linux-kernel - Re: [PATCH v3] scsi: ufs: core: skip UFS clkscale if host asynchronous scan in progress

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Message-ID: <5748d0cc-a603-4b44-bbfc-d39d684b2ea6@quicinc.com>
Date: Fri, 9 May 2025 13:02:46 +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>, <neil.armstrong@...aro.org>,
        <luca.weiss@...rphone.com>, <konrad.dybcio@....qualcomm.com>,
        <peter.wang@...iatek.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>,
        Manivannan Sadhasivam
	<manivannan.sadhasivam@...aro.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] scsi: ufs: core: skip UFS clkscale if host
 asynchronous scan in progress



On 5/9/2025 12:06 AM, Bart Van Assche wrote:
> On 5/8/25 2:38 AM, Ziqi Chen wrote:
>> diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
>> index 1c53ccf5a616..04f40677e76a 100644
>> --- a/drivers/ufs/core/ufshcd.c
>> +++ b/drivers/ufs/core/ufshcd.c
>> @@ -1207,6 +1207,9 @@ static bool 
>> ufshcd_is_devfreq_scaling_required(struct ufs_hba *hba,
>>       if (list_empty(head))
>>           return false;
>> +    if (hba->host->async_scan)
>> +        return false;
> 
> Testing a boolean is never a proper way to synchronize code sections.
> As an example, the SCSI core could set hba->host->async_scan after this
> check completed and before the code below is executed. I think we need a
> better solution.

Hi Bart,

I get your point, we have also taken this into consideration. That's why
we move ufshcd_devfreq_init() out of ufshd_add_lus().

Old sequence:

| ufshcd_async_scan()
   |ufshcd_add_lus()
     |ufshcd_devfreq_init()
     |  | enable UFS clock scaling
     |scsi_scan_host()
        |scsi_prep_async_scan()
        |    | set host->async_scan to '1'
        |async_schedule(do_scan_async, data)

With this old sequence , The ufs devfreq monitor started before the
scsi_prep_async_scan(),  the SCSI core could set hba->host->async_scan
after this check.

New sequence:

| ufshcd_async_scan()
   |ufshcd_add_lus()
   | |scsi_scan_host()
   |    |scsi_prep_async_scan()
   |    |    | set host->async_scan to '1'
   |    |async_schedule(do_scan_async, data)
   |ufshcd_devfreq_init()
   |    |enable UFS clock scaling

With the new sequence , it is guaranteed that host->async_scan
is set before the UFS clock scaling enabling.

I guess you might be worried about out-of-order execution will
cause this flag not be set before clock scaling enabling with
extremely low probability?
If yes, do you have any suggestion on this ?


BRs,
Ziqi


> 
> Bart.