linux-kernel - Re: [RESEND PATCH v4] clk/axs10x: Add I2S PLL clock driver

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Message-ID: <5718D13B.2040906@synopsys.com>
Date:	Thu, 21 Apr 2016 14:10:19 +0100
From:	Jose Abreu <Jose.Abreu@...opsys.com>
To:	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	"sboyd@...eaurora.org" <sboyd@...eaurora.org>
CC:	Carlos Palminha <CARLOS.PALMINHA@...opsys.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	"linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
	"linux-snps-arc@...ts.infradead.org" 
	<linux-snps-arc@...ts.infradead.org>
Subject: Re: [RESEND PATCH v4] clk/axs10x: Add I2S PLL clock driver

Hi Alexey,


On 21-04-2016 13:18, Alexey Brodkin wrote:
> Hi Jose,
>
> On Thu, 2016-04-21 at 10:51 +0100, Jose Abreu wrote:
>> Hi Alexey,
>
>>>>> Otherwise, I still prefer two DTS files for the two different FPGA
>>>>> versions. At the least, please use ioremap for any pointers that
>>>>> you readl/writel here.
>>>>>
>>>>> Beyond that, we should have a fixed rate source clk somewhere in
>>>>> the software view of the clk tree, because that reflects reality.
>>>>> Hardcoding the parent rate in the structure works, but doesn't
>>>>> properly express the clk tree.
>>>>>
>>>> Can I use a property in the DT to pass this reference clock? something like this:
>>>>     snps,parent-freq = <0xFBED9 27000000>, <0x0 28224000>; /* Tuple
>>>> <fpga-version reference-clock-freq>, fpga-version = 0 is default */
>>>>
>>>> Or use a parent clock? like:
>>>>     clk {
>>>>         compatible = "fixed-clock";
>>>>         clock-frequency = <27000000>;
>>>>         #clock-cells = <0>;
>>>>         snps,fpga-version = <0xFBED9>;
>>>>     }
>>>>
>>>> It is important to distinguish between the different versions automatically, is
>>>> any of these solutions ok?
>>> I do like that solution with a master clock but with some fine-tuning
>>> for simplification.
>>>
>>> We'll add master clock node for I2S as a fixed clock like that:
>>> ------------------->8------------------
>>> 	i2s_master_clock: clk {
>>> 		#clock-cells = <0>;
>>> 		compatible = "fixed-clock";
>>> 		clock-frequency = <27000000>;
>>> 	};
>>> ------------------->8------------------
>>>
>>> Note there's no mention of MB version, just a value of the frequency.
>>> And in the driver itself value of that master clock will be used for
>>> population of "pll_clk->ref_clk" directly.
>>>
>>> These are benefits we'll get with that approach:
>>>  [1] We escape any IOs not related to our clock device (I mean
>>>      "snps,i2s-pll-clock") itself.
>>>  [2] We'll use whatever reference clock value is given.
>>>      I.e. we'll be able to do a fix-up of that reference clock
>>>      value early in platform code depending on HW we're running on.
>>>      That's what people do here and there.
>>>  [3] Remember another clock driver for AXS10x board is right around
>>>      the corner. I mean the one for ARC PGU which uses exactly the same
>>>      master clock. So one fixup as mentioned above will work
>>>      at once for 2 clock drivers.
>>>
>>> Let me know if above makes sense.
>> That approach can't be used because the reference clock value will change in the
>> next firmware release.  The new release will have a reference clock of 28224000
>> Hz instead of the usual 27000000 Hz, so we need to have a way to distinguish
>> between them. Because of that we can't have only one master clock unless you
>> state to users that they have to change the reference clock value when using the
>> new firmware release. Stephen suggested to use two DT files (one for each
>> firmware release), but as Vineet said this would be annoying to the user so I am
>> trying to use another solution so that only one DT file is required.
> Ok reference clock will change.
> But I may guess we'll still be able to determine at least that new
> firmware version in run-time, right? If so we'll update a fix-up in
> early axs10x platform code so that reference clock will be set as 28224000 Hz.

Yes, there is a register where the FPGA version date is encoded, we can use that
to check which firmware is used (if date <= old_firmware_date then
clock=27000000; else clock=28224000). If that fix is acceptable it could be a
good solution without having to use custom parameters in the DT (no need to
encode the different clocks and we would only use one master clock) but I am not
sure where and how this can be encoded and I don't know how to change the DT on
runtime. Can you give me some guidelines?

>
> And indeed 2 DT files is a no go - we want to run the same one binary
> (with built-in .dtb) on all flavors of AXS boards. And fix-up I'm talking about
> will actually do transformation of .dtb early on kernel boot process so that will
> be a complete equivalent of different DT files.

And doing modifications on the DT can cause some misdirections to users.
Besides, we would have clock specific functions in init procedures which is
precisely what we are trying to avoid by submitting this driver.

>
> -Alexey

Best regards,
Jose Miguel Abreu