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

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Date:	Mon, 29 Sep 2014 16:18:21 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Bart Tanghe <bart.tanghe@...masmore.be>
Cc:	Stephen Warren <swarren@...dotorg.org>, matt.porter@...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 Mon, Sep 29, 2014 at 03:37:50PM +0200, Bart Tanghe wrote:
> On 2014-09-29 07:33, Thierry Reding wrote:
> > On Fri, Sep 26, 2014 at 08:45:57AM -0600, Stephen Warren wrote:
> >> On 09/26/2014 01:11 AM, Thierry Reding wrote:
> >>> On Thu, Sep 04, 2014 at 09:06:48AM -0600, Stephen Warren wrote:
> > [...]
> >>>> 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.
> > 
> > Given that this driver only queries the clock frequency adding a fixed-
> > rate clock to the device tree should work as well. Then the calls to
> > clk_prepare_enable() and clk_disable_unprepare() can still be added as
> > appropriate so that the driver doesn't need to change if a proper clock
> > driver ever gets added.
> > 
> > Or am I missing anything? Perhaps the issue is that the default clock
> > rate for the PWM clock isn't usable? That would still not prevent the
> > driver from being merged.
> > 
> > Thierry
> > 
> The pwm clock is default unusable. To let the pwm clock run, It's necessary to change some 
> clock registers. I've added the clk_prepare_enable and the clk_disable_unprepare functions. So the 
> driver is able to work with a proper clock driver in the future.

Okay. Sounds good to me.

Thierry

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