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Date:   Thu, 22 Oct 2020 17:53:39 -0700
From:   Jaegeuk Kim <jaegeuk@kernel.org>
To:     Can Guo <cang@codeaurora.org>
Cc:     linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com,
        Alim Akhtar <alim.akhtar@samsung.com>,
        Avri Altman <avri.altman@wdc.com>
Subject: Re: [PATCH v2 5/5] scsi: ufs: fix clkgating on/off correctly
Message-ID: <20201022201825.GA3329812@google.com>
References: <20201020195258.2005605-1-jaegeuk@kernel.org>
 <20201020195258.2005605-6-jaegeuk@kernel.org>
 <2a8ecc4185b3a5411077f4e3fc66000f@codeaurora.org>
 <20201021045213.GB3004521@google.com>
 <e3e58a89474d23f1b9446fe2e38a7426@codeaurora.org>
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Precedence: bulk

On 10/21, Can Guo wrote:
> On 2020-10-21 12:52, jaegeuk@kernel.org wrote:
> > 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.
> > 
> 
> If clocks cannot be gated after the last task is cleared from UFS host, then
> clk gating
> is broken, no? Assume UFS has completed the last task in its queue, as this
> change says,
> ufshcd_any_tag_in_use() is preventing ufshcd_release() from invoking
> gate_work().
> Similarly, ufshcd_any_tag_in_use() can prevent gate_work() from doing its
> real work -
> disabling the clocks. Do you agree?
> 
>         if (hba->clk_gating.active_reqs
>                 || hba->ufshcd_state != UFSHCD_STATE_OPERATIONAL
>                 || ufshcd_any_tag_in_use(hba) || hba->outstanding_tasks
>                 || hba->active_uic_cmd || hba->uic_async_done)
>                 goto rel_lock;

I see the point, but this happens only when clkgate_delay_ms is too short
to give enough time for releasing tag. If it's correctly set, I think there'd
be no problem, unless softirq was delayed by other RT threads which is just
a corner case tho.

> 
> Thanks,
> 
> Can Guo.
> 
> > > 
> > > 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@samsung.com>
> > > > Cc: Avri Altman <avri.altman@wdc.com>
> > > > Cc: Can Guo <cang@codeaurora.org>
> > > > Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.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;