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

[Geert Uytterhoeven]

Hi Biju,

On Wed, Jun 14, 2023 at 1:04 PM Biju Das <biju.das.jz at bp.renesas.com> wrote:
> > Subject: Re: [PATCH v5 01/11] i2c: Enhance i2c_new_ancillary_device API
> > 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:

> >   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?
>
> The ISL1208 driver doesn't support clock output. It is same as ISL1208, but difference is
> since same INT# pin used for PMIC, I guess we won't be able to use PMIC interrupt, if RTC configured for clock output.

Exactly.

The documentation confirms it can also be configured as clock signal
output in Frequency Output (FOUT) mode of the RTC on RAA215300.

> >   3. Are there other I2C addresses the chip listens to?
>
> No, only 2 address 0x12 and 0x6f.

Both addresses are programmable, and can even be the same!
Interesting, but more challenging for the Linux driver model...

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

Thanks, got access through the kind people behind the Renesas website.

Other things I noticed during a quick glance:
  - To activate the RTC, the host must first set the RTC EN bit =
    1 and the WRTC bit = 1.
    The RTC EN bit is located in the PMIC address space.
    IMHO this precludes using a separate DT node.
  - ISL1208 supports 400 kHz I2C, RAA215300 supports 1 MHz.

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