Re: [PATCH v4 3/4] media: i2c: Add MAX9286 driver

[jacopo mondi]

Hi Luca,

On Tue, Nov 20, 2018 at 11:51:37AM +0100, Luca Ceresoli wrote:
> Hi Kieran,
>
> On 20/11/18 01:32, Kieran Bingham wrote:
> > Hi Luca,
> >
> > My apologies for my travel induced delay in responding here,
>
> No problem.
>
> > On 14/11/2018 02:04, Luca Ceresoli wrote:
> [...]
> >>>>> +static int max9286_probe(struct i2c_client *client,
> >>>>> +			 const struct i2c_device_id *did)
> >>>>> +{
> >>>>> +	struct max9286_device *dev;
> >>>>> +	unsigned int i;
> >>>>> +	int ret;
> >>>>> +
> >>>>> +	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
> >>>>> +	if (!dev)
> >>>>> +		return -ENOMEM;
> >>>>> +
> >>>>> +	dev->client = client;
> >>>>> +	i2c_set_clientdata(client, dev);
> >>>>> +
> >>>>> +	for (i = 0; i < MAX9286_N_SINKS; i++)
> >>>>> +		max9286_init_format(&dev->fmt[i]);
> >>>>> +
> >>>>> +	ret = max9286_parse_dt(dev);
> >>>>> +	if (ret)
> >>>>> +		return ret;
> >>>>> +
> >>>>> +	dev->regulator = regulator_get(&client->dev, "poc");
> >>>>> +	if (IS_ERR(dev->regulator)) {
> >>>>> +		if (PTR_ERR(dev->regulator) != -EPROBE_DEFER)
> >>>>> +			dev_err(&client->dev,
> >>>>> +				"Unable to get PoC regulator (%ld)\n",
> >>>>> +				PTR_ERR(dev->regulator));
> >>>>> +		ret = PTR_ERR(dev->regulator);
> >>>>> +		goto err_free;
> >>>>> +	}
> >>>>> +
> >>>>> +	/*
> >>>>> +	 * We can have multiple MAX9286 instances on the same physical I2C
> >>>>> +	 * bus, and I2C children behind ports of separate MAX9286 instances
> >>>>> +	 * having the same I2C address. As the MAX9286 starts by default with
> >>>>> +	 * all ports enabled, we need to disable all ports on all MAX9286
> >>>>> +	 * instances before proceeding to further initialize the devices and
> >>>>> +	 * instantiate children.
> >>>>> +	 *
> >>>>> +	 * Start by just disabling all channels on the current device. Then,
> >>>>> +	 * if all other MAX9286 on the parent bus have been probed, proceed
> >>>>> +	 * to initialize them all, including the current one.
> >>>>> +	 */
> >>>>> +	max9286_i2c_mux_close(dev);
> >>>>> +
> >>>>> +	/*
> >>>>> +	 * The MAX9286 initialises with auto-acknowledge enabled by default.
> >>>>> +	 * This means that if multiple MAX9286 devices are connected to an I2C
> >>>>> +	 * bus, another MAX9286 could ack I2C transfers meant for a device on
> >>>>> +	 * the other side of the GMSL links for this MAX9286 (such as a
> >>>>> +	 * MAX9271). To prevent that disable auto-acknowledge early on; it
> >>>>> +	 * will be enabled later as needed.
> >>>>> +	 */
> >>>>> +	max9286_configure_i2c(dev, false);
> >>>>> +
> >>>>> +	ret = device_for_each_child(client->dev.parent, &client->dev,
> >>>>> +				    max9286_is_bound);
> >>>>> +	if (ret)
> >>>>> +		return 0;
> >>>>> +
> >>>>> +	dev_dbg(&client->dev,
> >>>>> +		"All max9286 probed: start initialization sequence\n");
> >>>>> +	ret = device_for_each_child(client->dev.parent, NULL,
> >>>>> +				    max9286_init);
> >>>>
> >>>> I can't manage to like this initialization sequence, sorry. If at all
> >>>> possible, each max9286 should initialize itself independently from each
> >>>> other, like any normal driver.
> >>>
> >>> Yes, I think we're in agreement here, but unfortunately this section is
> >>> a workaround for the fact that our devices share a common address space.
> >>>
> >>> We (currently) *must* disable both devices before we start the
> >>> initialisation process for either on our platform currently...
> >>
> >> The model I proposed in my review to patch 1/4 (split remote physical
> >> address from local address pool) allows to avoid this workaround.
> >
> >
> > Having just talked this through with Jacopo I think I see that we have
> > two topics getting intertwined here too.
> >
> >  - Address translation so that we can separate the camera addressing
> >
> >  - our 'device_is_bound/device_for_each_child()' workaround which lets
> >    us make sure the buses are closed before we power on any camera
> >    device.
> >
> >
> > For the upstream process of this driver - I will remove the
> > 'device_is_bound()/device_for_each_child()' workarounds.
> >
> >
> > It is /ugly/ and needs more consideration, but I believe we do still
> > need it (or something similar) for our platform currently.
> >
> >
> >
> > The other side of that is the address translation. I think I was wrong
> > earlier and may have said we have address translation on both sides.
> >
> >
> > I think I now see that we only have some minimal translation for two
> > addresses on the remote (max9271) side, not the local (max9286) side.
>
> Can the remote (max9271) translate addresses for transactions
> originating from the local side? This would make it possible to do a
> proper address translation, although 2 addresses is a quite small amount.
>

