Message-ID: <80b854b4-3c90-2d3c-4996-c13e8452b06f@kernel.org>
Date:   Sat, 3 Dec 2016 09:14:30 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Benjamin Gaignard <benjamin.gaignard@...aro.org>
Cc:     Lee Jones <lee.jones@...aro.org>, robh+dt@...nel.org,
        Mark Rutland <mark.rutland@....com>, alexandre.torgue@...com,
        devicetree@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-pwm@...r.kernel.org, knaack.h@....de,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Fabrice Gasnier <fabrice.gasnier@...com>,
        Gerald Baeza <gerald.baeza@...com>,
        Arnaud Pouliquen <arnaud.pouliquen@...com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
        Benjamin Gaignard <benjamin.gaignard@...com>
Subject: Re: [PATCH v2 1/7] MFD: add bindings for stm32 general purpose timer
 driver

On 29/11/16 08:48, Benjamin Gaignard wrote:
> 2016-11-27 16:41 GMT+01:00 Jonathan Cameron <jic23@...nel.org>:
>> On 27/11/16 14:10, Jonathan Cameron wrote:
>>> On 24/11/16 15:14, Benjamin Gaignard wrote:
>>>> Add bindings information for stm32 general purpose timer
>>>>
>>>> version 2:
>>>> - rename stm32-mfd-timer to stm32-gptimer
>>>> - only keep one compatible string
>>>>
>>>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...com>
>>>> ---
>>>>  .../bindings/mfd/stm32-general-purpose-timer.txt   | 43 ++++++++++++++++++++++
>>>>  1 file changed, 43 insertions(+)
>>>>  create mode 100644 Documentation/devicetree/bindings/mfd/stm32-general-purpose-timer.txt
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/mfd/stm32-general-purpose-timer.txt b/Documentation/devicetree/bindings/mfd/stm32-general-purpose-timer.txt
>>>> new file mode 100644
>>>> index 0000000..2f10e67
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/mfd/stm32-general-purpose-timer.txt
>>>> @@ -0,0 +1,43 @@
>>>> +STM32 general purpose timer driver
>>>> +
>>>> +Required parameters:
>>>> +- compatible: must be "st,stm32-gptimer"
>>>> +
>>>> +- reg:                      Physical base address and length of the controller's
>>>> +                    registers.
>>>> +- clock-names:              Set to "clk_int".
>>>> +- clocks:           Phandle to the clock used by the timer module.
>>>> +                    For Clk properties, please refer to ../clock/clock-bindings.txt
>>>> +
>>>> +Optional parameters:
>>>> +- resets:           Phandle to the parent reset controller.
>>>> +                    See ..reset/st,stm32-rcc.txt
>>>> +
>>>> +Optional subnodes:
>>>> +- pwm:                      See ../pwm/pwm-stm32.txt
>>>> +- iiotimer:         See ../iio/timer/stm32-iio-timer.txt
>>> Naming issue here.  Can't mention IIO as that's a linux subsystem and all
>>> bindings must be independent of OS.
>>>
>>> Perhaps adc-trigger-timer?
>>>> +
>>>> +Example:
>>>> +    gptimer1: gptimer1@...10000 {
>>>> +            compatible = "st,stm32-gptimer";
>>>> +            reg = <0x40010000 0x400>;
>>>> +            clocks = <&rcc 0 160>;
>>>> +            clock-names = "clk_int";
>>>> +
>>>> +            pwm1@0 {
>>>> +                    compatible = "st,stm32-pwm";
>>>> +                    st,pwm-num-chan = <4>;
>>>> +                    st,breakinput;
>>>> +                    st,complementary;
>>>> +            };
>>>> +
>>>> +            iiotimer1@0 {
>>>> +                    compatible = "st,stm32-iio-timer";
>>> Again, avoid the use of iio in here (same issue you had with mfd in the previous
>>> version).
>>>> +                    interrupts = <27>;
>>>> +                    st,input-triggers-names = TIM5_TRGO,
>>> Docs for these should be introduced before they are used in an example.
>>> Same for the PWM ones above.  Expand the detail fo the example as you add
>>> the other elements.
>>
>> I've just dived into the datasheet for these timers.
>> http://www.st.com/content/ccc/resource/technical/document/reference_manual/3d/6d/5a/66/b4/99/40/d4/DM00031020.pdf/files/DM00031020.pdf/jcr:content/translations/en.DM00031020.pdf
>>
> 
> I really appreciate that you do this effort, thanks.
> 
>>
>> I think you need a binding that describes the capabilities of each of the timers
>> explicitly.   Down to the level of whether there is a repetition counter or not.
>> Each should exists as a separate entity in device tree.
>>
>> They then have an existence as timers separate to the description of what they
>> are used for.
>>
>> Here the only way we are saying they exist is by their use which doesn't feel
>> right to me.
>>
>> So I think you need to move back to what you had in the first place.  The key
>> thing is that ever timer needs describing fully.  They are different enough
>> that for example the datasheet doesn't even try to describe them in one section.
>> (it has 4 separate chapters covering different sets of these hardware blocks).
>> The naming isn't really based on index, we are talking different hardware
>> that the datasheet authors have decided not to give different names to!
> 
> Even if the hardware are named differently in the documentation they
> all share the
> same registers definitions and mapping but configurations are
> different for each device.
> 
>>
>> If they'd called them
>> advanced timers
>> generic timers
>> basic timers
>> really basic timers meant for driving the DAC (6 and 7)
>>
>> We'd all have been quite happy with different compatible strings giving away
>> what they can do.
> 
> 4 compatible strings will not be enough to describe devices
> configuration, for example
> in the documentation general purpose timers could have a 16 or 32 bit
> counter, for 1 to 4
> pwm channels and triggers (accepted or generated by the device) are
> different for each device.
> 
> DAC could be drive by timers 2, 4, 5, 6, 7 and 8.
> ADC could be driver by 32 triggers
My point was more about the fact that though the naming appears to (and kind
of does describe an index) these devices are really as different as for example
different part numbers would imply on a range of ADCs (say the multitude supported
by the max1363 driver - all of which are very nearly register compatible)

