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Subjectlibata error handling

Tejun,

In an email I cannot find anymore, you asked why I was interested in
converting libata to use the fine-grained EH hooks in the SCSI layer,
rather than continued with the current ->eh_strategy_handler() method.

Several reasons:

1) The fine-grained hooks of the SCSI layer are somewhat standard for
block devices. The events they signify -- timeout, abort cmd, dev
reset, bus reset, and host reset -- map precisely to the events that we
must deal with at the ATA level.

But be warned of false sharing, as I talk about in #2...

2) When libata SAT translation layer becomes optional, and libata drives
a "true" block device, use of ->eh_strategy_handler() will actually be
an obstacle due to false sharing of code paths. ->eh_strategy_handler()
is indeed a single "do it all" EH entrypoint, but within that entrypoint
you must perform several SCSI-specific tasks.

3) ->eh_strategy_handler() has continually proven to be a method of
error handling poorly supported by the SCSI layer. There are many
assumption coded into the SCSI layer that this is -not- the path taken
by LLD EH code, and libata must constantly work around these assumptions.

4) libata is the -only- user of ->eh_strategy_handler(), and oddballs
must be stomped out. It creates a maintenance burden on the SCSI layer
that should be eliminated.


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