Yes, that's true for systems with a single max9286 [1]

We have a system with 2 de-serializers, and what happens is the
following:

The system starts with the following configuration:

1)
                    +------- max9271@40
                    +------- max9271@40
Soc ----> max9286 --+------- max9271@40
                    +------- max9271@40

with a single max9286 it would be easy. We operate on one channel at
the time, do the reprogramming (or set up the translation, for the TI
chip use case) when adding the adapter for the channel, and then we
can talk with all remotes, which now have a different address

2)
                    +-------- max9271@50
                    +--- / -- max9271@40
Soc ----> max9286 --+--- / -- max9271@40
                    +--- / -- max9271@40


                    +--- / -- max9271@50
                    +-------- max9271@51
Soc ----> max9286 --+--- / -- max9271@40
                    +--- / -- max9271@40

                    +--- / -- max9271@50
                    +--- / -- max9271@51
Soc ----> max9286 --+-------- max9271@52
                    +--- / -- max9271@40

                    +--- / -- max9271@50
                    +--- / -- max9271@51
Soc ----> max9286 --+--- / -- max9271@52
                    +-------- max9271@53

Of course, to do the reprogramming, we need to initially send messages
to the default 0x40 address each max9271 boots with. If we don't close
all channels but the one we intend to reprogram, all remotes would
receive the same message, and thus will be re-programmed to the same
address (not nice). [2]

Now, if you have two max9286, installed on the same i2c bus, then you
need to make sure all channels of the 'others' are closed, before you
can reprogram your remotes, otherwise, you would end up reprogramming
all the remotes of the 'others' when trying to reprogram yours, as our
local de-serializers, bounces everything they receives, not directed
to them, to their remote sides.

3)
                       +-------- max9271@50
                       +--- / -- max9271@40
Soc --+-> max9286@4c --+--- / -- max9271@40
      |                +--- / -- max9271@40
      |
      |-> max9286@6c --+-------- max9271@50  <-- not nice
                       +-------- max9271@50
                       +-------- max9271@50
                       +-------- max9271@50

                       +--- / -- max9271@50
                       +-------- max9271@51
Soc --+-> max9286@4c --+--- / -- max9271@40
      |                +--- / -- max9271@40
      |
      |-> max9286@6c --+-------- max9271@51 <-- not nice
                       +-------- max9271@51
                       +-------- max9271@51
                       +-------- max9271@51

....

