Implement ata_scr_{valid|read|write|write_flush}() and
ata_port_{online|offline}(). These functions replace
scr_{read|write}() and sata_dev_present().
Major difference between between the new SCR functions and the old
ones is that the new ones have a way to signal error to the caller.
This makes handling SCR-available and SCR-unavailable cases in the
same path easier. Also, it eases later PM implementation where SCR
access can fail due to various reasons.
ata_port_{online|offline}() functions return 1 only when they are
affirmitive of the condition. e.g. if SCR is unaccessible or
presence cannot be determined for other reasons, these functions
return 0. So, ata_port_online() != !ata_port_offline(). This
distinction is useful in many exception handling cases.
Signed-off-by: Tejun Heo <htejun@gmail.com>
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