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Subject[PATCH 0/1] Summary: hwmon driver for temperature sensors on SATA drives
In the past, several attempts have been made to add support for reporting
SCSI/[S]ATA drive temperatures to the Linux kernel. This is desirable to
have a means to report drive temperatures to userspace without root
privileges and in a standard format, but also to be able to tie reported
temperatures with the thermal subsystem.

The most recent attempt was [1] by Linus Walleij. It went through a total
of seven iterations. At the end, it was rejected for a number of reasons;
see the provided link for details. This implementation resides in the
SCSI core. It originally resided in libata but was moved to SCSI per
maintainer request, where it was ultimately rejected.

The feedback on this approach suggests to use the SCSI Temperature log page
[0x0d] as means to access drive temperature information. It is unknown
if this is implemented in any real SCSI drive. The feedback also suggests to
obtain temperature from ATA drives, convert it into the SCSI temperature log
page in libata-scsi, and to use that information in a hardware monitoring
driver. The format and method to do this is documented in [3]. This is not
currently implemented in the Linux kernel.

An earlier submission of a driver to report SCSI/SATA drive temperatures
was made back in 2009 by Constantin Baranov [2]. This submission resides
in the hardware monitoring subsystem. It does not rely on changes in the
SCSI subsystem or in libata-scsi. Instead, it registers itself with the
SCSI subsystem using scsi_register_interface(). It was rejected primarily
because it executes ATA passthrough commands without verification that it
is actually connected to an ATA drive.

Both submissions use SMART attributes to read drive temperature information.
[1] also tries to identify temperature limits from those attributes.
Unfortunately, SMART attributes are not well defined, resulting in relative
complex code trying to identify the exact format of the reported data.

With the available information and feedback, we can make a number of
observations and conclusions.
a) Using available (S)ATA drive temperature information and convert it to
a SCSI log page is an interesting idea. On the downside, it would add a
substantial amount of complexity to libata-scsi. The code would either
have to be optional, or it would have to be built into the kernel even
if it is never used on a given system. Without access to SCSI drives
supporting this feature, it would be all but impossible to test the code
against such a drive. It would neither be possible to test correctness
of the code in libata-scsi nor in the driver using that information.
Overall it would be much easier and much less risky to implement such
code on the receiving side (ie in a driver reporting the temperatures)
instead of trying to convert the information from one format to another
first. In summary, it is neither practical nor feasible. On top of that,
there is no guarantee that code implementing this functionality would
ever be accepted into the kernel for this very reason.
b) The code needed to read and analyze SCSI temperature log pages is quite
complex (see smartmontools [5]). There is no existing support code
in the Linux kernel; such code would have to be written. This makes
the approach discussed in a) even more risky and less practical.
c) Overall, any attempt to report temperature information for anything
but SATA drives in the kernel is not practical due to the complexity
involved, and due to the inability to test the resulting code with
non-SATA drives.
d) Using SMART data for anything but basic temperature reporting is not
really feasible due to the lack of standardization. Any attempt to do
this would add a substantial amount of code, ambiguity, and risk.

This submission implements a driver to report the temperature of SATA
drives through the hardware monitoring subsystem. It is implemented as
stand-alone driver in the hardware monitoring subsystem. The driver uses
the mechanism from submission [1] to register with the SCSI subsystem.
By using this mechanism, changes in the SCSI or ATA subsystems are not
required. To reduce risk and complexity, it only instantiates after
reliably validating that it is connected to a SATA drive. It does not
attempt to report the temperature of non-SATA drives.

The driver uses the SCT Command Transport feature set as specified in
ATA8-ACS [4] to read and report the temperature as well as temperature
limits and lowest/highest temperature information (if available) for
SATA drives. If a drive does not support SCT Command Transport, the driver
attempts to access a limited set of well known SMART attributes to read
the drive temperature. In that case, only the current drive temperature
is reported.

Information technology - SCSI / ATA Translation - 5 (SAT-5),
section 10.3.8 (Temperature log page).
ANS T13/1699-D "Information technology - AT Attachment 8 - ATA/ATAPI Command
Set (ATA8-ACS)"

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