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

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Message-ID: <ccf9079dc1767c7d200fe55b5a849ba0@codeaurora.org>
Date:   Mon, 26 Oct 2020 11:12:00 +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 2020-10-23 08:53, Jaegeuk Kim wrote:
> On 10/21, Can Guo wrote:
>> On 2020-10-21 12:52, jaegeuk@...nel.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.
> 

Yes, we are fixing corner cases, aren't we? I thought you would like to
address it since you are fixing clk gating.

Regards,

Can Guo.

>> 
>> 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@...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;