linux-kernel - Re: [PATCH v2 5/5] scsi: ufs: fix clkgating on/off correctly

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Date:   Wed, 21 Oct 2020 10:00:52 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>
Subject: Re: [PATCH v2 5/5] scsi: ufs: fix clkgating on/off correctly

On 2020-10-21 03:52, Jaegeuk Kim wrote:
> The below call stack prevents clk_gating at every IO completion.
> We can remove the condition, ufshcd_any_tag_in_use(), since 
> clkgating_work
> will check it again.
> 

I think checking ufshcd_any_tag_in_use() in either ufshcd_release() or
gate_work() can break UFS clk gating's functionality.

ufshcd_any_tag_in_use() was introduced to replace hba->lrb_in_use. 
However,
they are not exactly same - ufshcd_any_tag_in_use() returns true if any 
tag
assigned from block layer is still in use, but tags are released 
asynchronously
(through block softirq), meaning it does not reflect the real occupation 
of UFS host.
That is after UFS host finishes all tasks, ufshcd_any_tag_in_use() can 
still return true.

This change only removes the check of ufshcd_any_tag_in_use() in 
ufshcd_release(),
but having the check of it in gate_work() can still prevent gating from 
happening.
The current change works for you maybe because the tags are release 
before
hba->clk_gating.delay_ms expires, but if hba->clk_gating.delay_ms is 
shorter or
somehow block softirq is retarded, gate_work() may have chance to see 
ufshcd_any_tag_in_use()
returns true. What do you think?

Thanks,

Can Guo.

In __ufshcd_transfer_req_compl
Ihba->lrb_in_use is cleared immediately when UFS driver
finishes all tasks

> ufshcd_complete_requests(struct ufs_hba *hba)
>   ufshcd_transfer_req_compl()
>     __ufshcd_transfer_req_compl()
>       __ufshcd_release(hba)
>         if (ufshcd_any_tag_in_use() == 1)
>            return;
>   ufshcd_tmc_handler(hba);
>     blk_mq_tagset_busy_iter();
> 
> Cc: Alim Akhtar <alim.akhtar@...sung.com>
> Cc: Avri Altman <avri.altman@....com>
> Cc: Can Guo <cang@...eaurora.org>
> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> ---
>  drivers/scsi/ufs/ufshcd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> index b5ca0effe636..cecbd4ace8b4 100644
> --- a/drivers/scsi/ufs/ufshcd.c
> +++ b/drivers/scsi/ufs/ufshcd.c
> @@ -1746,7 +1746,7 @@ static void __ufshcd_release(struct ufs_hba *hba)
> 
>  	if (hba->clk_gating.active_reqs || hba->clk_gating.is_suspended ||
>  	    hba->ufshcd_state != UFSHCD_STATE_OPERATIONAL ||
> -	    ufshcd_any_tag_in_use(hba) || hba->outstanding_tasks ||
> +	    hba->outstanding_tasks ||
>  	    hba->active_uic_cmd || hba->uic_async_done)
>  		return;