Re: [PATCH] ata,scsi: do not issue START STOP UNIT on resume

[Damien Le Moal <dlemoal@kernel.org>]

On 9/13/23 19:34, Geert Uytterhoeven wrote:
> Hi Damien,
> 
> On Wed, Sep 13, 2023 at 12:21 PM Geert Uytterhoeven
> <geert@linux-m68k.org> wrote:
>> On Wed, Sep 13, 2023 at 12:58 AM Damien Le Moal <dlemoal@kernel.org> wrote:
>>> On 9/13/23 02:39, Geert Uytterhoeven wrote:
>>>> On Mon, 31 Jul 2023, Damien Le Moal wrote:
>>>>> During system resume, ata_port_pm_resume() triggers ata EH to
>>>>> 1) Resume the controller
>>>>> 2) Reset and rescan the ports
>>>>> 3) Revalidate devices
>>>>> This EH execution is started asynchronously from ata_port_pm_resume(),
>>>>> which means that when sd_resume() is executed, none or only part of the
>>>>> above processing may have been executed. However, sd_resume() issues a
>>>>> START STOP UNIT to wake up the drive from sleep mode. This command is
>>>>> translated to ATA with ata_scsi_start_stop_xlat() and issued to the
>>>>> device. However, depending on the state of execution of the EH process
>>>>> and revalidation triggerred by ata_port_pm_resume(), two things may
>>>>> happen:
>>>>> 1) The START STOP UNIT fails if it is received before the controller has
>>>>>   been reenabled at the beginning of the EH execution. This is visible
>>>>>   with error messages like:
>>>>>
>>>>> ata10.00: device reported invalid CHS sector 0
>>>>> sd 9:0:0:0: [sdc] Start/Stop Unit failed: Result: hostbyte=DID_OK driverbyte=DRIVER_OK
>>>>> sd 9:0:0:0: [sdc] Sense Key : Illegal Request [current]
>>>>> sd 9:0:0:0: [sdc] Add. Sense: Unaligned write command
>>>>> sd 9:0:0:0: PM: dpm_run_callback(): scsi_bus_resume+0x0/0x90 returns -5
>>>>> sd 9:0:0:0: PM: failed to resume async: error -5
>>>>>
>>>>> 2) The START STOP UNIT command is received while the EH process is
>>>>>   on-going, which mean that it is stopped and must wait for its
>>>>>   completion, at which point the command is rather useless as the drive
>>>>>   is already fully spun up already. This case results also in a
>>>>>   significant delay in sd_resume() which is observable by users as
>>>>>   the entire system resume completion is delayed.
>>>>>
>>>>> Given that ATA devices will be woken up by libata activity on resume,
>>>>> sd_resume() has no need to issue a START STOP UNIT command, which solves
>>>>> the above mentioned problems. Do not issue this command by introducing
>>>>> the new scsi_device flag no_start_on_resume and setting this flag to 1
>>>>> in ata_scsi_dev_config(). sd_resume() is modified to issue a START STOP
>>>>> UNIT command only if this flag is not set.
>>>>>
>>>>> Reported-by: Paul Ausbeck <paula@soe.ucsc.edu>
>>>>> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=215880
>>>>> Fixes: a19a93e4c6a9 ("scsi: core: pm: Rely on the device driver core for async power management")
>>>>> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
>>>>
>>>> Thanks for your patch, which is now commit 0a8589055936d8fe
>>>> ("ata,scsi: do not issue START STOP UNIT on resume") in v6.5-rc5.
>>>> Sorry for being late to the party, but this commit landed upstream
>>>> during my summer holidays, and apparently I wasn't that focussed on
>>>> noticing small behavioral changes after getting back to work...
>>>>
>>>> I noticed an oddity after s2idle or s2ram on Renesas Salvator-XS (R-Car
>>>> H3 ES2.0) with an old (spinning rust) SATA drive, and bisected it to
>>>> this commit: when accessing the drive after system resume, there is now
>>>> a delay of ca. 5s before data is returned, presumably due to starting
>>>> the drive, and having to wait for it to spin up to be able to read data.
>>>> But the good news is that the actual system resume takes less time than
>>>> before (reduced by even more than ca. 5s!), so this looks like a net
>>>> win...
>>>
>>> Thanks for the report. The delay for the first data access from user space right
>>> after resume is 100% expected, with or without this patch. The reason is that
>>> waking up the drive (spinning it up) is done asynchronously from the PM resume
>>> context, so when you get "PM suspend exit" message signaling that the system is
>>> resumed, the drive may not yet be spinning. Any access will wait for that to
>>> happen before proceeding. Depending on the drive that can take up to 10s or so.
>>
>> That does not match with what I am seeing: before this patch, there
>> was no delay on first data access from user space, as the drive is fully
>> spun up when system resume returns.
>> With this patch, system resume returns earlier, and the drive is only
>> spun up when user space starts accessing data.
>>
>> Note that I do not have any file system mounted, and use
>> "hd /dev/sda | head -70" to access the disk.
>>
>>> I am not entirely sure where the net win you see come from. But the patch you
>>> mention is in fact completely wrong and does not fix the underlying issues with
>>> ata suspend/resume and potential deadlocks in PM due to ata ports relationship
>>> with scsi devices. So I have been working on fixing this for the last 2 weeks,
>>> after another user also reported issues with the patch you mention [1].
>>>
>>> Could you try libata for-next branch on your system ? There are 7 fix patches in
>>> there that I plan to send out for 6.6-rc2 to fix the patch in question and other
>>> issues potentially causing deadlocks on resume. The patches were posted also [2].
>>>
>>> https://lore.kernel.org/linux-ide/20230912005655.368075-1-dlemoal@kernel.org/T/#t
>>
>> Unfortunately that didn't work, as /dev/sda no longer exists.
>> Will reply to the patch I bisected the issue to...
> 
> With libata/for-next (fa2259a59966c005 ("ata: libata: Cleanup inline
> DMA helper functions")) and commit 99626085d036ec32 ("ata: libata-scsi:
> link ata port and scsi device") reverted, it behaves as before (disk
> is spun up when system resume completes, no delay when accessing the
> disk from userspace).

I will check the ata platform driver for R-CAR. I may have overlooked something
in that area. I tested with AHCI and libsas adapters only as I do not have ATA
on the few ARM SBC boards I have. And I do not have an R-CAR board.

What surprises me is that you need to revert ata: libata: Cleanup inline DMA
helper functions". This patch has 0 functional changes and really is only a code
cleanup... Nothing should change with it. Can you confirm that you really need
to revert that patch to get things working again ?

-- 
Damien Le Moal
Western Digital Research