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

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Message-ID: <9337b005-f468-471c-97e1-f4059ed4283d@acm.org>
Date: Thu, 23 Jan 2025 10:02:32 -0800
From: Bart Van Assche <bvanassche@....org>
To: Ziqi Chen <quic_ziqichen@...cinc.com>, 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 1/22/25 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.