linux-kernel - Re: [PATCH v2] arm64: mm: convert __dma

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Date:	Fri, 29 Jul 2016 18:06:00 +0100
From:	Robin Murphy <robin.murphy@....com>
To:	"kwangwoo.lee@...com" <kwangwoo.lee@...com>,
	Russell King - ARM Linux <linux@...linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Mark Rutland <mark.rutland@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Cc:	"hyunchul3.kim@...com" <hyunchul3.kim@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"woosuk.chung@...com" <woosuk.chung@...com>
Subject: Re: [PATCH v2] arm64: mm: convert __dma_* routines to use start, size

On 28/07/16 01:08, kwangwoo.lee@...com wrote:
>> -----Original Message-----
>> From: Robin Murphy [mailto:robin.murphy@....com]
>> 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@...ts.infradead.org
>> Cc: 김현철(KIM HYUNCHUL) MS SW; linux-kernel@...r.kernel.org; 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@...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.

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?
> 
> /*
> - *     __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
>