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Date:   Tue, 12 Jan 2021 10:08:05 +0100
From:   Jacopo Mondi <jacopo@jmondi.org>
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     Jacopo Mondi <jacopo+renesas@jmondi.org>,
        kieran.bingham+renesas@ideasonboard.com,
        laurent.pinchart+renesas@ideasonboard.com,
        niklas.soderlund+renesas@ragnatech.se, geert@linux-m68k.org,
        linux-media@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
        linux-kernel@vger.kernel.org, Hyun Kwon <hyunk@xilinx.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
        sergei.shtylyov@gmail.com
Subject: Re: [PATCH v6 5/5] media: i2c: max9286: Configure reverse channel
 amplitude
Message-ID: <20210112090805.myglp2lpozo3blq5@uno.localdomain>
References: <20201215170957.92761-1-jacopo+renesas@jmondi.org>
 <20201215170957.92761-6-jacopo+renesas@jmondi.org>
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 <20210111104311.e6nyxhzhvlyjjxxw@uno.localdomain>
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 <20210111112023.brrhxgfedo5fer53@uno.localdomain>
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Hi Laurent,

On Tue, Jan 12, 2021 at 07:03:42AM +0200, Laurent Pinchart wrote:
> Hi Jacopo,
>
> On Mon, Jan 11, 2021 at 12:20:23PM +0100, Jacopo Mondi wrote:
> > On Mon, Jan 11, 2021 at 12:58:59PM +0200, Laurent Pinchart wrote:
> > > 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@ideasonboard.com>
> > >>>> Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.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@ideasonboard.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
> >
> > How so ? I mean, we can add an API for this, but currently it's
> > configured at probe time and that's it. Its configuration might as
> > well come from a DT property like we do on the deserializer here but I
> > fail to see why it's different. Both settings depends on the required
> > noise immunity of th system.
>
> The voltage level configuration need to match between the tserializer
> (transmitter) and the deserializer (receiver). The serializer is
> configured with a voltage level, and the deserializer needs to be
> configured with a corresponding threshold.
>

If I'm not mistaken it's actually the other way around, at least for
the chips we're dealing with.

The serializer (MAX9271) has an "Reverse Channel Receiver High
Threshold Enable" bit (register 0x08[0]) undocumented in the chip
manual but described in the "MAX9286 Programming Guide 2 10.pdf"
document in the "Important Registers" section.

The deserializer (MAX9286) has instead a configurable setting for the reverse
channel signal amplitude, which is what we are controlling in this
series.

The deserializer reverse channel amplitude has to match the remote
side 'high threshold enable' setting. If it is enabled the amplitude
has to be increased to be able to probe the remote side. If it's not
a lower amplitude has to be used to make comunication reliable.

As you said, some models (RDACM20) might be pre-programmed with the
'high threshold enable' bit set, and so the deserializer reverse
channel amplitude has to be adjusted accordingly to be able to
comunicate on the reverse channel.

> The voltage level of the serializer is configurable on the camera side
> when the system is powered up. The RDACM20 has a microcontroller which
> can configure the serializer, and other cameras may have similar
> mechanisms. As the deserializer can't query the information from the
> serializer (communication is unreliable if the threshold has an
> incorrect value), we need a DT property to tell the deserializer what
> threshold is initially used by the camera when it gets powered up.
>

That's what this series does, yes.

> This only covers initialization. A camera could boot up with a low
> voltage level, but we may want to increase the voltage level (and thus
> the threshold on the deserializer side) to increase noise immunity. Or,
> if the system environment isn't noisy, we may want to keep a low voltage
> level, or even decrease it if the camera boots up with a high voltage
> level. This runtime voltage level depends on the system design and its
> susceptibility to noise, and is thus a system property. Should we want
> to make it configurable, it should be specified in DT, and it's separate
> from the initial voltage level that is used to establish communication.
>

And that's what I meant. Assuming we handle initialization correctly
with this series, the serializers 'high threshold' configuration
-after- initialization can be specified with a DT property on the
-serializer- side. Then, to adjust the deserializer reverse channel
amplitude, once we the remote has probed and we have a subdevice
registered for it, we can query the 'high threshold' configuration
using get_mbus_config() (or another API if we think it's better) and
adjust the deserializer accordingly.

All in all:
- yes, I think there might be a need to control the noise immunity
  settings after initialization
- I think it should be done on the serializer side, possibly with a DT
  property, possibly something like a boolean 'maxim,high-threshold-enable'
- the deserializer can query that information with a kAPI like
  get_mbus_config() after the remote has probed
- Because of that there is no need for an additional deserializer property

Hope this makes sense