RE: [RFC/PATCH 0/1] New subdev sensor operation g_interface_parms

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Guennadi and Hans,

<snip>

> > The only static data I am concerned about are those that affect signal
> integrity.
> > After thinking carefully about this I realized that there is really only
> one
> > setting that is relevant to that: the sampling edge. The polarities do
> not
> > matter in this.

I respectfully disagree.

AFAIK, There is not such thing as sampling edge configuration for MIPI
Receivers, and the polarities DO matter, since it's a differential
signal.

> 
> Ok, this is much better! I'm still not perfectly happy having to punish
> all just for the sake of a couple of broken boards, but I can certainly
> much better live with this, than with having to hard-code each and every
> bit. Thanks, Hans!
> 
> So, I think, we can proceed with this, let's see the code now, shall we?;)
> 
> Currently soc-camera auto-configures the following parameters:
> 
> hsync polarity
> vsync polarity
> data polarity
> master / slave mode
> data bus width
> pixel clock polarity
> 
> (see include/media/soc_camera.h::soc_camera_bus_param_compatible() and
> drivers/media/video/soc_camera.c::soc_camera_apply_sensor_flags()).
> Removing the pixclk polarity, the rest we can use as a basis for a new
> subdev-based implementation.

Don't we need to move this out from soc_camera and make it available in
v4l2_subdev ops? (I'm talking about both parallel and the "new" MIPI
support)

That way both SoC_Camera, and Media Controller frameworks can use that.

Regards,
Sergio

> 
> Thanks
> Guennadi
> 
> > Unfortunately, if a subdev is set to 'sample at rising edge', then that
> does
> > not necessarily mean that the host should sample at the same edge.
> Depending
> > on the clock line routing and the integrity of the clock signal the host
> may
> > actually have to sample on the other edge. And yes, I've seen this.
> >
> > Anyway, this has been discussed to death already. I am very much opposed
> to
> > negotiating the sampling edge. During the Helsinki meeting in June last
> year
> > we decided to do this via platform data (see section 7 in the meeting
> > minutes: http://www.linuxtv.org/news.php?entry=2010-06-22.mchehab).
> >
> > I will formally NACK attempts to negotiate this. Mauro is of course free
> to
> > override me.
> >
> > Something simple like this for subdev platform_data might be enough:
> >
> > struct v4l2_bus_config {
> >         /* 0 - sample at falling edge, 1 - sample at rising edge */
> >         unsigned edge_pclock:1;
> >         /* 0 - host should use the same sampling edge, 1 - host should
> use the
> >            other sampling edge */
> >         unsigned host_invert_edge_pclock:1;
> > };
> >
> > The host can query the bus configuration and the subdev will return:
> >
> > 	edge = host_invert_edge_pclock ? !edge_pclock : edge_pclock;
> >
> > We might want to add bits as well to describe whether polarities are
> inverted.
> >
> > This old RFC gives a good overview of the possible polarities:
> >
> > http://www.mail-archive.com/linux-media@xxxxxxxxxxxxxxx/msg09041.html
> >
> > Regards,
> >
> > 	Hans
> >
> > > I personally like the Stanimir's proposal as the parameters to be
> negotiated
> > > are pretty dynamic. Only the number of lanes could be problematic as
> not all
> > > lanes might be routed across different boards. Perhaps we should
> consider specifying
> > > an AUTO value for some negotiated parameters. Such as in case of an
> attribute that
> > > need to be fixed on some boards or can be fully negotiated on others,
> a fixed
> > > value or "auto" could be respectively set up in the host's
> platform_data. This could
> > > be used to override some parameters in the host driver if needed.
> > >
> > > IMHO, as long as we negotiate only dynamic parameters there should be
> no special
> > > issues.
> > >
> > > Regards,
> > > Sylwester
> > >
> > > > about this if it wasn't for the fact that soc-camera doesn't do this
> but instead
> > > > negotiates it. Obviously, it isn't a pleasant prospect having to
> change all that.
> > > >
> > > > Normally this would be enough of an argument for me to just
> negotiate it. The
> > > > reason that I don't want this in this particular case is that I know
> from
> > > > personal experience that incorrect settings can be extremely hard to
> find.
> > > >
> > > > I also think that there is a reasonable chance that such bugs can
> happen. Take
> > > > a scenario like this: someone writes a new host driver. Initially
> there is only
> > > > support for positive polarity and detection on the rising edge,
> because that's
> > > > what the current board on which the driver was developed supports.
> This is quite
> > > > typical for an initial version of a driver.
> > > >
> > > > Later someone adds support for negative polarity and falling edge.
> Suddenly the
> > > > polarity negotiation on the previous board results in negative
> instead of positive
> > > > which was never tested. Now that board starts producing pixel errors
> every so
> > > > often. And yes, this type of hardware problems do happen as I know
> from painful
> > > > experience.
> > > >
> > > > Problems like this are next to impossible to debug without the aid
> of an
> > > > oscilloscope, so this isn't like most other bugs that are relatively
> easy to
> > > > debug.
> > > >
> > > > It is so much easier just to avoid this by putting it in platform
> data. It's
> > > > simple, unambiguous and above all, unchanging.
> > > >
> > > > Regards,
> > > >
> > > > 	Hans
> > > >
> > > >>
> > > >> Thanks
> > > >> Guennadi
> > > >> ---
> > > >> Guennadi Liakhovetski, Ph.D.
> > > >> Freelance Open-Source Software Developer
> > > >> http://www.open-technology.de/
> > > >> --
> > > >> To unsubscribe from this list: send the line "unsubscribe linux-
> media" in
> > > >> the body of a message to majordomo@xxxxxxxxxxxxxxx
> > > >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > > >>
> > > >
> > >
> > >
> >
> > --
> > Hans Verkuil - video4linux developer - sponsored by Cisco
> >
> 
> ---
> Guennadi Liakhovetski, Ph.D.
> Freelance Open-Source Software Developer
> http://www.open-technology.de/
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