On 9/11/23 06:02, Damien Le Moal wrote:
The introduction of a device link to create a consumer/supplier
relationship between the scsi device of an ATA device and the ATA port
of the ATA device fixed the ordering of the suspend and resume
operations. For suspend, the scsi device is suspended first and the ata
port after it. This is fine as this allows the synchronize cache and
START STOP UNIT commands issued by the scsi disk driver to be executed
before the ata port is disabled.
For resume operations, the ata port is resumed first, followed
by the scsi device. This allows having the request queue of the scsi
device to be unfrozen after the ata port restart is scheduled in EH,
thus avoiding to see new requests issued to the ATA device prematurely.
However, since libata sets manage_start_stop to 1, the scsi disk resume
operation also results in issuing a START STOP UNIT command to wakeup
the device. This is too late and that must be done before libata EH
resume handling starts revalidating the drive with IDENTIFY etc
commands. Commit 0a8589055936 ("ata,scsi: do not issue START STOP UNIT
on resume") disabled issuing the START STOP UNIT command to avoid
issues with it. However, this is incorrect as transitioning a device to
the active power mode from the standby power mode set on suspend
requires a media access command. The device link reset and subsequent
SET FEATURES, IDENTIFY and READ LOG commands executed in libata EH
context triggered by the ata port resume operation may thus fail.
Fix this by handling a device power mode transitions for suspend and
resume in libata EH context without relying on the scsi disk management
triggered with the manage_start_stop flag.
To do this, the following libata helper functions are introduced:
1) ata_dev_power_set_standby():
This function issues a STANDBY IMMEDIATE command to transitiom a device
to the standby power mode. For HDDs, this spins down the disks. This
function applies only to ATA and ZAC devices and does nothing otherwise.
This function also does nothing for devices that have the
ATA_FLAG_NO_POWEROFF_SPINDOWN or ATA_FLAG_NO_HIBERNATE_SPINDOWN flag
set.
For suspend, call ata_dev_power_set_standby() in
ata_eh_handle_port_suspend() before the port is disabled and frozen.
ata_eh_unload() is also modified to transition all enabled devices to
the standby power mode when the system is shutdown or devices removed.
2) ata_dev_power_set_active() and
This function applies to ATA or ZAC devices and issues a VERIFY command
for 1 sector at LBA 0 to transition the device to the active power mode.
For HDDs, since this function will complete only once the disk spin up.
Its execution uses the same timeouts as for reset, to give the drive
enough time to complete spinup without triggering a command timeout.
Neat. But why VERIFY?
Isn't there a dedicated command (ie the opposite of STANDBY IMMEDIATE)?
And can we be sure that VERIFY is implemented everywhere?
It's not that this command had been in active use until now ...
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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