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List:       linux-kernel
Subject:    RE: [PATCH v2] arm64: mm: convert __dma_* routines to use start, size
From:       "kwangwoo.lee () sk ! com" <kwangwoo ! lee () sk ! com>
Date:       2016-07-31 23:45:46
Message-ID: 1c4ec01bd3594485b6a4b386994dc6ca () nmail01 ! hynixad ! com
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Hi Robin,

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy@arm.com]
> Sent: Saturday, July 30, 2016 2:06 AM
> To: À̱¤¿ì(LEE KWANGWOO) MS SW; Russell King - ARM Linux; Catalin Marinas; Will \
> Deacon; Mark Rutland; linux-arm-kernel@lists.infradead.org
> Cc: ±èÇöö(KIM HYUNCHUL) MS SW; linux-kernel@vger.kernel.org; Á¤¿ì¼®(CHUNG WOO SUK) \
>                 MS SW
> Subject: Re: [PATCH v2] arm64: mm: convert __dma_* routines to use start, size
> 
> On 28/07/16 01:08, kwangwoo.lee@sk.com wrote:
> > > -----Original Message-----
> > > From: Robin Murphy [mailto:robin.murphy@arm.com]
> > > Sent: Wednesday, July 27, 2016 6:56 PM
> > > To: À̱¤¿ì(LEE KWANGWOO) MS SW; Russell King - ARM Linux; Catalin Marinas; Will \
> > > Deacon; Mark Rutland; linux-arm-kernel@lists.infradead.org
> > > Cc: ±èÇöö(KIM HYUNCHUL) MS SW; linux-kernel@vger.kernel.org; Á¤¿ì¼®(CHUNG WOO \
> > >                 SUK) MS SW
> > > Subject: Re: [PATCH v2] arm64: mm: convert __dma_* routines to use start, size
> > > 
> > > On 27/07/16 02:55, kwangwoo.lee@sk.com wrote:
> > > [...]
> > > > > > /*
> > > > > > - *	__dma_clean_range(start, end)
> > > > > > + *	__dma_clean_area(start, size)
> > > > > > *	- start   - virtual start address of region
> > > > > > - *	- end     - virtual end address of region
> > > > > > + *	- size    - size in question
> > > > > > */
> > > > > > -__dma_clean_range:
> > > > > > -	dcache_line_size x2, x3
> > > > > > -	sub	x3, x2, #1
> > > > > > -	bic	x0, x0, x3
> > > > > > -1:
> > > > > > +__dma_clean_area:
> > > > > > alternative_if_not ARM64_WORKAROUND_CLEAN_CACHE
> > > > > > -	dc	cvac, x0
> > > > > > +	dcache_by_line_op cvac, sy, x0, x1, x2, x3
> > > > > > alternative_else
> > > > > > -	dc	civac, x0
> > > > > > +	dcache_by_line_op civac, sy, x0, x1, x2, x3
> > > > > 
> > > > > dcache_by_line_op is a relatively large macro - is there any way we can
> > > > > still apply the alternative to just the one instruction which needs it,
> > > > > as opposed to having to patch the entire mostly-identical routine?
> > > > 
> > > > I agree with your opinion. Then, how do you think about using CONFIG_* \
> > > > options like below? I think that alternative_* macros seems to keep the space \
> > > > for unused instruction. Is it necessary? Please, share your thought about the
> > > > space. Thanks!
> > > > 
> > > > +__dma_clean_area:
> > > > +#if    defined(CONFIG_ARM64_ERRATUM_826319) || \
> > > > +       defined(CONFIG_ARM64_ERRATUM_827319) || \
> > > > +       defined(CONFIG_ARM64_ERRATUM_824069) || \
> > > > +       defined(CONFIG_ARM64_ERRATUM_819472)
> > > > +       dcache_by_line_op civac, sy, x0, x1, x2, x3
> > > > +#else
> > > > +       dcache_by_line_op cvac, sy, x0, x1, x2, x3
> > > > +#endif
> > > 
> > > That's not ideal, because we still only really want to use the
> > > workaround if we detect a CPU which needs it, rather than baking it in
> > > at compile time. I was thinking more along the lines of pushing the
> > > alternative down into dcache_by_line_op, something like the idea below
> > > (compile-tested only, may not actually be viable).
> > 
> > OK. Using the capability of CPU features seems to be preferred.
> > 
> > > Robin.
> > > 
> > > -----8<-----
> > > diff --git a/arch/arm64/include/asm/assembler.h
> > > b/arch/arm64/include/asm/assembler.h
> > > index 10b017c4bdd8..1c005c90387e 100644
> > > --- a/arch/arm64/include/asm/assembler.h
> > > +++ b/arch/arm64/include/asm/assembler.h
> > > @@ -261,7 +261,16 @@ lr	.req	x30		// link register
> > > 	add	\size, \kaddr, \size
> > > 	sub	\tmp2, \tmp1, #1
> > > 	bic	\kaddr, \kaddr, \tmp2
> > > -9998:	dc	\op, \kaddr
> > > +9998:
> > > +	.ifeqs "\op", "cvac"
> > > +alternative_if_not ARM64_WORKAROUND_CLEAN_CACHE
> > > +	dc	cvac, \kaddr
> > > +alternative_else
> > > +	dc	civac, \kaddr
> > > +alternative_endif
> > > +	.else
> > > +	dc	\op, \kaddr
> > > +	.endif
> > > 	add	\kaddr, \kaddr, \tmp1
> > > 	cmp	\kaddr, \size
> > > 	b.lo	9998b
> > 
> > I agree that it looks not viable because it makes the macro bigger and
> > conditional specifically with CVAC op.
> 
> Actually, having had a poke around in the resulting disassembly, it
> looks like this does work correctly. I can't think of a viable reason
> for the whole dcache_by_line_op to ever be wrapped in yet another
> alternative (which almost certainly would go horribly wrong), and it
> would mean that any other future users are automatically covered for
> free. It's just horrible to look at at the source level.

Then, Are you going to send a patch for this? Or should I include this change?

> Robin.
> 
> > 
> > Then.. if the number of the usage of alternative_* macros for erratum is
> > few (just one in this case for cache clean), I think only small change like
> > below seems to be optimal and there is no need to create a variant macro of
> > dcache_cache_by_line_op. How do you think about it?
[...]

Regards,
Kwangwoo


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