Re: [PATCH] [media] v4l2-subdev: return -EPIPE instead of -EINVAL in link validate default

Hans Verkuil
2015-08-10T14:07:58Z

On 07/14/2015 04:32 PM, Laurent Pinchart wrote:
> Hi Hans,
>
> On Tuesday 14 July 2015 16:19:50 Hans Verkuil wrote:
>> On 07/13/15 10:03, Sakari Ailus wrote:
>>> Helen Fornazier wrote:
>>>> On Tue, Jun 30, 2015 at 4:26 PM, Helen Fornazier wrote:
>>>>> On Tue, Jun 30, 2015 at 6:19 AM, Sakari Ailus wrote:
>>>>>> Laurent Pinchart wrote:
>>>>>>> On Monday 29 June 2015 10:23:34 Sakari Ailus wrote:
>>>>>>>> Helen Fornazier wrote:
>>>>>>>>> According to the V4L2 API, the VIDIOC_STREAMON ioctl should return
>>>>>
>>>>> EPIPE
>>>>>
>>>>>>>>> when the pipeline configuration is invalid.
>>>>>>>>>
>>>>>>>>> As the .vidioc_streamon in the v4l2_ioctl_ops usually forwards the
>>>>>
>>>>> error
>>>>>
>>>>>>>>> caused by the v4l2_subdev_link_validate_default (if it is in use),
>>>>>>>>> it
>>>>>>>>> should return -EPIPE if it detects a format mismatch in the pipeline
>>>>>>>>> configuration
>>>>>>>>
>>>>>>>> Only link configuration errors have yielded -EPIPE so far, sub-device
>>>>>>>> format configuration error has returned -INVAL instead as you
>>>>>>>> noticed.
>>>>>>>
>>>>>>> It should also be noted that while v4l2_subdev_link_validate() will
>>>>>
>>>>> return -
>>>>>
>>>>>>> EINVAL in case of error, the only driver that performs custom link
>>>>>
>>>>> validation
>>>>>
>>>>>>> (omap3isp/ispccdc.c) will return -EPIPE.
>>>>>>
>>>>>> Good point. That has escaped me until now.
>>>>>>
>>>>>>>> There are not many sources of -EINVAL while enabling streaming and
>>>>>>>> all others are directly caused by the application; I lean towards
>>>>>>>> thinking the code is good as it was. The documentation could be
>>>>>>>> improved though. It may not be clear which error codes could be
>>>>>>>> caused by different conditions.
>>>>>>>>
>>>>>>>> The debug level messages from media module
>>>>>>>> (drivers/media/media-entity.c) do provide more information if needed,
>>>>>>>> albeit this certainly is not an application interface.
>>>>>>>>
>>>>>>>> I wonder what others think.
>>>>>>>
>>>>>>> There's a discrepancy between the implementation and the
>>>>>>> documentation, so at least one of them need to be fixed. -EPIPE would
>>>>>>> be coherent with the documentation and seems appropriately named, but
>>>>>>> another error code would allow userspace to tell link configuration
>>>>>>> and format configuration problems apart.
>>>>>>
>>>>>> That was the original intent, I think.
>>>>>>
>>>>>>> Do you think -EINVAL is the most appropriate error code for format
>>>>>>> configuration ? It's already used to indicate that the stream type is
>>>>>>> invalid or that not enough buffers have been allocated, and is also
>>>>>>> used by drivers directly for various purposes.
>>>>>>
>>>>>> That's true, it's been used also for that purpose. At the time this
>>>>>> certainly was not the primary concern. If you can think of a better
>>>>>> error code for the purpose (than EINVAL) I'm certainly fine with using
>>>>>> one. I still think that -EPIPE is worse for telling about incorrect
>>>>>> format configuration than -EINVAL since it's relatively easy to avoid -
>>>>>> EINVAL for the documented reasons.
>>>>>
>>>>> I'd like just to point out where in the docs EPIPE for format mismatch
>>>>> is specified, as it is not described in the streamon page as I thought
>>>>> it would, but it is in the subdev page in case anyone is looking for
>>>>> it (as I took some time to find it too):
>>>>>
>>>>> http://linuxtv.org/downloads/v4l-dvb-apis/subdev.html
>>>>> "Applications are responsible for configuring coherent parameters on
>>>>> the whole pipeline and making sure that connected pads have compatible
>>>>> formats. The pipeline is checked for formats mismatch at
>>>>> VIDIOC_STREAMON time, and an EPIPE error code is then returned if the
>>>>> configuration is invalid"
>>>>>
>>>>> So maybe the doc should be improved as you already stated.
>>>>
>>>> I would like to revive this subject.
>>>>
>>>> Should we change the docs? Change the -EINVAL to -EPIPE, or create
>>>> another error code? What are your opinion?
>>>>
>>>> I read in the docs of dev-kmsg that EPIPE is returned when messages get
>>>> overwritten, and in other parts of the code EPIPE is returned when there
>>>> is an error in the pipeline communication level while trying to send
>>>> information through the pipe or a pipe broken error.
>>>>
>>>> But in the error-codes.txt files, the EPIPE error is defined as:
>>>> *EPIPE "The pipe type specified in the URB doesn't match the endpoint's
>>>> actual type"*
>>>
>>> This exact definition sound USB specific to me.
>>>
>>>> Then, if EPIPE is used when types don't match between two endpoints, it
>>>> seems reasonable to me to use EPIPE when formats don't match either. Or
>>>> do "types" in this context have a specific definition? I don't know much
>>>> about URB, you may be able to judge this better.
>>>
>>> A short recap of the current situation as far as I understand it:
>>>
>>> - MC link validation failure yields EPIPE to the user space,
>>>
>>> - V4L2 sub-device format validation failure generally results in EINVAL,
>>> except that
>>
>> I think that returning EINVAL here is wrong. EINVAL is returned when you
>> set e.g. a format and the format is invalid. In this case the format for
>> each subdev is perfectly fine, it's just that the sink and source formats
>> do not match.
>>
>> Rather than returning EINVAL I think ENOLINK would work well here as you
>> can't setup a link between two entities. And EPIPE can still be used
>> for other higher-level pipeline errors.
>>
>>> - omap3isp CCDC driver returns EPIPE instead and
>>
>> That seems definitely the wrong thing to do.

I think I was ambiguous here. What is wrong here is that it replaces the
actual error code with EPIPE instead of passing it along to userspace.

>
> The VIDIOC_STREAMON documentation states that
>
> "EPIPE
>
> The driver implements pad-level format configuration and the pipeline
> configuration is invalid."
>
> According to the documentation, EINVAL is clearly wrong, and EPIPE should be
> used. I'm open to the idea of using different error codes to indicate severed
> link errors and links with an invalid configuration, but how about backward
> compatibility ?

Applications should *always* check for any error. An application that only
checks for a specific error and assumes that any other error is the same as
'Success' is obviously broken. I have no problem with adding a separate error
for link validation errors (keeping EPIPE for format validation errors).

My preferences for such a link validation error are (in descending order):

- ENOLINK
- EMLINK

Personally I feel that if you can't validate the video pipeline, then you
can't setup the video data links, i.e. ENOLINK.

EMLINK is when you have too many links which feels wrong to me, although
Sakari prefers it. Could this actually be a separate error? I.e. if you
make too many links active? Perhaps we should allow both errors?

Regards,

Hans