RE: [PATCH v2] arm64: mm: convert __dma_* routines to use start, size

[kwangwoo.lee@xxxxxx]

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy@xxxxxxx]
> Sent: Wednesday, July 27, 2016 6:56 PM
> To: 이광우(LEE KWANGWOO) MS SW; Russell King - ARM Linux; Catalin Marinas; Will Deacon; Mark Rutland;
> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: 김현철(KIM HYUNCHUL) MS SW; linux-kernel@xxxxxxxxxxxxxxx; 정우석(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@xxxxxx 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.

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?

/*
- *     __dma_clean_range(start, end)
+ *     __clean_dcache_area_poc(kaddr, size)
+ *
+ *     Ensure that any D-cache lines for the interval [kaddr, kaddr+size)
+ *     are cleaned to the PoC.
+ *
+ *     - kaddr   - kernel address
+ *     - size    - size in question
+ */
+ENTRY(__clean_dcache_area_poc)
+       /* FALLTHROUGH */
+
+/*
+ *     __dma_clean_area(start, size)
  *     - start   - virtual start address of region
- *     - end     - virtual end address of region
+ *     - size    - size in question
  */
-__dma_clean_range:
+__dma_clean_area:
+       add     x1, x1, x0
        dcache_line_size x2, x3
        sub     x3, x2, #1
        bic     x0, x0, x3
@@ -158,24 +172,21 @@ alternative_endif
        b.lo    1b
        dsb     sy
        ret
-ENDPROC(__dma_clean_range)
+ENDPIPROC(__clean_dcache_area_poc)
+ENDPROC(__dma_clean_area)

Regards,
Kwangwoo Lee