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

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Date:	Thu, 11 Dec 2014 14:26:27 -0800
From:	Mike Turquette <mturquette@...aro.org>
To:	Raymond Tan <raymond.tan@...el.com>,
	"Lee Jones" <lee.jones@...aro.org>,
	"Samuel Ortiz" <sameo@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, "Alvin Chen" <alvin.chen@...el.com>,
	"Andriy Shevchenko" <andriy.shevchenko@...el.com>,
	"Raymond Tan" <raymond.tan@...el.com>
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@...ux.intel.com>
> Signed-off-by: Weike Chen <alvin.chen@...el.com>
> Signed-off-by: Raymond Tan <raymond.tan@...el.com>
> ---
>  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.

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?

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.

Regards,
Mike
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