linux-kernel - Re: [PATCH v2] scsi: libsas: Allocation SMP request is aligned to ARCH_DMA

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Date: Thu, 28 Mar 2024 16:23:22 +0900
From: Damien Le Moal <dlemoal@...nel.org>
To: Yihang Li <liyihang9@...wei.com>, Christoph Hellwig <hch@...radead.org>,
 John Garry <john.g.garry@...cle.com>
Cc: yanaijie@...wei.com, jejb@...ux.ibm.com, martin.petersen@...cle.com,
 chenxiang66@...ilicon.com, linux-scsi@...r.kernel.org,
 linux-kernel@...r.kernel.org, linuxarm@...wei.com, prime.zeng@...wei.com,
 yangxingui@...wei.com
Subject: Re: [PATCH v2] scsi: libsas: Allocation SMP request is aligned to
 ARCH_DMA_MINALIGN

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