Subject: [PATCH] arm64: Implement cache_line_size() based on CTR_EL0.CWG

From: [email-masked] (Catalin Marinas)
Date: Fri, 2 May 2014 18:31:21 +0100

On Fri, May 02, 2014 at 06:12:47PM +0100, Will Deacon wrote:
> On Fri, May 02, 2014 at 04:55:32PM +0100, Catalin Marinas wrote:
> > The hardware provides the maximum cache line size in the system via the
> > CTR_EL0.CWG bits. This patch implements the cache_line_size() function
> > to read such information, together with a sanity check if the statically
> > defined L1_CACHE_BYTES is smaller than the hardware value.
> > 
> > Signed-off-by: Catalin Marinas <[email-masked]>
> > ---
> >  arch/arm64/Kconfig                 |  3 +++
> >  arch/arm64/include/asm/cache.h     | 13 ++++++++++++-
> >  arch/arm64/include/asm/cachetype.h | 11 +++++++++++
> >  arch/arm64/kernel/setup.c          | 15 +++++++++++++++
> >  4 files changed, 41 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index e759af5d7098..9a5b5fea86ba 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -242,6 +242,9 @@ config ARCH_WANT_HUGE_PMD_SHARE
> >  config HAVE_ARCH_TRANSPARENT_HUGEPAGE
> >  	def_bool y
> >  
> > +config ARCH_HAS_CACHE_LINE_SIZE
> > +	def_bool y
> > +
> >  source "mm/Kconfig"
> >  
> >  config XEN_DOM0
> > diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
> > index 390308a67f0d..88cc05b5f3ac 100644
> > --- a/arch/arm64/include/asm/cache.h
> > +++ b/arch/arm64/include/asm/cache.h
> > @@ -16,6 +16,8 @@
> >  #ifndef __ASM_CACHE_H
> >  #define __ASM_CACHE_H
> >  
> > +#include <asm/cachetype.h>
> > +
> >  #define L1_CACHE_SHIFT		6
> >  #define L1_CACHE_BYTES		(1 << L1_CACHE_SHIFT)
> >  
> > @@ -27,6 +29,15 @@
> >   * the CPU.
> >   */
> >  #define ARCH_DMA_MINALIGN	L1_CACHE_BYTES
> > -#define ARCH_SLAB_MINALIGN	8
> > +
> > +#ifndef __ASSEMBLY__
> > +
> > +static inline int cache_line_size(void)
> > +{
> > +	u32 cwg = cache_type_cwg();
> > +	return cwg ? 4 << cwg : L1_CACHE_BYTES;
> > +}
> 
> Hmmm, but the CWG is not the same thing as the L1 cache line size, so
> something is amiss here. If I have an L2 cache with bigger lines, then
> reporting L1_CACHE_BYTES is wrong here.

L1 is usually DMinSize while CWG is the maximum in the system. If a
system has a system cache, it should be covered by CWG (especially those
semi-transparent system caches).

Using this macro is misleading indeed but Linux has lots of assumptions
about the cache line size and only using L1_CACHE_BYTES, including cache
line aligned sections. I consider L1 in this context to actually mean
any cache line flushed by the standard DC instructions.

-- 
Catalin