Hence I'd be less quick to dismiss the option of a number of compatible strings
rather than the wealth of description you'll otherwise have to put in the device
tree.
> 
>> What you have here is far too specific to what you are trying to do with them
>> right now.
>>
>> These things are separately capable of timing capture (which is I guess where
>> the IIO device later comes in).
>>
>> So my expectation is that we end up potentially instantiating:
>>
>> 1) An MFD to handle the shared elements of the timers.
>> 2) Up to 12ish timers each with separate existence as a device in the driver model
>> and in device tree.
>> (nasty corner cases here are using timers as perscalers for other timers - I'd be
>> tempted to leave that for now)
>> Note that each of these devices has a different register set I think? Any shared
>> bits are handled via the mfd above (if we even need that MFD which I'm starting
>> to doubt looking at the datasheet).
>>
> 
> pwm and trigger share the same registers but not the same bits.
> With regmap write functions I don't have sharing problems.
> 
>> 3) Up to N pwms again with there own existence in the device model.  These don't
>> do much other than wrap the timer and stick it in output mode.
>> 4) Up to N iio triggers - this is basically using the timer as a periodic interrupt
>> (though without the interrupt having visibility to the kernel) which fires off
>> sampling on associated ADCs.
>> 5) Up to N iio capture devices for all channels that support timing capture.
>> Note there is also hardware encoder capture support in these which should be
>> correctly handled as well.  This comes back to an ancient discussion on the
>> TI ecap units which have similar capabilities (driver never went anywhere but
>> I think that was because the author left TI).
>>
>> Certainly for the IIO devices these should no be bound up into one instance
>> as you have done here.
>>
>> Anyhow, I fear that right now this discussion is missing the key ingredient
>> that the hardware is horrendously variable in it's capabilities and really
>> is 4-5 different types of hardware that just happen to share a few bits of
>> their offsets in their register maps.
> 
> Hardware really share the same registers mapping that why I have wrote
> one only driver
> per framework. Only the configurations are different
One driver is fine, but the difference to my mind are sufficient that
we may need to use compatible strings for the various options.  Worth
a go at trying to fully describe them first though!
> 
>>
>> So after all that I'm almost more confused than I was at the start!
>>
>> Jonathan
>>
>>
>>>> +                                              TIM2_TRGO,
>>>> +                                              TIM4_TRGO,
>>>> +                                              TIM3_TRGO;
>>>> +                    st,output-triggers-names = TIM1_TRGO;
>>>> +            };
>>>> +    };
>>>>
>>>
>>> --
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>>>
>>
> 
> 
> 

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