Howdy!
Some IDE drives destined for use in server class or datacenter
machines will come with "write cache" disabled. With the current
code, the setting of the drive is effectively ignored, and the cache
is always enabled if the drive has cache.
It is hard to define or enable certain behaviors that will keep
everyone happy, but hey, this is my (and a certain vendor's) take on
this. If you are happy with the status quo, no behavior change. If
the user desires to have write cache disabled by default (per the
drive), she can enable a more server friendly behavior.
These patches are against BK5. No attempt has been made to integrate
with Jens' latest "scsi/sata write barrier" patches, but will look
into that when the dust has settled for both.
With BLK_DEV_HDWC at the default of 0, the current driver behavior is
maintained, i.e., the drive will always use the write cache per the
current code. With the setting enabled, the driver will use the
default value of the drive write cache.
One area this does not touch on is locking. There does appear to be
some issues in changing the setting on the fly, but this is ignored in
the existing code. Bart has been making some effort.
This set of patches changes the way the write cache setting of IDE
drives is handled to:
1) move the cache_write code to ide-io, where it will be callable from
kernels built without ide-disk.
2) allow the drive to define the default setting of the cache when
BLK_DEV_HDWC is set.
3) correctly handle the change of write cache setting from hdparm.
This means that the call also changes the blk_queue_ordered()
visible variable.
1/7 - add ide_write_cache() to ide-io.c. This is in preparation for
the following patches that remove the current write_cache() from
ide-disk.c. Moving to ide-io is allow ide_write_cache() to
handle calls (no-op) on systems without ide disks (IDE may not
have been built for disks).
2/7 - add hook in ide-taskfile:ide_cmd_ioctl() to call the new
function ide_write_cache().
3/7 - Add documentation for the use of the Kbuild variable
BLK_DEV_HDWC.
4/7 - use new config variable BLK_DEV_HDWC in idedisk_setup code.
5/7 - remove (now obsolete) write_cache() from ide-disk.c. replace calls in
ide-disk.c with calls to ide_write_cache().
6/7 - change the way that ide_write_cache controls the use of flushing
via blk_queue_ordered() to match the setting of the drive's
write cache setting. This patch leaves the idedisk_issue_flush
in ide-disk.c, but the func could be moved to ide-io to complete
the break of the dependency between ide-io and ide-disk.
7/7 - move idedisk_issue_flush to ide-io.c supporting
ide_write_cache()
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