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= SAT with UAS under Linux =


== Overview ==

As mentioned on the [wiki:USB USB devices and smartmontools] page, in certain situations the Linux "uas" driver disables SAT transfers, which prevents `smartmontools` (and other tools, e.g. `hdparm`) from communicating properly with the attached SATA device.  See [query:?keywords=~uas+linux related tickets] for examples.

This is because the Linux kernel rejects SAT ATA pass-through commands for certain devices with broken pass-through support.
This depends on Kernel version and may include
[https://github.com/torvalds/linux/commit/7fee72d5e8f1e7b8d8212e28291b1a0243ecf2f1 all Seagate enclosures],
[https://github.com/torvalds/linux/commit/bda13e35d584dabf52c9f77e0fe62683ac4d9f86 Initio INIC-3069] and
[https://github.com/torvalds/linux/commit/673029fe9c16c95600bdaca4760673527af32edf VIA VL711].

== Technical Details ==

The problem is that certain USB-SATA-bridge chipsets do not properly pass SAT commands on to the SATA device when running in UAS mode, and to work around this problem Linux automatically enables the NO_ATA_1X flag for those chipsets.  This turns off SAT passthrough... which in turn means that smartmontools is prevented from communicating with the device.

For many of these chipsets, SAT passthrough works correctly is the old [https://en.wikipedia.org/wiki/USB_mass_storage_device_class BOT] (`usb-storage` driver) mode, and so smartmontools may suddenly stop working when the kernel is upgraded to a version which supports UAS, or from one which did did not apply the flag for that device to a newer kernel which does.

For more information on the usb-storage flags, search for the 'usb-storage.quirks' entry in the [https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html Kernel Parameters Documentation].

The detailed selection logic used by the kernel to decide which devices get this flag can be found in the `uas_use_uas_driver()` function in the [https://github.com/torvalds/linux/blob/master/drivers/usb/storage/uas-detect.h drivers/usb/storage/uas-detect.h]  combined with the list of entries found in the [https://github.com/torvalds/linux/blob/master/drivers/usb/storage/unusual_uas.h drivers/usb/storage/unusual_uas.h]
file.  (As far as smartmontools/SAT passthrough is concerned, only the NO_ATA_1X flag is significant.)

There doesn't seem to be a definitive way to confirm that the NO_ATA_1X flag is in fact in effect for a particular device, but if it is the symptom will be that commands which rely on SAT show invalid data, e.g.

{{{
# smartctl -d sat -a /dev/sdb
smartctl 6.6 2016-05-31 r4324 [x86_64-linux-4.15.0-33-generic] (local build)
Copyright (C) 2002-16, Bruce Allen, Christian Franke, www.smartmontools.org

Read Device Identity failed: scsi error unsupported field in scsi command

A mandatory SMART command failed: exiting. To continue, add one or more '-T permissive' options.


# smartctl -d sat -T permissive -a /dev/sdb
smartctl 6.6 2016-05-31 r4324 [x86_64-linux-4.15.0-33-generic] (local build)
Copyright (C) 2002-16, Bruce Allen, Christian Franke, www.smartmontools.org

Read Device Identity failed: scsi error unsupported field in scsi command

=== START OF INFORMATION SECTION ===
Device Model:     [No Information Found]
Serial Number:    [No Information Found]
Firmware Version: [No Information Found]
Device is:        Not in smartctl database [for details use: -P showall]
ATA Version is:   [No Information Found]
Local Time is:    Mon Aug 27 20:44:01 2018 CEST
SMART support is: Ambiguous - ATA IDENTIFY DEVICE words 82-83 don't show if SMART supported.
SMART support is: Ambiguous - ATA IDENTIFY DEVICE words 85-87 don't show if SMART is enabled.
A mandatory SMART command failed: exiting. To continue, add one or more '-T permissive
}}}

or

{{{
# hdparm -I /dev/sdc

/dev/sdc:
SG_IO: bad/missing sense data, sb[]:  70 00 05 00 00 00 00 0a 00 00 00 00 24 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

ATA device, with non-removable media
Standards:
        Likely used: 1
Configuration:
        Logical         max     current
        cylinders       0       0
        heads           0       0
        sectors/track   0       0
        --
        Logical/Physical Sector size:           512 bytes
        device size with M = 1024*1024:           0 MBytes
        device size with M = 1000*1000:           0 MBytes 
        cache/buffer size  = unknown
Capabilities:
        IORDY not likely
        Cannot perform double-word IO
        R/W multiple sector transfer: not supported
        DMA: not supported
        PIO: pio0
}}}


== Permanent Workarounds ==



== Short-term Workarounds ==