[systemd-devel] Fix for - udev/ata_id failing on SATA drives behind JMicron USB bridge

[Harendra Kumar]

I have a few drives in an enclosure (mediasonic pro box) connected via USB
using a Jmicron USB bridge (ID 152d:0567 JMicron Technology Corp.). I am
not able to create the desired names ("ata-<model>-<serial>") for these
devices in /dev/disk/by-id because ata_id is unable to retrieve the ATA
device identity information for these drives.

I did some root causing and found where the problem lies. The ATA Pass
Through command (
https://github.com/systemd/systemd/blob/master/src/udev/ata_id/ata_id.c#L130)
sets the check condition flag (CK_COND=1). In the response it expects and
checks for sense data -
https://github.com/systemd/systemd/blob/master/src/udev/ata_id/ata_id.c#L178
.

It seems the JMicron USB bridge successfully responds to the ATA IDENTIFY
DEVICE command but it does not return any sense data along with it and
therefore the disk_identify_command() function returns EIO even though the
command was successful and we got all that we needed. I checked by removing
this condition and the ata_id output was found to be accurate.

The behavior implemented by ata_id seems to be correct strictly speaking as
per the SAT draft. It seems the Jmicron USB hardware does not comply with
the standard. Here is an excerpt from page 123 of the SAT draft I found by
googling (
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.394.3605&rep=rep1&type=pdf
).

-----snip-----
The CK_COND (Check Condition) bit may be used to request the SATL to return
a copy of ATA register information in the sense data upon command
completion. If the CK_COND bit is set to one the SATL shall return a status
of CHECK CONDITION when the ATA command completes, even if the command
completes successfully, and return the ATA Normal Output fields (see
ATA8-ACS) in the sense data using the ATA Return descriptor (see 12.2.6).
If the CK_COND bit is set to zero, then the SATL shall terminate the
command with CHECK CONDITION status only if an error occurs in processing
the command. See clause 11 for a description of ATA error conditions.
-----snip-----

Can anyone suggest what the right fix for this would be? One way I can
think of is to not set the CK_COND flag in the command and check for sense
error only if check condition status/sense data was returned by the device.
This seems to be the way smartctl works for SAT devices and that's why it
works for my JMicron as well. Will this cause any problems? Why was the
code written this way in the first place? Is there a better fix for this?

Thanks,
harendra
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