linux-kernel - Re: [PATCH v4 1/6] Documentation: DT: Consolidate SP805 binding docs

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Date:   Wed, 20 Jun 2018 10:39:16 -0700
From:   Ray Jui <ray.jui@...adcom.com>
To:     Rob Herring <robh@...nel.org>, Guenter Roeck <linux@...ck-us.net>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Mark Rutland <mark.rutland@....com>,
        Frank Rowand <frowand.list@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Robin Murphy <robin.murphy@....com>,
        linux-watchdog@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>
Subject: Re: [PATCH v4 1/6] Documentation: DT: Consolidate SP805 binding docs

Hi Guenter/Rob,

Kindly let me know how you want to proceed with this?

Thanks,

Ray

On 6/6/2018 4:39 PM, Ray Jui wrote:
> 
> 
> On 6/6/2018 9:33 AM, Rob Herring wrote:
>> On Wed, Jun 6, 2018 at 11:19 AM, Guenter Roeck <linux@...ck-us.net> 
>> wrote:
>>> On 06/05/2018 12:41 PM, Rob Herring wrote:
>>>>
>>>> On Mon, May 28, 2018 at 11:01:32AM -0700, Ray Jui wrote:
>>>>>
>>>>> Consolidate two SP805 binding documents "arm,sp805.txt" and
>>>>> "sp805-wdt.txt" into "arm,sp805.txt" that matches the naming of the
>>>>> desired compatible string to be used
>>>>>
>>>>> Signed-off-by: Ray Jui <ray.jui@...adcom.com>
>>>>> ---
>>>>>    .../devicetree/bindings/watchdog/arm,sp805.txt     | 27
>>>>> ++++++++++++++-----
>>>>>    .../devicetree/bindings/watchdog/sp805-wdt.txt     | 31
>>>>> ----------------------
>>>>>    2 files changed, 20 insertions(+), 38 deletions(-)
>>>>>    delete mode 100644
>>>>> Documentation/devicetree/bindings/watchdog/sp805-wdt.txt
>>>>
>>>>
>>>> Would be good to get a ACK from FSL/NXP person on this. It looks to me
>>>> like the driver fetches the wrong clock as it gets the first one and 
>>>> the
>>>> driver really wants 'wdog_clk'. In any case, their dts files should be
>>>> updated.
>>>>
>>>
>>> This is really confusing, since he deleted file lists apb_pclk first.
>>> Does the watchdog driver need apb_pclk or wdog_clk ? That isn't clear 
>>> to me.
>>> arch/arm64/boot/dts/hisilicon/hi3660.dtsi only provides apb_pclk, or at
>>> least
>>> it says so.
>>
>> Note that that clock source is 32KHz. That is obviously a mistake
>> because no one clocks their bus/register interface at 32KHz. Someone
>> just filled in something that happened to work.
>>
>>> The fsl dts files all have apb_pclk first.
>>
>> It's all kind of a mess, but fortunately one we should be able to 
>> clean-up.
>>
> 
> It is indeed a mess. Note the SP805 driver only derive one clock from 
> DT, and that's not done based on name. As a result, the first clock 
> defined in DT will be fetched and the rate calculation will be carried 
> out based on that clock rate.
> 
> I assumed the clock entries and their names defined in the binding 
> document are just placeholders, at least for the 2nd clock.
> 
> Based on how the current driver is, the first clock needs to be the 
> WDOGCLK for things to work properly.
> 
> According to the SP805 TRM, APB clock is the PCLK, that drives the bus 
> for register access.
> 
> The relationship between WDOGCLK and PCLK is defined as:
> 
> - the rising edges of WDOGCLK must be synchronous and
> balanced with a rising edge of PCLK
> 
> - the WDOGCLK frequency cannot be greater than the PCLK
> frequency
> 
>> The compatible string changes too, but AMBA bus devices don't actually
>> use the compatible string as they use the ID registers to match. I
>> suppose some other OS could do things differently. Worth the risk to
>> clean-up IMO.
>>
>>>
>>> Either case, why are two clocks asked for in the first place ? Are there
>>> situations where the second clock is actually used/useful ?
>>
>> For clocks, the bus needs "apb_pclk" and the driver just gets the
>> first clock. The driver is obviously going to want the functional
>> clock that determines the counter rate. That should
>>
>> Primecell peripherals are about the only ones that have clear specs
>> WRT clock inputs. Yet we've still managed to screw them up. There are
>> 2 clocks in the spec, so the DT has (or should have) 2 clocks.
>>
>> Rob
>>
> 
> Let me know how you guys want to proceed with this?
> 
> Thanks,
> 
> Ray