linux-kernel - Re: [PATCH] arm64: mm: take CWG into account in __inval_cache

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Message-ID: <20160419153238.GE8482@e104818-lin.cambridge.arm.com>
Date:	Tue, 19 Apr 2016 16:32:38 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Mark Rutland <mark.rutland@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	James Morse <james.morse@....com>, robin.murphy@....com,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: mm: take CWG into account in __inval_cache_range()

On Tue, Apr 19, 2016 at 04:48:32PM +0200, Ard Biesheuvel wrote:
> On 19 April 2016 at 16:13, Catalin Marinas <catalin.marinas@....com> wrote:
> > The best we could do is to warn if ARCH_DMA_MINALIGN is smaller than CWG
> > (as Robin suggested, we could do this only if we have non-coherent DMA
> > masters via arch_setup_dma_ops()). Quick hack below:
> >
> > -------------------------------8<-----------------------------------
> > diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
> > index 5082b30bc2c0..5967fcbb617a 100644
> > --- a/arch/arm64/include/asm/cache.h
> > +++ b/arch/arm64/include/asm/cache.h
> > @@ -28,7 +28,7 @@
> >   * cache before the transfer is done, causing old data to be seen by
> >   * the CPU.
> >   */
> > -#define ARCH_DMA_MINALIGN      L1_CACHE_BYTES
> > +#define ARCH_DMA_MINALIGN      128
> >
> >  #ifndef __ASSEMBLY__
> >
> > @@ -37,7 +37,7 @@
> >  static inline int cache_line_size(void)
> >  {
> >         u32 cwg = cache_type_cwg();
> > -       return cwg ? 4 << cwg : L1_CACHE_BYTES;
> > +       return cwg ? 4 << cwg : ARCH_DMA_MINALIGN;
> 
> Unrelated, but this does not look right: if the CWG field is zero, we
> should either assume 2 KB, or iterate over all the CCSIDR values and
> take the maximum linesize.

It may be a better guess but even that is not always relevant since
CCSIDR may not present the real hardware information. It's only meant to
give enough information to be able to do cache maintenance by set/way
and we've seen CPU implementations where this has nothing to do with the
actual cache geometry.

So I don't think we can do anything more than just hard-coding and hope
that implementations where CWG is 0 (or higher than 128) are only
deployed in a fully coherent configuration.

-- 
Catalin