linux-kernel - Re: [PATCH v3 05/11] memory: add Atmel EBI (External Bus Interface) driver

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Message-ID: <20141201212820.6a7fc4b0@bbrezillon>
Date:	Mon, 1 Dec 2014 21:28:20 +0100
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Andrew Victor <linux@...im.org.za>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	devicetree@...r.kernel.org,
	Jean-Jacques Hiblot <jjhiblot@...phandler.com>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>
Subject: Re: [PATCH v3 05/11] memory: add Atmel EBI (External Bus Interface)
 driver

On Mon, 01 Dec 2014 20:43:31 +0100
Arnd Bergmann <arnd@...db.de> wrote:

> On Monday 01 December 2014 19:29:23 Boris Brezillon wrote:
> > Hi Arnd,
> > 
> > On Mon, 01 Dec 2014 17:26:27 +0100
> > Arnd Bergmann <arnd@...db.de> wrote:
> > 
> > > On Monday 01 December 2014 11:27:21 Boris Brezillon wrote:
> > > > The EBI (External Bus Interface) is used to access external peripherals
> > > > (NOR, SRAM, NAND, and other specific devices like ethernet controllers).
> > > > Each device is assigned a CS line and an address range and can have its
> > > > own configuration (timings, access mode, bus width, ...).
> > > > This driver provides a generic DT binding to configure a device according
> > > > to its requirements.
> > > > For specific device controllers (like the NAND one) the SMC timings
> > > > should be configured by the controller driver through the matrix and
> > > > smc syscon regmaps.
> > > 
> > > Nice!
> > > 
> > > > +
> > > > +#define AT91_EBICSA_REGFIELD(soc)			\
> > > > +	REG_FIELD(soc ## _MATRIX_EBICSA_OFF, 0,		\
> > > > +		  AT91_MATRIX_EBI_NUM_CS - 1)
> > > > +
> > > > +#define AT91_MULTI_EBICSA_REGFIELD(soc, n)		\
> > > > +	REG_FIELD(soc ## _MATRIX_EBI ## n ## CSA_OFF,	\
> > > > +		  0, AT91_MATRIX_EBI_NUM_CS - 1)
> > > 
> > > I don't like the use macros that concatenate symbol names like
> > > this. Why not do either
> > > 
> > > - open-code the macro contents in the few uses, to allow
> > >   grepping for them, or
> > 
> > I'm not sure to get this one, are you suggesting to do something like
> > this:
> > 
> > #define AT91_EBICSA_REGFIELD(off)			\
> > 	REG_FIELD(ebicsa_off, AT91_MATRIX_EBI_NUM_CS - 1)
> > 
> 
> That would be acceptable too, but what I really meant is one step further:
> 
> static const struct reg_field at91sam9260_ebi_csa = 
> 	REG_FIELD(AT91SAM9260_MATRIX_EBICSA_OFF, 0, AT91_MATRIX_EBI_NUM_CS - 1);

That's what I did in the first place (in a version I didn't submitted),
and I guess I'll go for that one unless you really prefer the
alternative (I ran into a lot of trouble with DT bindings ABI
stability, and I'd prefer to keep DT bindings as simple as
possible).

> 
> > > - put the register number in the syscon reference and look it
> > >   up from there (this would be slightly more complicated for the
> > >   second macro)
> > 
> > I've told several times not to encode register offsets or register ids

    ^ "I've been told" :-).

> > in the DT :-) (and if I'm not mistaken that's what you're suggesting
> > here).
> 
> I think it's actually fine for syscon references, although in general
> I would agree with that. The difference in my opinion is that syscon
> by nature is a set of registers.

Okay, good to know.



-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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