The Linux ahci driver has historically implemented a configuration fixup
for platforms / platform-firmware that fails to enable the ports prior
to OS hand-off at boot. The fixup was originally implemented way back
before ahci moved from drivers/scsi/ to drivers/ata/, and was updated in
2007 via commit 49f29090 "ahci: update PCS programming". The quirk
sets a port-enable bitmap in the PCS register at offset 0x92.

This quirk could be applied generically up until the arrival of the
Denverton (DNV) platform. The DNV AHCI controller architecture supports
more than 6 ports and along with that the PCS register location and
format were updated to allow for more possible ports in the bitmap. DNV
AHCI expands the register to 32-bits and moves it to offset 0x94.

As it stands there are no known problem reports with existing Linux
trying to set bits at offset 0x92 which indicates that the quirk is not
applicable. Likely it is not applicable on a wider range of platforms,
but it is difficult to discern which platforms if any still depend on
the quirk.

Rather than try to fix the PCS quirk to consider the DNV register layout
instead require explicit opt-in. The assumption is that the OS driver
need not touch this register, and platforms can be added with a new
boad_ahci_pcs7 board-id when / if problematic platforms are found in the
future. The logic in ahci_intel_pcs_quirk() looks for all Intel AHCI
instances with "legacy" board-ids and otherwise skips the quirk if the
board was matched by class-code.
Reported-by: Stephen Douthit <stephend@silicom-usa.com>
Cc: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Stephen Douthit <stephend@silicom-usa.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>