Hi Guennadi, On Mon, Nov 07, 2011 at 05:17:23PM +0100, Guennadi Liakhovetski wrote: > Hi all > > At the V4L/DVB workshop in Prague a couple of weeks ago possible merits of > supporting multiple camera sensor contexts have been discussed. Such > contexts are often promoted by camera manufacturers as a hardware > optimization to support fast switching to the snapshot mode. Such a switch > is often accompanied by a change of the frame format. Typically, a smaller > frame is used for the preview mode and a larger frame is used for photo > shooting. Those sensors provide 2 (or more) sets of frame size and data > format registers and a single command to switch between them. The > decision, whether or not to support these multiple camera contexts has > been postponed until some measurements become available, how much time > such a "fast switching" implementation would save us. > > I took the mt9m111 driver, that supports mt9m111, mt9m131, and mt9m112 > camera sensors from Aptina. They do indeed implement two contexts, > however, the driver first had to be somewhat reorganised to make use of > them. I pushed my (highly!) experimental tree to > > git://linuxtv.org/gliakhovetski/v4l-dvb.git staging-3.3 > > with the addition of the below debugging diff, that pre-programs a fixed > format into the second context registers and switches to it, once a > matching S_FMT is called. On the i.MX31 based pcm037 board, that I've got, > this sensor is attached to the I2C bus #2, running at 20kHz. The explicit > programming of the new format parameters measures to take around 27ms, > which is also about what we win, when using the second context.
27 ms isn't a lot. May I ask what's the reason for such an unusual I2C speed? Even the relatively low speed 400 kHz spec has been available for almost 20 years by now. > As for interpretation: firstly 20kHz is not much, I expect many other set > ups to run much faster. But even if we accept, that on some hardware > > 20kHz doesn't work and we really lose 27ms when not using multiple > register contexts, is it a lot? Thinking about my personal photographing > experiences with cameras and camera-phones, I don't think, I'd notice a > 27ms latency;-) I don't think anything below 200ms really makes a > difference and, I think, the major contributor to the snapshot latency is > the need to synchronise on a frame, and, possibly skip or shoot several > frames, instead of just one. > > So, my conclusion would be: when working with "sane" camera sensors, i.e., > those, where you don't have to reprogram 100s of registers from some magic > tables to configure a different frame format (;-)), supporting several > register contexts doesn't bring a huge advantage in terms of snapshot > latency. OTOH, it can well happen, that at some point we anyway will have > to support those multiple register contexts for some other reason. Sensor have seldom anything which would require passing a huge amount of data to configure the sensor. I personally don't see need for supporting different contects in the foreseeable future --- but of course I could be wrong. Also knowing much of a future desired configuration is a difficult guess. The hardware people will hopefully rather provide a faster bus to transfer the settings to the sensor to make the configuration delays smaller. Kind regards, -- Sakari Ailus e-mail: sakari.ai...@iki.fi jabber/XMPP/Gmail: sai...@retiisi.org.uk -- 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
