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

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Message-ID: <20190116172849.h5qdfa5wc3vaprbn@pengutronix.de>
Date:   Wed, 16 Jan 2019 18:28:49 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Paul Walmsley <paul.walmsley@...ive.com>
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>
Subject: Re: [PATCH 2/2] pwm: sifive: Add a driver for SiFive SoC PWM

On Wed, Jan 16, 2019 at 09:18:45AM -0800, Paul Walmsley wrote:
> 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.

The dependency serves two purposes:

 a) ensure that the build requirements are fulfilled.
 b) restrict configuration to actually existing machines

When considering b) it doesn't make sense to enable the driver for (say)
a machine that is powered by an ARM SoC by TI. If you still want to
compile test the sifive driver for ARM, enable COMPILE_TEST. That's what
this symbol is there for.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |