Re: [RFC 1/4] pwm: sifive: Add DT documentation for SiFive PWM Controller.

From: Atish Patra <atish.patra@wdc.com>
To: Thierry Reding <thierry.reding@gmail.com>,
	Paul Walmsley <paul.walmsley@sifive.com>
Cc: Rob Herring <robh+dt@kernel.org>,
	mark.rutland@arm.com, linux-pwm@vger.kernel.org,
	devicetree@vger.kernel.org, Wesley Terpstra <wesley@sifive.com>,
	linus.walleij@linaro.org, palmer@sifive.com,
	linux-kernel@vger.kernel.org, hch@infradead.org,
	linux-gpio@vger.kernel.org, linux-riscv@lists.infradead.org
Subject: Re: [RFC 1/4] pwm: sifive: Add DT documentation for SiFive PWM Controller.
Date: Tue, 16 Oct 2018 15:20:34 -0700	[thread overview]
Message-ID: <7fc1168d-a840-032a-c0a9-2a610127c484@wdc.com> (raw)
In-Reply-To: <20181016220437.GB31973@mithrandir>

On 10/16/18 3:04 PM, Thierry Reding wrote:
> On Tue, Oct 16, 2018 at 10:31:42AM -0700, Paul Walmsley wrote:
>>
>> On 10/16/18 4:01 AM, Thierry Reding wrote:
>>> On Mon, Oct 15, 2018 at 03:57:35PM -0700, Atish Patra wrote:
>>>> On 10/10/18 6:49 AM, Thierry Reding wrote:
>>>>> On Tue, Oct 09, 2018 at 11:51:22AM -0700, Atish Patra wrote:
>>>>>> +Required properties:
>>>>>> +- compatible: should be one of
>>>>>> +	"sifive,fu540-c000-pwm0","sifive,pwm0".
>>>>> What's the '0' in here? A version number?
>>>>>
>>>> I think yes. Since fu540 is the first Linux capable RISC-V core, SiFive Guys
>>>> decided mark it as version 0.
>>>>
>>>> @Wesly: Please correct me if I am wrong.
>>> It seems fairly superfluous to me to have a version number in additon to
>>> the fu540-c000, which already seems to be the core plus some sort of
>>> part number. Do you really expect there to be any changes in the SoC
>>> that would require a different compatible string at this point? If the
>>> SoC has taped out, how will you ever get a different version of the PWM
>>> IP in it?
>>>
>>> I would expect any improvements or changes to the PWM IP to show up in a
>>> different SoC generation, at which point it would be something like
>>> "sifive,fu640-c000" maybe, or perhaps "sifive,fu540-d000", or whatever
>>> the numbering is.
>>
>>
>> The "0" suffix refers to a revision number for the underlying PWM IP block.
>>
>> It's certainly important to keep that version number on the "sifive,pwm0"
>> compatible string that doesn't have the chip name associated with it.
> 
> Isn't the hardware identified by "sifive,pwm0" and "sifive,fu540-c000"
> effectively identical? 

Yes.

Is there a need to have two compatible strings
> that refer to the exact same hardware?
> 

The DT in the hardware has only sifive,pwm0. I have added 
"sifive,fu540-c000" as that was concluded as the correct compatible 
string from platform level interrupt controller patch(PLIC) discussion.

(http://lists.infradead.org/pipermail/linux-riscv/2018-August/001135.html)

"sifive,pwm0" is required to until all the Unleashed SoC gets an updated 
firmware with correct compatible string "sifive,fu540-c000". I agree 
this is a mess. But we have to carry it until all every DT(corresponding 
to each driver) is finalized. I guess SiFive will release a firmware 
update that contains all the updated DT once that is done. We can get 
rid of all the redundant compatible strings at that time.

Regards,
Atish
>> As to whether there could ever be a FU540-C000 part with different IP block
>> versions on it: FU540-C000 is ultimately a marketing name.  While
>> theoretically we shouldn't have another "FU540-C000" chip with different
>> peripheral IP block versions on it, I don't think any engineer can guarantee
>> that it won't happen.
> 
> I would argue that if at some point there was indeed a chip with the
> same name but a different IP block version in it, we can figure out what
> to call it. Sure there are no guarantees, but it's still fairly unlikely
> in my opinion, so I personally wouldn't worry about this up front.
> 
> Anyway, I don't feel strongly either way, I'm just pointing out that
> this is somewhat unusual. If you want to keep it, feel free to.
> 
> Thierry
> 