linux-kernel - Re: [PATCH v2 02/18] media: i2c: rdacm20: Enable noise immunity

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Date:   Tue, 16 Mar 2021 21:24:03 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Jacopo Mondi <jacopo@...ndi.org>
Cc:     Jacopo Mondi <jacopo+renesas@...ndi.org>,
        kieran.bingham+renesas@...asonboard.com,
        niklas.soderlund+renesas@...natech.se, geert@...ux-m68k.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/18] media: i2c: rdacm20: Enable noise immunity

Hi Jacopo,

On Tue, Mar 16, 2021 at 01:56:07PM +0100, Jacopo Mondi wrote:
> On Mon, Mar 15, 2021 at 11:37:26PM +0200, Laurent Pinchart wrote:
> > On Mon, Mar 15, 2021 at 02:14:56PM +0100, Jacopo Mondi wrote:
> > > Enable the noise immunity threshold at the end of the rdacm20
> > > initialization routine.
> > >
> > > The rdacm20 camera module has been so far tested with a startup
> > > delay that allowed the embedded MCU to program the serializer. If
> > > the initialization routine is run before the MCU programs the
> > > serializer and the image sensor and their addresses gets changed
> > > by the rdacm20 driver it is required to manually enable the noise
> > > immunity threshold to make the communication on the control channel
> > > more reliable.
> >
> > I'm still worried by the race with the MCU. Any update on dumping the
> > MCU configuration to check what it initializes ?
> 
> Not yet, you're right ...
> 
> I mainly focused on testing with rdacm21, what if I strip the rdacm20
> changes out from this series ? I will have to keep the init()
> operation introduction to maintain compatibility with max9286 changes,
> and in case of no regressions, we can keep the 8 seconds delay in the
> .dtsi. However it will break upstream support on Eagle for rdacm20 as
> we don't have a regulator where to insert the startup delay there, and
> a downstream patch that waits for 8 seconds in the deserializer driver
> should be used instead...

I don't think the rdacm20 changes need to wait. Even this one could be
merged as-is, as long as we consider it to be a temporary workaround and
don't build anything on top that would make it more difficult to address
the issue properly (a TODO comment in the code could help).

> > > Reviewed-by: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
> > > Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
> > > ---
> > >  drivers/media/i2c/rdacm20.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/media/i2c/rdacm20.c b/drivers/media/i2c/rdacm20.c
> > > index 90eb73f0e6e9..f7fd5ae955d0 100644
> > > --- a/drivers/media/i2c/rdacm20.c
> > > +++ b/drivers/media/i2c/rdacm20.c
> > > @@ -541,7 +541,13 @@ static int rdacm20_initialize(struct rdacm20_device *dev)
> > >
> > >  	dev_info(dev->dev, "Identified MAX9271 + OV10635 device\n");
> > >
> > > -	return 0;
> > > +	/*
> > > +	 * Set reverse channel high threshold to increase noise immunity.
> > > +	 *
> > > +	 * This should be compensated by increasing the reverse channel
> > > +	 * amplitude on the remote deserializer side.
> > > +	 */
> > > +	return max9271_set_high_threshold(&dev->serializer, true);
> > >  }
> > >
> > >  static int rdacm20_probe(struct i2c_client *client)

-- 
Regards,

Laurent Pinchart