linux-kernel - Re: [PATCH v2 08/31] arm64: CPU support

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Message-ID: <20120820155740.GA912@arm.com>
Date:	Mon, 20 Aug 2012 16:57:40 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Olof Johansson <olof@...om.net>
Cc:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH v2 08/31] arm64: CPU support

On Wed, Aug 15, 2012 at 01:10:43AM +0100, Olof Johansson wrote:
> On Tue, Aug 14, 2012 at 06:52:09PM +0100, Catalin Marinas wrote:
> 
> > diff --git a/arch/arm64/include/asm/cputype.h b/arch/arm64/include/asm/cputype.h
> > new file mode 100644
> > index 0000000..ef54125
> > --- /dev/null
> > +++ b/arch/arm64/include/asm/cputype.h
> > @@ -0,0 +1,49 @@
> > +#define ID_MIDR_EL1		"midr_el1"
> > +#define ID_CTR_EL0		"ctr_el0"
> > +
> > +#define ID_AA64PFR0_EL1		"id_aa64pfr0_el1"
> > +#define ID_AA64DFR0_EL1		"id_aa64dfr0_el1"
> > +#define ID_AA64AFR0_EL1		"id_aa64afr0_el1"
> > +#define ID_AA64ISAR0_EL1	"id_aa64isar0_el1"
> > +#define ID_AA64MMFR0_EL1	"id_aa64mmfr0_el1"
> > +
> > +#define read_cpuid(reg) ({						\
> > +	u64 __val;							\
> > +	asm("mrs	%0, " reg : "=r" (__val));			\
> > +	__val;								\
> > +})
> > +
> > +/*
> > + * The CPU ID never changes at run time, so we might as well tell the
> > + * compiler that it's constant.  Use this function to read the CPU ID
> > + * rather than directly reading processor_id or read_cpuid() directly.
> > + */
> > +static inline u32 __attribute_const__ read_cpuid_id(void)
> > +{
> > +	return read_cpuid(ID_MIDR_EL1);
> > +}
> > +
> > +static inline u32 __attribute_const__ read_cpuid_cachetype(void)
> > +{
> > +	return read_cpuid(ID_CTR_EL0);
> > +}
> 
> Is this perhaps a carry-over from arch/arm? Abstracting out read_cpuid()
> doesn't seem to buy anything here, just opencode the one-line assembly
> in each.

It doesn't buy much but it's more readable to use read_cpuid() in places
like hw_breakpoint.c than open coding the assembly.

I could get rid of the ID_* macros and just pass the register name
direcly to read_cpuid().

> Might as well cleanup the naming a little too while you're at it, i.e.
> read_cpu_id() and read_cpu_cachetype().

These were defined for convenience, a bit less typing. But they have the
intended name.

> > --- /dev/null
> > +++ b/arch/arm64/mm/proc-syms.c
...
> > +EXPORT_SYMBOL(__cpuc_flush_kern_all);
> > +EXPORT_SYMBOL(__cpuc_flush_user_all);
> > +EXPORT_SYMBOL(__cpuc_flush_user_range);
> > +EXPORT_SYMBOL(__cpuc_coherent_kern_range);
> > +EXPORT_SYMBOL(__cpuc_flush_dcache_area);
> 
> See comment on other email about putting function pointers in a struct
> instead.

There is no need to support multiple CPU architectures with different
implementations, so allowing these functions to be called without
indirection is better.

> > diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> > new file mode 100644
> > index 0000000..453f517
> > --- /dev/null
> > +++ b/arch/arm64/mm/proc.S
> > @@ -0,0 +1,193 @@
> > +	.section ".proc.info.init", #alloc, #execinstr
> > +
> > +	.type	__v8_proc_info, #object
> > +__v8_proc_info:
> > +	.long	0x000f0000			// Required ID value
> > +	.long	0x000f0000			// Mask for ID
> > +	b	__cpu_setup
> > +	nop
> > +	.quad	cpu_name
> > +	.long	0
> > +	.size	__v8_proc_info, . - __v8_proc_info
> 
> I know this is a carry-over from arch/arm, but how about moving this
> to more of a C construct similar to arch/powerpc/kernel/cputable.c
> instead? It's considerably easier to read that way, and it's convenient
> to have the definitions all in one place, making it easier to share some
> of the functions, etc.

I can do this, it would be indeed cleaner.

-- 
Catalin
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