From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from perceval.ideasonboard.com ([95.142.166.194]:47644 "EHLO perceval.ideasonboard.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756729Ab2I0Kka (ORCPT ); Thu, 27 Sep 2012 06:40:30 -0400 From: Laurent Pinchart To: Sylwester Nawrocki Cc: linux-media@vger.kernel.org, Sakari Ailus , Hans Verkuil , remi@remlab.net, Kamil Debski Subject: Re: [RFC] Timestamps and V4L2 Date: Thu, 27 Sep 2012 12:41:08 +0200 Message-ID: <21756413.MuViFDO8NB@avalon> In-Reply-To: <50638219.7020105@gmail.com> References: <20120920202122.GA12025@valkosipuli.retiisi.org.uk> <32114057.tIVjSTYujk@avalon> <50638219.7020105@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-media-owner@vger.kernel.org List-ID: Hi Sylwester, On Thursday 27 September 2012 00:30:49 Sylwester Nawrocki wrote: > On 09/25/2012 02:35 AM, Laurent Pinchart wrote: > > Does the clock type need to be selectable for mem-to-mem devices ? Do > > device- specific timestamps make sense there ? > > I'd like to clarify one thing here, i.e. if we select device-specific > timestamps how should the v4l2_buffer::timestamp field behave ? > > Are these two things exclusive ? Or should v4l2_buffer::timestamp be > valid even if device-specific timestamps are enabled ? That's a very good question. The use cases I have in mind don't need both at the same time. The point of device-specific timestamps is to get a precise timestamp corresponding to the frame capture time, instead of the frame transfer time. They need to be correlated with system timestamps, but for that we need device-specific APIs to pass correlation information to userspace. Passing a "transfer time" system timestamp along with the device timestamp would be useless, as there would be no good correlation between the two. > With regards to your question, I think device-specific timestamps make > sense for mem-to-mem devices. Maybe not for the very simple ones, that > process buffers 1-to-1, but codecs may need it. I was told the Exynos/ > S5P Multi Format Codec device has some register the timestamps could > be read from, but it's currently not used by the s5p-mfc driver. Kamil > might provide more details on that. What kind of timestamps are they ? > I guess if capture and output devices can have their timestamping clocks > selectable it should be also possible for mem-to-mem devices. -- Regards, Laurent Pinchart