Date:   Mon, 11 Jan 2021 12:58:59 +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,
        laurent.pinchart+renesas@...asonboard.com,
        niklas.soderlund+renesas@...natech.se, geert@...ux-m68k.org,
        linux-media@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Hyun Kwon <hyunk@...inx.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        sergei.shtylyov@...il.com
Subject: Re: [PATCH v6 5/5] media: i2c: max9286: Configure reverse channel
 amplitude

Hi Jacopo,

On Mon, Jan 11, 2021 at 11:43:11AM +0100, Jacopo Mondi wrote:
> On Wed, Dec 16, 2020 at 07:22:17PM +0200, Laurent Pinchart wrote:
> > On Tue, Dec 15, 2020 at 06:09:57PM +0100, Jacopo Mondi wrote:
> > > Adjust the initial reverse channel amplitude parsing from
> > > firmware interface the 'maxim,reverse-channel-microvolt'
> > > property.
> > >
> > > This change is required for both rdacm20 and rdacm21 camera
> > > modules to be correctly probed when used in combination with
> > > the max9286 deserializer.
> > >
> > > Reviewed-by: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
> > > Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
> > > ---
> > >  drivers/media/i2c/max9286.c | 23 ++++++++++++++++++++++-
> > >  1 file changed, 22 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/media/i2c/max9286.c b/drivers/media/i2c/max9286.c
> > > index 021309c6dd6f..9b40a4890c4d 100644
> > > --- a/drivers/media/i2c/max9286.c
> > > +++ b/drivers/media/i2c/max9286.c
> > > @@ -163,6 +163,8 @@ struct max9286_priv {
> > >  	unsigned int mux_channel;
> > >  	bool mux_open;
> > >
> > > +	u32 reverse_channel_mv;
> > > +
> > >  	struct v4l2_ctrl_handler ctrls;
> > >  	struct v4l2_ctrl *pixelrate;
> > >
> > > @@ -557,10 +559,14 @@ static int max9286_notify_bound(struct v4l2_async_notifier *notifier,
> > >  	 * All enabled sources have probed and enabled their reverse control
> > >  	 * channels:
> > >  	 *
> > > +	 * - Increase the reverse channel amplitude to compensate for the
> > > +	 *   remote ends high threshold, if not done already
> > >  	 * - Verify all configuration links are properly detected
> > >  	 * - Disable auto-ack as communication on the control channel are now
> > >  	 *   stable.
> > >  	 */
> > > +	if (priv->reverse_channel_mv < 170)
> > > +		max9286_reverse_channel_setup(priv, 170);
> >
> > I'm beginning to wonder if there will be a need in the future to not
> > increase the reverse channel amplitude (keeping the threshold low on the
> > remote side). An increased amplitude increases power consumption, and if
> > the environment isn't noisy, a low amplitude would work. The device tree
> > would then need to specify both the initial amplitude required by the
> > remote side, and the desired amplitude after initialization. What do you
> > think ? Is it overkill ? We don't have to implement this now, so
> >
> > Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> >
> > but if this feature could be required later, we may want to take into
> > account in the naming of the new DT property to reflect the fact that it
> > is the initial value.
> 
> I had the same thought when I initially proposed
> "maxim,initial-reverse-channel-mV"
> 
> Having to use the standard unit suffix that would have become
> "maxim,initial-reverse-channel-microvolt"
> which is extremely long.
> 
> I can't tell if there will be any need to adjust the amplitude later.
> In any case, I would not rely on a DTS property to do so, as once we
> have probed the remote we have a subdev where to call
> 'get_mbus_config()' on, and from there we can report the high threshold
> status of the serializer and adjust the deser amplitude accordingly.

I don't think that's the point. The threshold of the serializer is
something we can configure at runtime. What voltage level to use after
initialization time is a system property as it depends on noise
immunity, so we'll have to specify it in DT.

> The property documentation clearly says the there specified amplitude
> is 'initial' many times, so I don't think it is strictly necessary to
> report it in the name too.
> 
> Would this work for you ?

I don't mind either way.

> > >  	max9286_check_config_link(priv, priv->source_mask);
> > >
> > >  	/*
> > > @@ -967,7 +973,7 @@ static int max9286_setup(struct max9286_priv *priv)
> > >  	 * only. This should be disabled after the mux is initialised.
> > >  	 */
> > >  	max9286_configure_i2c(priv, true);
> > > -	max9286_reverse_channel_setup(priv, 170);
> > > +	max9286_reverse_channel_setup(priv, priv->reverse_channel_mv);
> > >
> > >  	/*
> > >  	 * Enable GMSL links, mask unused ones and autodetect link
> > > @@ -1131,6 +1137,7 @@ static int max9286_parse_dt(struct max9286_priv *priv)
> > >  	struct device_node *i2c_mux;
> > >  	struct device_node *node = NULL;
> > >  	unsigned int i2c_mux_mask = 0;
> > > +	u32 reverse_channel_microvolt;
> > >
> > >  	/* Balance the of_node_put() performed by of_find_node_by_name(). */
> > >  	of_node_get(dev->of_node);
> > > @@ -1221,6 +1228,20 @@ static int max9286_parse_dt(struct max9286_priv *priv)
> > >  	}
> > >  	of_node_put(node);
> > >
> > > +	/*
> > > +	 * Parse the initial value of the reverse channel amplitude from
> > > +	 * the firmware interface and convert it to millivolts.
> > > +	 *
> > > +	 * Default it to 170mV for backward compatibility with DTBs that do not
> > > +	 * provide the property.
> > > +	 */
> > > +	if (of_property_read_u32(dev->of_node,
> > > +				 "maxim,reverse-channel-microvolt",
> > > +				 &reverse_channel_microvolt))
> > > +		priv->reverse_channel_mv = 170;
> > > +	else
> > > +		priv->reverse_channel_mv = reverse_channel_microvolt / 1000U;
> > > +
> > >  	priv->route_mask = priv->source_mask;
> > >
> > >  	return 0;

-- 
Regards,

Laurent Pinchart

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