linux-kernel - Re: [PATCH 16/16] media: i2c: gmsl: Use 339Kbps I2C bit-rate

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Date:   Mon, 22 Feb 2021 04:06:48 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Jacopo Mondi <jacopo+renesas@...ndi.org>
Cc:     Kieran Bingham <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 16/16] media: i2c: gmsl: Use 339Kbps I2C bit-rate

Hi Jacopo,

Thank you for the patch.

On Thu, Feb 18, 2021 at 04:07:30PM +0000, Kieran Bingham wrote:
> On 16/02/2021 17:41, Jacopo Mondi wrote:
> > With the camera modules initialization routines now running with
> > the noise immunity threshold enabled, it is possible to restore
> > the bit rate of the I2C transactions transported on the GMSL control
> > channel to 339 Kbps.
> > 
> > The 339 Kbps bit rate represents the default setting for the serializer
> > and the deserializer chips, and the setup/hold time and slave timeout
> > time in use are calibrate to support that rate.
> 
> s/calibrate/calibrated/
> 
> Does that mean the setup/hold time and timeouts should be adjusted based
> on the i2c speed? (which we have not been doing?)
> 
> With all of your other reliability improvements, does *this* change
> alone have any difference or impact on reliability?
> 
> I.e. if we go slower (stay at current speed) - would we be more reliable?
> 
> Is there a reliability improvement by making this speed faster?
> 
> I presume we don't have a way to convey the i2c bus speed between the
> max9286 and the max9271 currently? Seems a bit like a bus parameter....

There's a max bus speed DT parameter that we can use. And I think we
should, as, in theory at least, there's no guarantee that all the
devices behind the serializer can operate at 400kbps.

> > Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
> > ---
> >  drivers/media/i2c/max9286.c | 2 +-
> >  drivers/media/i2c/rdacm20.c | 2 +-
> >  drivers/media/i2c/rdacm21.c | 2 +-
> >  3 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/media/i2c/max9286.c b/drivers/media/i2c/max9286.c
> > index aa01d5bb79ef..0b620f2f8c41 100644
> > --- a/drivers/media/i2c/max9286.c
> > +++ b/drivers/media/i2c/max9286.c
> > @@ -330,7 +330,7 @@ static int max9286_i2c_mux_init(struct max9286_priv *priv)
> >  static void max9286_configure_i2c(struct max9286_priv *priv, bool localack)
> >  {
> >  	u8 config = MAX9286_I2CSLVSH_469NS_234NS | MAX9286_I2CSLVTO_1024US |
> > -		    MAX9286_I2CMSTBT_105KBPS;
> > +		    MAX9286_I2CMSTBT_339KBPS;
> >  
> >  	if (localack)
> >  		config |= MAX9286_I2CLOCACK;
> > diff --git a/drivers/media/i2c/rdacm20.c b/drivers/media/i2c/rdacm20.c
> > index 0632ef98eea7..d45e8b0e52a0 100644
> > --- a/drivers/media/i2c/rdacm20.c
> > +++ b/drivers/media/i2c/rdacm20.c
> > @@ -450,7 +450,7 @@ static int rdacm20_init(struct v4l2_subdev *sd, unsigned int val)
> >  	ret = max9271_configure_i2c(&dev->serializer,
> >  				    MAX9271_I2CSLVSH_469NS_234NS |
> >  				    MAX9271_I2CSLVTO_1024US |
> > -				    MAX9271_I2CMSTBT_105KBPS);
> > +				    MAX9271_I2CMSTBT_339KBPS);
> >  	if (ret)
> >  		return ret;
> >  
> > diff --git a/drivers/media/i2c/rdacm21.c b/drivers/media/i2c/rdacm21.c
> > index 80b6f16f87a8..552985026458 100644
> > --- a/drivers/media/i2c/rdacm21.c
> > +++ b/drivers/media/i2c/rdacm21.c
> > @@ -442,7 +442,7 @@ static int rdacm21_init(struct v4l2_subdev *sd, unsigned int val)
> >  	ret = max9271_configure_i2c(&dev->serializer,
> >  				    MAX9271_I2CSLVSH_469NS_234NS |
> >  				    MAX9271_I2CSLVTO_1024US |
> > -				    MAX9271_I2CMSTBT_105KBPS);
> > +				    MAX9271_I2CMSTBT_339KBPS);
> >  	if (ret)
> >  		return ret;
> >  

-- 
Regards,

Laurent Pinchart