linux-kernel - Re: [PATCH v2 5/8] scsi: ufs: core: Enable multi-level gear scaling

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Message-ID: <929ee551-e7ed-4dbc-9c9a-b2b02585a960@quicinc.com>
Date: Fri, 24 Jan 2025 10:41:41 +0800
From: Ziqi Chen <quic_ziqichen@...cinc.com>
To: Bart Van Assche <bvanassche@....org>, <quic_cang@...cinc.com>,
        <mani@...nel.org>, <beanhuo@...ron.com>, <avri.altman@....com>,
        <junwoo80.lee@...sung.com>, <martin.petersen@...cle.com>,
        <quic_nguyenb@...cinc.com>, <quic_nitirawa@...cinc.com>,
        <quic_rampraka@...cinc.com>
CC: <linux-arm-msm@...r.kernel.org>, <linux-scsi@...r.kernel.org>,
        Alim Akhtar
	<alim.akhtar@...sung.com>,
        "James E.J. Bottomley"
	<James.Bottomley@...senPartnership.com>,
        Peter Wang
	<peter.wang@...iatek.com>,
        Manivannan Sadhasivam
	<manivannan.sadhasivam@...aro.org>,
        Andrew Halaney <ahalaney@...hat.com>,
        Maramaina Naresh <quic_mnaresh@...cinc.com>,
        open list
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/8] scsi: ufs: core: Enable multi-level gear scaling



On 1/24/2025 2:02 AM, Bart Van Assche wrote:
> On 1/22/25 11:41 PM, Ziqi Chen wrote:
>> We use memcpy() here is due to memcpy() can be faster than direct 
>> assignment. We don't worry about safety because they are same struct 
>> "ufs_pa_layer_attr" so that we can ensure the accuracy of number of 
>> bytes and member type.
> 
> The memcpy() call we are discussing is not in the hot path so it doesn't
> have to be hyper-optimized. Making the compiler perform type checking is
> more important in this code path than micro-optimizing the code.
> 
> Additionally, please do not try to be smarter than the compiler. 
> Compilers are able to convert struct assignments into a memcpy() call if
> there are good reasons to assume that the memcpy() call will be faster.
> 
> Given the small size of struct ufs_pa_layer_attr (7 * 4 = 28 bytes),
> memberwise assignment probably is faster than a memcpy() call. The trunk
> version of gcc (ARM64) translates a memberwise assignment of struct 
> ufs_pa_layer_attr into the following four assembler instructions (x0 and
> x1 point to struct ufs_pa_layer_attr instances, q30 and q31 are 128 bit
> registers):
> 
>          ldr     q30, [x1]
>          ldr     q31, [x1, 12]
>          str     q30, [x0]
>          str     q31, [x0, 12]
> 
> Thanks,
> 
> Bart.
> 
Sure , Let me try and test it. If works fine , I will update in next 
version.

-Ziqi