linux-kernel - Re: [PATCH 2/2] pwm: sifive: Add a driver for SiFive SoC PWM

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Message-ID: <alpine.DEB.2.21.9999.1901160909540.19681@viisi.sifive.com>
Date:   Wed, 16 Jan 2019 09:18:45 -0800 (PST)
From:   Paul Walmsley <paul.walmsley@...ive.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
cc:     Yash Shah <yash.shah@...ive.com>,
        Palmer Dabbelt <palmer@...ive.com>, linux-pwm@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        Thierry Reding <thierry.reding@...il.com>, robh+dt@...nel.org,
        mark.rutland@....com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sachin Ghadi <sachin.ghadi@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>
Subject: Re: [PATCH 2/2] pwm: sifive: Add a driver for SiFive SoC PWM

On Wed, 16 Jan 2019, Uwe Kleine-König wrote:

> On Wed, Jan 16, 2019 at 04:40:42PM +0530, Yash Shah wrote:
> > On Wed, Jan 16, 2019 at 3:30 AM Uwe Kleine-König
> > <u.kleine-koenig@...gutronix.de> wrote:
> > > On Fri, Jan 11, 2019 at 01:52:44PM +0530, Yash Shah wrote:
> > > > diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> > > > index a8f47df..3bcaf6a 100644
> > > > --- a/drivers/pwm/Kconfig
> > > > +++ b/drivers/pwm/Kconfig
> > > > @@ -380,6 +380,16 @@ config PWM_SAMSUNG
> > > >         To compile this driver as a module, choose M here: the module
> > > >         will be called pwm-samsung.
> > > >
> > > > +config PWM_SIFIVE
> > > > +     tristate "SiFive PWM support"
> > > > +     depends on OF
> > > > +     depends on COMMON_CLK
> > >
> > > I'd say add:
> > >
> > >         depends on MACH_SIFIVE || COMPILE_TEST
> > >
> > > (I guess "MACH_SIFIVE" is wrong, but I assume you get what I mean.)
> > 
> > As of now, MACH_SIFIVE/ARCH_SIFIVE isn't available.
> > @Paul, Do you have any comments on this?
> 
> If this is not going to be available at least protect it by
> 
> 	depends RISCV || COMPILE_TEST

There's nothing RISC-V or SiFive SoC-specific about this driver or IP 
block.  The HDL for this IP block is open-source and posted on Github.  
The IP block and driver would work unchanged on an ARM or MIPS SoC, and in 
fact, SiFive does design ARM-based SoCs as well.  Likewise, any other SoC 
vendor could take the HDL for this IP block from the git tree and 
implement it on their own SoC.

More generally: it's a basic principle of Linux device drivers that they 
should be buildable for any architecture.  The idea here is to prevent 
developers from burying architecture or SoC-specific hacks into the 
driver.  So there shouldn't be any architecture or SoC-specific code in 
any device driver, unless it's abstracted in some way - ideally through a 
common framework.

So from this point of view, neither "depends MACH_SIFIVE" nor "depends 
RISCV" would be appropriate.  Similarly, the equivalents for other 
architectures (e.g. "ARCH_ARM") or SoC manufacturers (e.g., 
"MACH_SAMSUNG") wouldn't be appropriate for any generic IP block device 
driver like this one.

- Paul