUR is also affected then userspace can trigger
this warning. So in that case I think we really should use
dev_warn_once() or have a flag per ctrl to track this
and only warn once per ctrl if we want to know which
ctrls exactly are buggy.

What we really do not want is userspace repeatedly calling
VIDIOC_G_CTRL / VIDIOC_G_EXT_CTRLS resulting in a message
in dmesg every call.

>  		return 0;
> +	}
>  
>  	if (ret != -EPIPE) {
>  		dev_err(&dev->udev->dev,
>  			"Failed to query (%s) UVC control %u on unit %u: %d (exp. %u).\n",
>  			uvc_query_name(query), cs, unit, ret, size);
> -		return ret < 0 ? ret : -EPIPE;
> +		return ret ? ret : -EPIPE;

It took me a minute to wrap my brain around this and even
though I now understand this change I do not like it.

There is no need to optimize an error-handling path like this
and IMHO the original code is much easier to read:

		return ret < 0 ? ret : -ESOMETHING;

is a well known pattern to check results from functions which
return a negative errno, or the amount of bytes read, combined
with an earlier success check for ret == amount-expected .

By changing this to:

		return ret ? ret : -EPIPE;

You are breaking the pattern recognition people familiar with
this kinda code have and IMHO this is not necessary.

Also not changing this reduces the patch-size / avoids code-churn
which also is a good thing.

Please drop this part of the patch.

Regards,

Hans