Re: [PATCH v4 2/8] media: v4l2: abstract timeval handling in v4l2_buffer

[Hans Verkuil]

On 11/26/19 12:34 PM, Arnd Bergmann wrote:
> On Mon, Nov 25, 2019 at 4:52 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>>
>> On 11/11/19 9:38 PM, Arnd Bergmann wrote:
>>> As a preparation for adding 64-bit time_t support in the uapi,
>>> change the drivers to no longer care about the format of the
>>> timestamp field in struct v4l2_buffer.
>>>
>>> The v4l2_timeval_to_ns() function is no longer needed in the
>>> kernel after this, but there may be userspace code relying on
>>> it because it is part of the uapi header.
>>
>> There is indeed userspace code that relies on this.
> 
> Ok, good to know. I rephrased the changelog text as
> 
> The v4l2_timeval_to_ns() function is no longer needed in the
> kernel after this, but there is userspace code relying on
> it to be part of the uapi header.
> 
>>>
>>> +static inline u64 v4l2_buffer_get_timestamp(const struct v4l2_buffer *buf)
>>> +{
>>> +     return buf->timestamp.tv_sec * NSEC_PER_SEC +
>>> +            (u32)buf->timestamp.tv_usec * NSEC_PER_USEC;
>>
>> Why the (u32) cast?
> 
> Simple question, long answer:
> 
> on 32-bit architectures, the tv_usec member may be 32-bit wide plus
> padding in user space when interpreted as a regular 'struct timeval',
> but the kernel implementation now sees it as a 64-bit member,
> with half of it being possibly uninitialized user space data.
> 
> The 32-bit cast avoids that uninitialized data and ensures user space
> passing garbage in the upper half gets ignored, as it has to be on 32-bit
> user space.

But that's only valid for little endian 32 bit systems, right?
Is this only an issue for x86 platforms?

> 
> On 64-bit native user space, the tv_usec field is always 64 bit wide,
> so this is a change in behavior for denormalized timeval data
> with tv_usec > U32_MAX, but the current behavior does not appear
> worth preserving either.
> 
> The correct way would probably be to return an error for
>  tv_usec >USEC_PER_SEC, but as the code never did that, this
> would risk a regression for user space that relies on passing
> invalid timestamps without getting an error.

This long answer needs to be added to a comment to that function.
Because otherwise someone will come along later and remove that
seemingly unnecessary cast.

It's OK if it is a long comment, it's a non-trivial reason.

> 
>>> +static inline void v4l2_buffer_set_timestamp(struct v4l2_buffer *buf,
>>> +                                          u64 timestamp)
>>> +{
>>> +     struct timespec64 ts = ns_to_timespec64(timestamp);
>>> +
>>> +     buf->timestamp.tv_sec  = ts.tv_sec;
>>> +     buf->timestamp.tv_usec = ts.tv_nsec / NSEC_PER_USEC;
>>> +}
>>> +
>>
>> This does not belong in the public header. This is kernel specific,
> 
> Note: this is not the uapi header but the in-kernel one.

Ah, I missed that.

> 
>> so media/v4l2-common.h would be a good place.
> 
> Ok, sounds good. I wasn't sure where to put it, and ended up
> with include/linux/videodev2.h as the best replacement for
> include/uapi/linux/videodev2.h, changed it to
> include/media/v4l2-common.h now.

Never use include/linux/videodev2.h. It's just a wrapper around
the uapi header and should not contain any 'real' code.

It's also why I missed that you modified that header since we never
touch it.

Regards,

	Hans

> 
>        Arnd
>