Date:   Tue, 14 Mar 2017 19:26:46 +0100
From:   Pavel Machek <pavel@....cz>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Steve Longerbeam <slongerbeam@...il.com>, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, kernel@...gutronix.de,
        fabio.estevam@....com, mchehab@...nel.org, nick@...anahar.org,
        markus.heiser@...marIT.de, p.zabel@...gutronix.de,
        laurent.pinchart+renesas@...asonboard.com, bparrot@...com,
        geert@...ux-m68k.org, arnd@...db.de, sudipm.mukherjee@...il.com,
        minghsiu.tsai@...iatek.com, tiffany.lin@...iatek.com,
        jean-christophe.trotin@...com, horms+renesas@...ge.net.au,
        niklas.soderlund+renesas@...natech.se, robert.jarzmik@...e.fr,
        songjun.wu@...rochip.com, andrew-ct.chen@...iatek.com,
        gregkh@...uxfoundation.org, shuah@...nel.org,
        sakari.ailus@...ux.intel.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-media@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCH v5 15/39] [media] v4l2: add a frame interval error event

On Mon 2017-03-13 10:45:38, Russell King - ARM Linux wrote:
> On Mon, Mar 13, 2017 at 11:02:34AM +0100, Hans Verkuil wrote:
> > On 03/11/2017 07:14 PM, Steve Longerbeam wrote:
> > > The event must be user visible, otherwise the user has no indication
> > > the error, and can't correct it by stream restart.
> > 
> > In that case the driver can detect this and call vb2_queue_error. It's
> > what it is there for.
> > 
> > The event doesn't help you since only this driver has this issue. So nobody
> > will watch this event, unless it is sw specifically written for this SoC.
> > 
> > Much better to call vb2_queue_error to signal a fatal error (which this
> > apparently is) since there are more drivers that do this, and vivid supports
> > triggering this condition as well.
> 
> So today, I can fiddle around with the IMX219 registers to help gain
> an understanding of how this sensor works.  Several of the registers
> (such as the PLL setup [*]) require me to disable streaming on the
> sensor while changing them.
> 
> This is something I've done many times while testing various ideas,
> and is my primary way of figuring out and testing such things.
> 
> Whenever I resume streaming (provided I've let the sensor stop
> streaming at a frame boundary) it resumes as if nothing happened.  If I
> stop the sensor mid-frame, then I get the rolling issue that Steve
> reports, but once the top of the frame becomes aligned with the top of
> the capture, everything then becomes stable again as if nothing happened.
> 
> The side effect of what you're proposing is that when I disable streaming
> at the sensor by poking at its registers, rather than the capture just
> stopping, an error is going to be delivered to gstreamer, and gstreamer
> is going to exit, taking the entire capture process down.
> 
> This severely restricts the ability to be able to develop and test
> sensor drivers.

Well, but kernel should do what is best for production, not what is
best for driver debugging.

And yes, I guess you can have #ifdef or module parameter or something
switching for behaviour you prefer when you are debugging. But for
production, vb2_queue_error() seems to be the right solution.

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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