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

Can Guo
2020-10-26T03:12:04Z

On 2020-10-23 08:53, Jaegeuk Kim wrote:
> 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.
>

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@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;