linux-kernel - Re: [PATCH v7 1/2] phy: Add new Exynos5 USB 3.0 PHY driver

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Message-ID: <CAFp+6iEGvy71s=-Vt37TKkVDUX1wO6GZdwL=uBfPjng78yE5-A@mail.gmail.com>
Date:	Fri, 9 May 2014 16:21:19 +0530
From:	Vivek Gautam <gautam.vivek@...sung.com>
To:	Sylwester Nawrocki <s.nawrocki@...sung.com>
Cc:	Linux USB Mailing List <linux-usb@...r.kernel.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>, kishon <kishon@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-doc@...r.kernel.org,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Felipe Balbi <balbi@...com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Tomasz Figa <t.figa@...sung.com>,
	Kamil Debski <k.debski@...sung.com>
Subject: Re: [PATCH v7 1/2] phy: Add new Exynos5 USB 3.0 PHY driver

Hi Sylwester,


On Fri, May 9, 2014 at 3:43 PM, Sylwester Nawrocki
<s.nawrocki@...sung.com> wrote:
> Hi Vivek,
>
> On 08/05/14 11:03, Vivek Gautam wrote:
>>>>> diff --git a/Documentation/devicetree/bindings/phy/samsung-phy.txt b/Documentation/devicetree/bindings/phy/samsung-phy.txt
>>>>> >>> index b422e38..51efe4c 100644
>>>>> >>> --- a/Documentation/devicetree/bindings/phy/samsung-phy.txt
>>>>> >>> +++ b/Documentation/devicetree/bindings/phy/samsung-phy.txt
>>>>> >>> @@ -114,3 +114,43 @@ Example:
>>>>> >>>               compatible = "samsung,exynos-sataphy-i2c";
>>>>> >>>               reg = <0x38>;
>>>>> >>>       };
>>>>> >>> +
>>>>> >>> +Samsung Exynos5 SoC series USB DRD PHY controller
>>>>> >>> +--------------------------------------------------
>>>>> >>> +
>>>>> >>> +Required properties:
>>>>> >>> +- compatible : Should be set to one of the following supported values:
>>>>> >>> +     - "samsung,exynos5250-usbdrd-phy" - for exynos5250 SoC,
>>>>> >>> +     - "samsung,exynos5420-usbdrd-phy" - for exynos5420 SoC.
>>>>> >>> +- reg : Register offset and length of USB DRD PHY register set;
>>>>> >>> +- clocks: Clock IDs array as required by the controller
>>>>> >>> +- clock-names: names of clocks correseponding to IDs in the clock property;
>>>>> >>> +            Required clocks:
>>>>> >>> +     - phy: main PHY clock (same as USB DRD controller i.e. DWC3 IP clock),
>>>>> >>> +            used for register access.
>>>>> >>> +     - ref: PHY's reference clock (usually crystal clock), used for
>>>>> >>> +            PHY operations, associated by phy name. It is used to
>>>>> >>> +            determine bit values for clock settings register.
>>>>> >>> +            For Exynos5420 this is given as 'sclk_usbphy30' in CMU.
>>>>> >>> +- samsung,pmu-syscon: phandle for PMU system controller interface, used to
>>>>> >>> +                   control pmu registers for power isolation.
>>>>> >>> +- samsung,pmu-offset: phy power control register offset to pmu-system-controller
>>>>> >>> +                   base.
>>>> >>
>>>> >> It doesn't seem right to have register offset encoded in the device tree
>>>> >> like this. I think it'd be more appropriate to associate such an offset
>>>> >> with the compatible string's value in the driver.
>>> >
>>> > Ok, it makes more sense.
>>> > Just out of curiosity, what difference would this make ?
>>
>> Moreover, in case of Exynos5420 (and may be in future SoCs), where we
>> have 2 USB DRD PHY controllers,
>> we will need to have a way around to deal with two separate offsets in
>> the driver for one compatible string.
>
> It could be handled by creating a list of offsets per compatible string and
> then adding some way to identify the PHY device instance. So I believe that's
> not a big issue.

True, we had tried to make use of 'aliases' for multiple controllers
on Exynos5420,
in earlier version V3 of this patch, to handle the pmu-register
offsets in _somewhat_ similar fashion. [1]
But then we had to drop that as per the suggestions in the mailing list.

> Now you'd be encoding a list of registers offsets in the
> device tree, without encoding bit layout of each register. It's unlikely that
> each instance would have different bits layout, but describing individual
> registers in DT I thought is something that we're not supposed to do.

Ok, so this is information to me, i was not very much aware of this.
Thanks for explaining it here.

>
>> Getting the offsets from DT seems a cleaner way to handle this case of
>> multi controllers.
>
> I think it's easier from the driver POV, but isn't it violating the device
> tree rules ?
> Anyway. I'm just pointing this minor issue in the binding as it appears
> to me. The final word of course belongs to a DT binding maintainer.

Thanks Sylwester, for pointing this out here.
Meantime, please let me know if the approach using aliases is one
possible solution for this or not ?


[1] [PATCH v3] phy: Add new Exynos5 USB 3.0 PHY driver
     https://lkml.org/lkml/2014/1/21/46



-- 
Best Regards
Vivek Gautam
Samsung R&D Institute, Bangalore
India
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