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Message-ID: <03ed6449-eb57-4a55-b2bf-ecbb9787feca@kernel.org>
Date: Thu, 28 Mar 2024 16:23:22 +0900
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Subject: Re: [PATCH v2] scsi: libsas: Allocation SMP request is aligned to
 ARCH_DMA_MINALIGN
To: Yihang Li <liyihang9@huawei.com>, Christoph Hellwig <hch@infradead.org>,
 John Garry <john.g.garry@oracle.com>
Cc: yanaijie@huawei.com, jejb@linux.ibm.com, martin.petersen@oracle.com,
 chenxiang66@hisilicon.com, linux-scsi@vger.kernel.org,
 linux-kernel@vger.kernel.org, linuxarm@huawei.com, prime.zeng@huawei.com,
 yangxingui@huawei.com
References: <20240326124358.2466259-1-liyihang9@huawei.com>
 <5b5b9392-7fd2-4c87-8e41-5e54adf20003@kernel.org>
 <0ba9914d-7060-498a-beac-2b19770e1963@oracle.com>
 <ZgUPpwhkE9bRwHec@infradead.org>
 <75df3e2d-10c3-5370-3cd8-fe2fb0ff2acc@huawei.com>
From: Damien Le Moal <dlemoal@kernel.org>
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Organization: Western Digital Research
In-Reply-To: <75df3e2d-10c3-5370-3cd8-fe2fb0ff2acc@huawei.com>
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On 3/28/24 15:59, Yihang Li wrote:
> 
> 
> On 2024/3/28 14:35, Christoph Hellwig wrote:
>> On Tue, Mar 26, 2024 at 01:32:09PM +0000, John Garry wrote:
>>>>> +	u8 *p;
>>>>> +
>>>>> +	size = ALIGN(size, ARCH_DMA_MINALIGN);
>>>
>>>
>>> If this is a hisi_sas requirement, then why use ARCH_DMA_MINALIGN and not
>>> 16B as minimum alignment?
>>>
>>> Or are we really talking about an arch requirement?
>>
>> One thing is that we should never allocate unaligned memory for
>> anything DMA mapped, or data will be corrupted by non-coherent DMA.
>> So ARCH_DMA_MINALIGN needs to be here.  If specific hardware has
>> further requirements we'll need to communicated it through a field
>> or op vector.
> 
> Got it. Looks like it's still going to be aligned to ARCH_DMA_MINALIGN.

But I thought that the original issue was that some arch have ARCH_DMA_MINALIGN
down to 8B but hisi driver needs at least 16 ?

So in the end, you need something like:

	size = ALIGN(size, max(16, ARCH_DMA_MINALIGN));

no ?

And define a macro for the "16" value to document it.
Something like:

#define SAS_SMP_REQ_ALIGN	16

Not sure about the name... Naming is hard :)

> 
> Thanks,
> Yihang
> 
>>
>>
>> .
>>

-- 
Damien Le Moal
Western Digital Research