[PATCH v5 01/11] i2c: Enhance i2c_new_ancillary_device API

[Geert Uytterhoeven]

Hi Biju,

On Wed, Jun 14, 2023 at 10:21 AM Biju Das <biju.das.jz at bp.renesas.com> wrote:
> > Subject: Re: [PATCH v5 01/11] i2c: Enhance i2c_new_ancillary_device API
> > On Tue, Jun 13, 2023 at 07:31:46PM +0000, Biju Das wrote:
> > > > Subject: RE: [PATCH v5 01/11] i2c: Enhance i2c_new_ancillary_device
> > > > API
> > > > > Subject: RE: [PATCH v5 01/11] i2c: Enhance
> > > > > i2c_new_ancillary_device API
> > > > > > Subject: Re: [PATCH v5 01/11] i2c: Enhance
> > > > > > i2c_new_ancillary_device API
> > > > > >
> > > > > > Hi everyone,
> > > > > >
> > > > > > > Perhaps we should first think through what an ancillary device
> > > > > > > really is.  My understanding is that it is used to talk to
> > > > > > > secondary addresses of a multi-address I2C slave device.
> > > > > >
> > > > > > As I mentioned somewhere before, this is not the case. Ancillary
> > > > > > devices are when one *driver* handles more than one address.
> > > > > > Everything else has been handled differently in the past (for
> > > > > > all the uses I am aware of).
> > > > > >
> > > > > > Yet, I have another idea which is so simple that I wonder if it
> > > > > > maybe has already been discussed so far?
> > > > > >
> > > > > > * have two regs in the bindings
> > > > >
> > > > > OK, it is inline with DT maintainers expectation as it is matching
> > > > > with real hw as single device node having two regs.
> > > > >
> > > > > > * use the second reg with i2c_new_client_device to instantiate the
> > > > > >   RTC sibling. 'struct i2c_board_info', which is one parameter,
> > should
> > > > > >   have enough options to pass data, e.g it has a software_node.
> > > > >
> > > > > OK, I can see the below can be passed from PMIC to new client
> > device.
> > > > >
> > > > >         client->addr = info->addr;
> > > > >
> > > > >         client->init_irq = info->irq;
> > > > >
> > > > > >
> > > > > > Should work or did I miss something here?
> > > > >
> > > > > I guess it will work. We instantiate appropriate device based On
> > > > > PMIC revision and slave address and IRQ resource passed through
> > > > > 'struct i2c_board_info'
> > > > >
> > > > > Will check this and update you.
> > > >
> > > > info.irq = irq; -->Irq fine
> > > > info.addr = addr; -->slave address fine size = strscpy(info.type,
> > > > name, sizeof(info.type)); -->instantiation based on PMIC version
> > > > fine.
> > > >
> > > > 1) How do we share clk details on instantiated device to find is it
> > > > connected to external crystal or external clock source? as we cannot
> > > > pass of_node between PMIC and "i2c_board_info" as it results in
> > > > pinctrl failure. info->platformdata and
> > > > Client->dev.platformdata to retrieve this info??
> > >
> > > Or
> > >
> > > I2C instantiation based on actual oscillator bit value, ie, two
> > > i2c_device_id's with one for setting oscillator bit and another for
> > > clearing oscillator bit
> > >
> > > PMIC driver parses the clock details. Based on firmware version and
> > > clock, It instantiates either i2c_device_id with setting oscillator
> > > bit or clearing oscillator bit.
> >
> > I don't like that hack. I still think that two DT nodes is the best
> > option, I think you're trying hard to hack around a problem that is
> > actually not a problem.
>
> Why do you think it is a hack? I believe rather it is actual solution
>
> PMIC is a single device, with 2 regs, clocks, pinctrl and IRQ properties.
> So it will be represented as single node with single compatible.
>
> By instating a client device, we are sharing the relevant resources to RTC device driver.

Exactly.  RAA215300 is a PMIC with an integrated ISL1208-derivative.
My biggest concern with using 2 separate nodes in DT is that one day
we might discover another integration issue, which needs communication
between the two parts.

Things from the top of my head:
  1. The device has a single interrupt pin.  Is there any interaction
     or coordination between PMIC and RTC interrupts?
  2. On the real ISL1208, the interrupt pin can also be used as a clock
     output.  Perhaps this is fed to some PMIC part in the
     RAA215300, too?
  2. Does the battery charger circuit in the PMIC impact the VBAT
     input of the RTC?
  3. Are there other I2C addresses the chip listens to?

I only have access to the Short-Form Datasheet for the RAA215300,
so I cannot check myself...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds