linux-kernel - Re: [PATCH v4 1/2] i2c: qup: add ACPI support

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Date:	Fri, 01 Jul 2016 11:57:05 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Christopher Covington <cov@...eaurora.org>,
	Philip Elcan <pelcan@...eaurora.org>,
	Austin Christ <austinwc@...eaurora.org>, wsa@...-dreams.de,
	linux-arm-msm@...r.kernel.org, timur@...eaurora.org,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org, rruigrok@...eaurora.org,
	linux-i2c@...r.kernel.org, Sricharan R <sricharan@...eaurora.org>
Subject: Re: [PATCH v4 1/2] i2c: qup: add ACPI support

On Thursday, June 30, 2016 2:50:44 PM CEST Christopher Covington wrote:
> Hi Arnd,
> 
> On 06/30/2016 07:52 AM, Arnd Bergmann wrote:
> > On Thursday, June 30, 2016 7:35:14 AM CEST Christopher Covington wrote:
> >> Hi Arnd,
> >>
> >> On 06/08/2016 05:02 PM, Arnd Bergmann wrote:
> >>> On Wednesday, June 8, 2016 12:19:44 PM CEST Austin Christ wrote:
> >>>> +               ret = device_property_read_u32(qup->dev,
> >>>> +                               "src-clock-hz", &src_clk_freq);
> >>>> +               if (ret) {
> >>>> +                       dev_warn(qup->dev, "using default src-clock-hz %d",
> >>>> +                               DEFAULT_SRC_CLK);
> >>>> +                       src_clk_freq = DEFAULT_SRC_CLK;
> >>>> +               }
> >>>>
> >>>
> >>> Where is this property documented?
> >>
> >> We plan on submitting documentation via dsd@...ica.org to
> >> https://github.com/ahs3/dsd once it is operational. As I understand it
> >> the project is brand new. It may take several months to begin accepting
> >> submissions. In the mean time, we could potentially include
> >> documentation in a reply to this thread, the cover of the next series, a
> >> wiki page on codeaurora.org, a file in Documentation (perhaps to be
> >> replaced by ACPICA style imports of the OS-neutral DSD project or a git
> >> submodule) or potentially other means. Please let us know what you think
> >> is sufficient.
> > 
> > As you are reusing part of the DT binding, it seems appropriate to put
> > the documentation for this into the binding documentation in the kernel.
> 
> My understanding is that in the device tree case, the input/source clock
> frequency is assumed to be run-time managed through Global Clock
> Controller (GCC) code and the common clock framework.
> 
> drivers/i2c/busses/i2c-qup.c:1549
> 
> src_clk_freq = clk_get_rate(qup->clk);
> 
> So a given I2C device tree entry points to the GCC device tree entry,
> but there isn't an explicit, fixed input/source clock frequency value.

Correct.

> arch/arm64/boot/dts/qcom/msm8916.dtsi:310
> 
> blsp_i2c2: i2c@...6000 {
>         compatible = "qcom,i2c-qup-v2.2.1";
>         reg = <0x78b6000 0x1000>;
>         interrupts = <GIC_SPI 96 0>;
>         clocks = <&gcc GCC_BLSP1_AHB_CLK>,
>                 <&gcc GCC_BLSP1_QUP2_I2C_APPS_CLK>;
>         clock-names = "iface", "core";
>         pinctrl-names = "default", "sleep";
>         pinctrl-0 = <&i2c2_default>;
>         pinctrl-1 = <&i2c2_sleep>;
>         #address-cells = <1>;
>         #size-cells = <0>;
>         status = "disabled";
> };
> 
> On the other hand, when ACPI is in use, the driver assumes a fixed
> input/source clock frequency value, which it tries to look up as a
> device property.

Also correct.
 
> (I'm out of my depth here, so somebody please correct me if I've
> described this wrong.)

What I'm saying was that the binding file could just document both
cases as alternatives. We prefer to specify the clocks directly
when using a dtb, but for the driver one of the two is ok, and
the ACPI documentation will already have to refer to the DT binding
for the other properties, so I think it makes sense to describe
both the "clocks" and "src-clock-hz" properties in the same document
as optional properties and clarify that at least one of the two
is requried.

Note that we have traditionally used "clock-frequency" as the
property name for the input clock frequency in DT bindings that
predate the generic clock handling (e.g. 8250 uarts on IBM Power
servers), so we could use the same here.

	Arnd