Hi, On 09/15/2012 11:31 AM, Laurent Pinchart wrote: > Hi Hans, > > On Saturday 15 September 2012 09:41:59 Hans Verkuil wrote: >> On Fri September 14 2012 23:05:45 Sakari Ailus wrote: >>> Rémi Denis-Courmont wrote: >>>> Le vendredi 14 septembre 2012 23:25:01, Sakari Ailus a écrit : >>>>> I had a quick discussion with Laurent, and what he suggested was to use >>>>> the kernel version to figure out the type of the timestamp. The drivers >>>>> that use the monotonic time right now wouldn't be affected by the new >>>>> flag on older kernels. If we've decided we're going to switch to >>>>> monotonic time anyway, why not just change all the drivers now and >>>>> forget the capability flag. >>>> >>>> That does not work In Real Life. >>>> >>>> People do port old drivers forward to new kernels. >>>> People do port new drivers back to old kernels >>> >>> Why would you port a driver from an old kernel to a new kernel? Or are >>> you talking about out-of-tree drivers? >> >> More likely the latter. >> >>> If you do port drivers across different kernel versions I guess you're >>> supposed to use the appropriate interfaces for those kernels, too. Using >>> a helper function helps here, so compiling a backported driver would >>> fail w/o the helper function that produces the timestamp, forcing the >>> backporter to notice the situation. >>> >>> Anyway, I don't have a very strict opinion on this, so I'm okay with the >>> flag, too, but I personally simply don't think it's the best choice we >>> can make now. Also, without converting the drivers now the user space >>> must live with different kinds of timestamps much longer. >> >> There are a number of reasons why I want to go with a flag: >> >> - Out-of-tree drivers which are unlikely to switch to monotonic in practice >> - Backporting drivers >> - It makes it easy to verify in v4l2-compliance and enforce the use of >> the monotonic clock. >> - It's easy for apps to check. >> >> The third reason is probably the most important one for me. There tends to >> be a great deal of inertia before changes like this are applied to new >> drivers, and without being able to (easily) check this in v4l2-compliance >> more drivers will be merged that keep using gettimeofday. It's all too easy >> to miss in a review. > > If we switch all existing drivers to monotonic timestamps in kernel release > 3.x, v4l2-compliance can just use the version it gets from VIDIOC_QUERYCAP and > enforce monotonic timestamps verification if the version is>= 3.x. This isn't > more difficult for apps to check than a dedicated flag (although it's less > explicit). > > My concern is identical to Sakari's, I'm not very keen on introducing a flag > that all drivers will set in the very near future and that we will have to > keep around forever. > >> That doesn't mean that it isn't a good idea to convert existing drivers >> asap. But it's not something I'm likely to take up myself. > > Sakari, are you volunteering for that ? ;-) > >> Creating a small helper function as you suggested elsewhere is a good idea >> as well. I'll write something for that.
IMHO, using a flag is going to more reliable, i.e. it is going to be reliable. It shouldn't be a big deal to set the flag, unless we're running out of free bits in the caps field. Once all drivers are converted it could be set in v4l2-core. And applications would always know what they get. -- Regards, Sylwester -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
