Messages in this thread | | | Date | Tue, 15 Sep 2015 18:27:19 +0200 | From | Hans Verkuil <> | Subject | Re: [PATCH 6/7] [RFC] [media]: v4l2: introduce v4l2_timeval |
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On 09/15/2015 05:49 PM, Arnd Bergmann wrote: > The v4l2 API uses a 'struct timeval' to communicate time stamps to user > space. This is broken on 32-bit architectures as soon as we have a C library > that defines time_t as 64 bit, which then changes the structure layout of > struct v4l2_buffer. > > Fortunately, almost all v4l2 drivers use monotonic timestamps and call > v4l2_get_timestamp(), which means they don't also have a y2038 problem. > This means we can keep using the existing binary layout of the structure > and do not need to worry about defining a new kernel interface for > userland with 64-bit time_t. > > A possible downside of this approach is that it breaks any user space > that tries to assign the timeval structure returned from the kernel > to another timeval, or to pass a pointer to it into a function that > expects a timeval. Those will cause a build-time warning or error > that can be fixed up in a backwards compatible way. > > The alternative to this patch is to leave the structure using > 'struct timeval', but then we have to rework the kernel to let > it handle both 32-bit and 64-bit time_t for 32-bit user space > processes.
Cool. Only this morning I was thinking about what would be needed in v4l2 to be y2038 safe, and here it is!
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h > index 3228fbebcd63..b02cf054fbb8 100644 > --- a/include/uapi/linux/videodev2.h > +++ b/include/uapi/linux/videodev2.h > @@ -803,6 +803,12 @@ struct v4l2_plane { > __u32 reserved[11]; > }; > > +/* used for monotonic times, therefore y2038 safe */ > +struct v4l2_timeval { > + long tv_sec; > + long tv_usec; > +}; > + > /** > * struct v4l2_buffer - video buffer info > * @index: id number of the buffer > @@ -839,7 +845,7 @@ struct v4l2_buffer { > __u32 bytesused; > __u32 flags; > __u32 field; > - struct timeval timestamp; > + struct v4l2_timeval timestamp; > struct v4l2_timecode timecode; > __u32 sequence; > >
I suspect that quite a few apps use assign the timestamp to another timeval struct. A quick grep in v4l-utils (which we maintain) shows at least two of those assignments. Ditto for xawtv3.
So I don't think v4l2_timeval is an option as it would break userspace too badly.
An alternative to supporting a 64-bit timeval for 32-bit userspace is to make a new y2038-aware struct and a new set of ioctls and use this opportunity to clean up and extend the v4l2_buffer struct.
So any 32-bit app that needs to be y2038 compliant would just use the new struct and ioctls.
But this is something to discuss among the v4l2 developers.
Regards,
Hans
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