linux-kernel - Re: [PATCH v4] scsi: libsas: Allocation SMP request is aligned to ARCH_DMA

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Message-ID: <225d75ff-c7a6-45ee-a149-51eb720c5420@oracle.com>
Date: Thu, 28 Mar 2024 09:32:23 +0000
From: John Garry <john.g.garry@...cle.com>
To: Yihang Li <liyihang9@...wei.com>, yanaijie@...wei.com, jejb@...ux.ibm.com,
        martin.petersen@...cle.com, dlemoal@...nel.org,
        chenxiang66@...ilicon.com
Cc: linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com, prime.zeng@...wei.com, yangxingui@...wei.com
Subject: Re: [PATCH v4] scsi: libsas: Allocation SMP request is aligned to
 ARCH_DMA_MINALIGN

On 28/03/2024 09:06, Yihang Li wrote:
> This series [1] reducing the kmalloc() minimum alignment on arm64 to 8
> (from 128). In libsas, this will cause SMP requests to be 8-byte-aligned
> through kmalloc() allocation. However, for the hisi_sas hardware, all
> commands address must be 16-byte-aligned. Otherwise, the commands fail to
> be executed.
> 
> ARCH_DMA_MINALIGN represents the minimum (static) alignment for safe DMA
> operations, so use ARCH_DMA_MINALIGN as the alignment for SMP request.

I'd say that we must align to at least ARCH_DMA_MINALIGN for memory 
which we want to map for DMA. ARCH_DMA_MINALIGN would never be less than 
the 16B requirement for hisi_sas, so just align to ARCH_DMA_MINALIGN.

> 
> Link: https://urldefense.com/v3/__https://lkml.kernel.org/r/20230612153201.554742-1-catalin.marinas@arm.com__;!!ACWV5N9M2RV99hQ!LbkCJSfklWds8mk0UGjtg0qRNpWuAc-ibudDMl7sI_Qip4Hd6WNqHZmRbuJfObd4fcZXj0sC8nGTs1UbA5PlCA$  [1]
> Signed-off-by: Yihang Li <liyihang9@...wei.com>
> Reviewed-by: Damien Le Moal <dlemoal@...nel.org>

Reviewed-by: John Garry <john.g.garry@...cle.com>

> ---
> Changes since v3:
> - Still aligned to ARCH_DMA_MINALIGN for safe DMA operations.
> 
> Changes since v2:
> - Use 16B as alignment for SMP requests instead of ARCH_DMA_MINALIGN.
> 
> Changes since v1:
> - Directly modify alloc_smp_req() instead of using handler callback.
> ---
>   drivers/scsi/libsas/sas_expander.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/libsas/sas_expander.c b/drivers/scsi/libsas/sas_expander.c
> index a2204674b680..c989d182fc75 100644
> --- a/drivers/scsi/libsas/sas_expander.c
> +++ b/drivers/scsi/libsas/sas_expander.c
> @@ -135,7 +135,7 @@ static int smp_execute_task(struct domain_device *dev, void *req, int req_size,
>   
>   static inline void *alloc_smp_req(int size)
>   {
> -	u8 *p = kzalloc(size, GFP_KERNEL);
> +	u8 *p = kzalloc(ALIGN(size, ARCH_DMA_MINALIGN), GFP_KERNEL);
>   	if (p)
>   		p[0] = SMP_REQUEST;
>   	return p;