linux-kernel - Re: [resend rfc v3] pwm: add BCM2835 PWM driver

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Date:	Fri, 26 Sep 2014 08:45:57 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Thierry Reding <thierry.reding@...il.com>
CC:	Bart Tanghe <bart.tanghe@...masmore.be>, tim.kryger@...aro.org,
	linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org,
	linux-rpi-kernel@...ts.infradead.org
Subject: Re: [resend rfc v3] pwm: add BCM2835 PWM driver

On 09/26/2014 01:11 AM, Thierry Reding wrote:
> On Thu, Sep 04, 2014 at 09:06:48AM -0600, Stephen Warren wrote:
>> On 09/04/2014 04:05 AM, Bart Tanghe wrote:
>>> No problem. Thanks for the feedback.
>>> I've got some question below.
>>>
>>> On 2014-08-25 15:19, Thierry Reding wrote:
>>>> Sorry for taking so long to reply to this, I had completely forgotten.
>>>>
>>>> On Mon, Apr 28, 2014 at 02:54:46PM +0200, Bart Tanghe wrote:
>>>>> 	Add some better error handling and Device table support
>>>>> 	Added Documentation/devicetree/bindings/pwm/pwm-bcm2835.txt
>>>>> 	
>>>>> Signed-off-by: Bart Tanghe <bart.tanghe@...masmore.be>
>>
>>>>> diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
>>>>> index 22f2f28..20341a3 100644
>>>>> --- a/drivers/pwm/Kconfig
>>>>> +++ b/drivers/pwm/Kconfig
>>>>> @@ -62,6 +62,18 @@ config PWM_ATMEL_TCB
>>>>>   	  To compile this driver as a module, choose M here: the module
>>>>>   	  will be called pwm-atmel-tcb.
>>>>>
>>>>> +config PWM_BCM2835
>>>>> +	tristate "BCM2835 PWM support"
>>>>> +	depends on MACH_BCM2835 || MACH_BCM2708
>>>>> +	help
>>>>> +	  PWM framework driver for BCM2835 controller (raspberry pi)
>>>>
>>>> I think the correct capitalization would be "Raspberry Pi".
>>>>
>>>>> +	  Only 1 channel is implemented.
>>>>
>>>> How many can it take? Why haven't all been implemented?
>>>
>>> BCM2835 can take 2 pwm channels.
>>> I can implement 2 channels but can't physically test the second channel. Is that a problem?
>>
>> I don't think that's a problem; I would expect the channels to be identical,
>> so testing 1 should be fine.
>
> Agreed. If it turns out not to work it can always be fixed.
>
>>>> I notice that you never prepare or enable the clock here. Perhaps this
>>>> isn't required because it's always on, but I think you should still call
>>>> clk_prepare_enable() here (and clk_disable_unprepare() in .remove()) to
>>>> make sure the driver is more portable.
>>> The frequency can be minimized by a clock_divider ( the pwm clock is default disabled). this has to be done by
>>> a clock driver, as mentioned in a previous comment by Stephen Warren.
>>>
>>> Any clock programming should be performed by a clock driver. We don't
>>> have one of those upstream yet, mainly because it would rely on talking
>>> to the firmware (running on the VideoCore) to manipulate the clocks, and
>>> we don't have a firmware protocol driver either.
>>>
>>> Nowadays, I'm using a userspace program to change the clock_divider, but would like to implement this in a clock driver.
>>> The clock hardware description isn't implemented in the datasheet. I can convert the userspace prog to a clock driver but this is very experimental.
>>> If anyone has some suggestions?
>>
>> Oh dear. It sounds like we need at least some form of clock driver for the
>> platform then. I still don't think there's complete documentation for the
>> HW, even though a lot of register docs were published which presumably cover
>> the clock HW? Equally, given that the VC firmware assumes it owns most of
>> the HW, it seems best to manipulate the clocks through the firmware
>> interface rather than directly touching the HW. Unfortunately, I don't
>> believe there's any ABI guarantee on the firmware interface. Perhaps we can
>> get one?
>
> Urgs... this VC firmware seems to be more of a headache that I thought
> it was. How is this handled in other drivers? Surely PWM isn't the first
> one that needs clocks?

For the other clocks, I've set up dummy fixed-rate clocks in the DT 
and/or "clock driver" code to satisfy references by phandle or clock 
name respectively. Since the other drivers don't actually manipulate the 
clock rates etc., this is enough for the drivers.

I always hoped that enough HW information would either be reverse 
engineered or released to somehow disable the VC firmware, and implement 
all the drivers within Linux talking to HW directly.
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