RE: [PATCH v3 1/1] mfd: intel_quark_i2c_gpio: Add Intel Quark X1000 I2C-GPIO MFD Driver

[Mike Turquette]

Quoting Tan, Raymond (2014-12-21 18:33:42)
> Hi Mike,
> 
> Thanks for your reply. I've answered the questions as below.
> 
> Warm Regards, 
> 
> Raymond Tan
> 
> > -----Original Message-----
> > From: Mike Turquette [mailto:mturquette@xxxxxxxxxx]
> > Sent: Friday, December 12, 2014 6:26 AM
> > To: Tan, Raymond; Lee Jones; Samuel Ortiz
> > Cc: linux-kernel@xxxxxxxxxxxxxxx; Chen, Alvin; Shevchenko, Andriy; Tan,
> > Raymond
> > Subject: Re: [PATCH v3 1/1] mfd: intel_quark_i2c_gpio: Add Intel Quark
> > X1000 I2C-GPIO MFD Driver
> > 
> > Quoting Raymond Tan (2014-12-11 01:38:30)
> > > In Quark X1000, there's a single PCI device that provides both an I2C
> > > controller and a GPIO controller. This MFD driver will split the 2
> > > devices for their respective drivers.
> > >
> > > This patch is based on Josef Ahmad's initial work for Quark enabling.
> > >
> > > Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Weike Chen <alvin.chen@xxxxxxxxx>
> > > Signed-off-by: Raymond Tan <raymond.tan@xxxxxxxxx>
> > > ---
> > >  drivers/mfd/Kconfig                |   12 ++
> > >  drivers/mfd/Makefile               |    1 +
> > >  drivers/mfd/intel_quark_i2c_gpio.c |  279
> > > ++++++++++++++++++++++++++++++++++++
> > >  3 files changed, 292 insertions(+)
> > >  create mode 100644 drivers/mfd/intel_quark_i2c_gpio.c
> > 
> > <snip>
> > 
> > > +static int intel_quark_register_i2c_clk(struct intel_quark_mfd
> > > +*quark_mfd) {
> > > +       struct pci_dev *pdev = quark_mfd->pdev;
> > > +       struct clk_lookup *i2c_clk_lookup;
> > > +       struct clk *i2c_clk;
> > > +       int retval;
> > > +
> > > +       i2c_clk_lookup = devm_kcalloc(
> > > +               &pdev->dev, INTEL_QUARK_I2C_NCLK,
> > > +               sizeof(*i2c_clk_lookup), GFP_KERNEL);
> > > +
> > > +       if (!i2c_clk_lookup)
> > > +               return -ENOMEM;
> > > +
> > > +       i2c_clk_lookup[0].dev_id = INTEL_QUARK_I2C_CONTROLLER_CLK;
> > > +
> > > +       i2c_clk = clk_register_fixed_rate(
> > > +               &pdev->dev, INTEL_QUARK_I2C_CONTROLLER_CLK, NULL,
> > > +               CLK_IS_ROOT, INTEL_QUARK_I2C_CLK_HZ);
> > > +
> > > +       quark_mfd->i2c_clk_lookup = i2c_clk_lookup;
> > > +       quark_mfd->i2c_clk = i2c_clk;
> > > +
> > > +       retval = clk_register_clkdevs(i2c_clk, i2c_clk_lookup,
> > > +                                     INTEL_QUARK_I2C_NCLK);
> > 
> > Lee asked about this in V2, so I'll follow up here in V3. It is OK for a driver to
> > use the clock provider api to register clocks with the clk framework if that
> > device truly is the provider of that clock signal. A good example can be found
> > here:
> > 
> > drivers/media/platform/omap3isp/isp.c
> > 
> > The OMAP3 ISP receives a clock signal as a input. Within the image signal
> > processor IP block it also has some basic clock controls of it's own which it
> > feeds to downstream IP blocks. As such it is both a clock consumer and a
> > provider and this is a common pattern amongst SoC designs.
> 
> Thanks for the reference, however the mfd driver is purely a clk provider in this case.
> 
> > 
> > So my question for this driver is if i2c_clk is provided by whatever the hell this
> > mfd device is supposed to be, or if it's just a convenient place to call the code?
> 
> As you've noticed, this is a fixed clock which only consumed by the I2C controller. 
> Following the structure of the designware i2c controller device driver, a clk is needed for it, 
> and on this platform, it is a fixed clk. 
> I'm putting the clk functions in this mfd driver is due to the fact that, this mfd driver
> is splitting the function of the PCI device to 2 controllers downstream. 
> 
> > 
> > Another concern is that fact that this is a fixed clock. For architectures that
> > use device tree to desribe board topology (ARM, MIPS,
> > PPC) it is common to simply put the fixed-rate clocks there and not directly
> > into the drive code. This prevents having to hack a lot of conditionals into
> > your driver when rev 2.0 of your hardware comes out with a faster fixed rate
> > clock, but you still need to support 1.0 hardware users at the slower rate. I
> > don't know if x86 has a similar way of describing board topology but it might
> > something to look into.
> 
> I checked the kernel source for x86 arch, sadly there's no similar implementation of
> fixed clk being developed/written on the architectures code. 
> That being said, for this platform, we do have a separate platform board file for those 
> onboard peripherals, do you think that it's better I put the clk function under the
> board file instead? My reasoning behind is if I were to introduce clk in general to x86
> in this way, it's effect will be on x86 unless I introduce further checking during
> compilation / runtime. 

Thanks for the explanation. One final question, who consumes this clock?

The clk bits of the driver look good to me so please add my:

Acked-by: Michael Turquette <mturquette@xxxxxxxxxx>

Thanks,
Mike

> 
> > 
> > Regards,
> > Mike
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