Re: [PATCH 6/7] [RFC] [media]: v4l2: introduce v4l2_timeval
From: Hans Verkuil <hidden>
Date: 2015-09-15 16:28:42
Also in:
linux-media, linux-samsung-soc, lkml
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!
quoted hunk ↗ jump to hunk
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