linux-kernel - Re: [PATCH 7/9] ARM64: kernel: add support for cpu cache information

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Date:	Fri, 27 Jun 2014 11:36:11 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Sudeep Holla <sudeep.holla@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Will Deacon <Will.Deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 7/9] ARM64: kernel: add support for cpu cache information

Hi Sudeep,

On Wed, Jun 25, 2014 at 06:30:42PM +0100, Sudeep Holla wrote:
> From: Sudeep Holla <sudeep.holla@....com>
> 
> This patch adds support for cacheinfo on ARM64.
> 
> On ARMv8, the cache hierarchy can be identified through Cache Level ID
> (CLIDR) register while the cache geometry is provided by Cache Size ID
> (CCSIDR) register.
> 
> Since the architecture doesn't provide any way of detecting the cpus
> sharing particular cache, device tree is used for the same purpose.
> 
> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
> Cc: linux-arm-kernel@...ts.infradead.org
> ---
>  arch/arm64/kernel/Makefile    |   3 +-
>  arch/arm64/kernel/cacheinfo.c | 135 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 137 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/kernel/cacheinfo.c

[...]

> +static inline enum cache_type get_cache_type(int level)
> +{
> +	unsigned int clidr;
> +
> +	if (level > MAX_CACHE_LEVEL)
> +		return CACHE_TYPE_NOCACHE;
> +	asm volatile ("mrs     %0, clidr_el1" : "=r" (clidr));

Can't that allocate a w register?

You can make clidr a u64 to avoid that.

> +	return CLIDR_CTYPE(clidr, level);
> +}
> +
> +/*
> + * NumSets, bits[27:13] - (Number of sets in cache) - 1
> + * Associativity, bits[12:3] - (Associativity of cache) - 1
> + * LineSize, bits[2:0] - (Log2(Number of words in cache line)) - 2
> + */
> +#define CCSIDR_WRITE_THROUGH	BIT(31)
> +#define CCSIDR_WRITE_BACK	BIT(30)
> +#define CCSIDR_READ_ALLOCATE	BIT(29)
> +#define CCSIDR_WRITE_ALLOCATE	BIT(28)
> +#define CCSIDR_LINESIZE_MASK	0x7
> +#define CCSIDR_ASSOCIAT_SHIFT	3
> +#define CCSIDR_ASSOCIAT_MASK	0x3FF

ASSOCIAT doesn't quite roll off of the tongue...

> +#define CCSIDR_NUMSETS_SHIFT	13
> +#define CCSIDR_NUMSETS_MASK	0x7FF
> +
> +/*
> + * Which cache CCSIDR represents depends on CSSELR value
> + * Make sure no one else changes CSSELR during this
> + * smp_call_function_single prevents preemption for us
> + */
> +static inline u32 get_ccsidr(u32 csselr)
> +{
> +	u32 ccsidr;
> +
> +	/* Put value into CSSELR */
> +	asm volatile("msr csselr_el1, %x0" : : "r" (csselr));

This looks a little dodgy. I think GCC can leave the upper 32 bits in a
random state. Why not cast csselr to a u64 here?

> +	isb();
> +	/* Read result out of CCSIDR */
> +	asm volatile("mrs %0, ccsidr_el1" : "=r" (ccsidr));
> +
> +	return ccsidr;

Similarly it might make sense to make the temporary variable a u64.

[...]

> +int init_cache_level(unsigned int cpu)
> +{
> +	unsigned int ctype, level = 1, leaves = 0;
> +	struct cpu_cacheinfo *this_cpu_ci = get_cpu_cacheinfo(cpu);
> +
> +	if (!this_cpu_ci)
> +		return -EINVAL;
> +
> +	do {
> +		ctype = get_cache_type(level);
> +		if (ctype == CACHE_TYPE_NOCACHE)
> +			break;
> +		/* Separate instruction and data caches */
> +		leaves += (ctype == CACHE_TYPE_SEPARATE) ? 2 : 1;
> +	} while (++level <= MAX_CACHE_LEVEL);

I think this would be clearer with:

for (level = 1; level <= MAX_CACHE_LEVEL; level++)

We do something like that in populate_cache_leaves below.

> +
> +	this_cpu_ci->num_levels = level - 1;
> +	this_cpu_ci->num_leaves = leaves;
> +	return 0;
> +}
> +
> +int populate_cache_leaves(unsigned int cpu)
> +{
> +	unsigned int level, idx;
> +	enum cache_type type;
> +	struct cpu_cacheinfo *this_cpu_ci = get_cpu_cacheinfo(cpu);
> +	struct cacheinfo *this_leaf = this_cpu_ci->info_list;
> +
> +	for (idx = 0, level = 1; level <= this_cpu_ci->num_levels &&
> +	     idx < this_cpu_ci->num_leaves; idx++, level++) {
> +		if (!this_leaf)
> +			return -EINVAL;
> +
> +		type = get_cache_type(level);
> +		if (type == CACHE_TYPE_SEPARATE) {
> +			ci_leaf_init(this_leaf++, CACHE_TYPE_DATA, level);
> +			ci_leaf_init(this_leaf++, CACHE_TYPE_INST, level);
> +		} else {
> +			ci_leaf_init(this_leaf++, type, level);
> +		}
> +	}
> +	return 0;
> +}

Cheers,
Mark.
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