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

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Message-ID: <20201021045213.GB3004521@google.com>
Date:   Tue, 20 Oct 2020 21:52:13 -0700
From:   jaegeuk@...nel.org
To:     Can Guo <cang@...eaurora.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 10/21, Can Guo wrote:
> 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?

I don't think this breaks clkgating, but fix the wrong condition check which
prevented gate_work at all. As you mentioned, even if this schedules gate_work
by racy conditions, gate_work will handle it as a last resort.

> 
> 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;