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Subject: Re: [PATCH] scsi: libsas: Add SMP request allocation handler callback
To: Damien Le Moal <dlemoal@kernel.org>, Yihang Li <liyihang9@huawei.com>,
	<jejb@linux.ibm.com>, <martin.petersen@oracle.com>,
	<john.g.garry@oracle.com>, <chenxiang66@hisilicon.com>
CC: <linux-scsi@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<linuxarm@huawei.com>, <prime.zeng@huawei.com>, <yangxingui@huawei.com>
References: <20240325131751.1840329-1-liyihang9@huawei.com>
 <da4de4e0-6d13-4509-b288-a9d122d1c2d3@kernel.org>
From: Jason Yan <yanaijie@huawei.com>
Message-ID: <e7612e81-dc44-7bc6-3bd2-0b71367408ec@huawei.com>
Date: Mon, 25 Mar 2024 22:21:47 +0800
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On 2024/3/25 22:03, Damien Le Moal wrote:
> On 3/25/24 22:17, Yihang Li wrote:
>> This series [1] reducing the kmalloc() minimum alignment on arm64 to 8
>> (from 128).
> 
> And ? What is the point you are trying to convey here ?
> 
>> The hisi_sas has special requirements on the memory address alignment
>> (must be 16-byte-aligned) of the command request frame, so add a SMP
>> request allocation callback and fill it in for the hisi_sas driver.
> 
> 128 is aligned to 16. So what is the problem you are trying to solve here ?
> Can you clarify ? I suspect this is all about memory allocation optimization ?

After series [1] been merged, kmalloc is 8-byte-aligned, however 
hisi_sas hardware needs 16-byte-aligned. That's the problem.

> 
>>
>> Link: https://lkml.kernel.org/r/20230612153201.554742-1-catalin.marinas@arm.com [1]
>> Signed-off-by: Yihang Li <liyihang9@huawei.com>
>> ---
>>   drivers/scsi/hisi_sas/hisi_sas_main.c | 14 ++++++++++++
>>   drivers/scsi/libsas/sas_expander.c    | 31 ++++++++++++++++++---------
>>   include/scsi/libsas.h                 |  3 +++
>>   3 files changed, 38 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/scsi/hisi_sas/hisi_sas_main.c b/drivers/scsi/hisi_sas/hisi_sas_main.c
>> index 097dfe4b620d..40329558d435 100644
>> --- a/drivers/scsi/hisi_sas/hisi_sas_main.c
>> +++ b/drivers/scsi/hisi_sas/hisi_sas_main.c
>> @@ -2031,6 +2031,19 @@ static int hisi_sas_write_gpio(struct sas_ha_struct *sha, u8 reg_type,
>>   				reg_index, reg_count, write_data);
>>   }
>>   
>> +static void *hisi_sas_alloc_smp_req(int size)
>> +{
>> +	u8 *p;
>> +
>> +	/* The address must be 16-byte-aligned. */
> 
> ARCH_DMA_MINALIGN is not always 16, right ?
> 
>> +	size = ALIGN(size, ARCH_DMA_MINALIGN);
>> +	p = kzalloc(size, GFP_KERNEL);
>> +	if (p)
>> +		p[0] = SMP_REQUEST;
>> +
>> +	return p;
>> +}
>> +
>>   static void hisi_sas_phy_disconnected(struct hisi_sas_phy *phy)
>>   {
>>   	struct asd_sas_phy *sas_phy = &phy->sas_phy;
>> @@ -2130,6 +2143,7 @@ static struct sas_domain_function_template hisi_sas_transport_ops = {
>>   	.lldd_write_gpio	= hisi_sas_write_gpio,
>>   	.lldd_tmf_aborted	= hisi_sas_tmf_aborted,
>>   	.lldd_abort_timeout	= hisi_sas_internal_abort_timeout,
>> +	.lldd_alloc_smp_req	= hisi_sas_alloc_smp_req,
> 
> Why this complexity ? Why not simply modify alloc_smp_req() to have the required
> alignment ? This will avoid a costly indirect function call.

Yeah, I think it's simpler to modify alloc_smp_req() directly too. 
Yihang, Can you please cook a new one?

Thansk,
Jason