With the (not nice) 'max9286_is_bound()' we make sure we close all
channels on all max9286 first

4)
                       +--- / -- max9271@40
                       +--- / -- max9271@40
Soc --+-> max9286@4c --+--- / -- max9271@40
      |                +--- / -- max9271@40
      |
      |-> max9286@6c --+--- / -- max9271@40
                       +--- / -- max9271@40
                       +--- / -- max9271@40
                       +--- / -- max9271@40

And then only the last one to probe calls the re-programming
phase for all its fellows de-serializers on the bus.

5)
                       +-------- max9271@50
                       +--- / -- max9271@40
Soc --+-> max9286@4c --+--- / -- max9271@40
      |                +--- / -- max9271@40
      |
      |-> max9286@6c --+--- / -- max9271@40
                       +--- / -- max9271@40
                       +--- / -- max9271@40
                       +--- / -- max9271@40
    ....


                       +--- / -- max9271@50
                       +--- / -- max9271@51
Soc --+-> max9286@4c --+--- / -- max9271@52
      |                +--- / -- max9271@53
      |
      |-> max9286@6c --+-------- max9271@54
                       +--- / -- max9271@40
                       +--- / -- max9271@40
                       +--- / -- max9271@40

When addr reprogramming is done, we enter the image streaming phase,
with all channels open, as now, all remotes, have a different i2c
address assigned.

Suggestions on how to better handle this are very welcome. The point
here is that, to me, this is a gmsl-specific implementation thing.

Do you think for your chips, if they do translations, can you easy mask
them with the i2c address you want (being that specified in the remote
node or selected from an i2c-addr-pool, or something else) without
having to care about others remotes to be accidentally programmed to
an i2c address they're not intended to be assigned to.

Hope this helps clarify your concerns, and I think the actual issue
to discuss, at least on bindings, would be the i2c-address assignment
method, as this impacts GMSL, as well as other implementation that
would use the same binding style as this patches.

Thanks
   j

[1] I still don't get why 'addr translation' >> 'addr reprogramming'.
Even the GMSL application development examples uses addr reprogramming,
so I guess this is how those chips are supposed to work.

[2] If your local side supports address translation, you don't need to
talk with the remote side to 'mask' it, so you don't need this workaround.

> BTW all the TI chips I'm looking at can do address translation but, as
> far as I understand, only when acting as "slave proxy", i.e. when
> attached to the bus master. If the Maxim chips do the same, the "remote
> translation" would be unusable.
>
> > We have the ability to reprogram addresses through, and that's what we
> > are using.
>
> Sadly, it looks pretty much unavoidable...
>
> > There's a lot more local discussion going on here that I may have missed
> > so I hope Jacopo, Niklas, or Laurent may add more here if relevant :)
> >
> >
> >
> >
> >>> That said - I think this section needs to be removed from the upstream
> >>> part at least for now. I think we should probably carry this
> >>> 'workaround' separately.
> >>>
> >>> This part is the core issue that I talked about in my presentation at
> >>> ALS-Japan [0]
> >>>
> >>>  [0] https://sched.co/EaXa
> >>
> >> Oh, interesting, I hadn't noticed that you gave this talk -- at the same
> >> conference as Vladimir's talk! No video recording apparently, but are
> >> slides available at least?
> >
> > Hrm ... I was sure I uploaded to the conference so that they should have
> > been available on that URL - but they are not showing.
> >
> > They are available here:
> >
> > 	https://www.slideshare.net/KieranBingham/gmsl-in-linux
>
> Thanks.
>
> > (Please excuse the incorrect date on the first slide :D)
> >
> > I had put a proposal in to give this talk again at ELCE but
> > unfortunately it didn't get accepted.
> >
> > Seems it really would have been useful to have a slot. Lets hope next
> > ELCE is too late and we work out a good system by then :)
>
> Indeed it would have been!
>
> But hey, The FOSDEM CFPs are still open!
>
> Bye,
> --
> Luca

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