[PATCH/RFC 0/9] libata: irq_on/off restructuring

[Albert Lee <albertcc@tw.ibm.com>]

For ATA, there are two levels of mechanism available to turn irq on/off.
 - device level: nIEN bit in the control register. nIEN masks INTRQ from the device.
 - host adapter level: some controller can mask out per-port irq from the host adapter.

Currently various parts of libata deal with irq on/off.
  ex. tf_load() can alter the nIEN bit.
  ex. irq_on() also alters the device level nIEN bit.
  ex. freeze()/thaw() deal with the host adapter irq mask.

It seems these irq on/off codes could be better structured.
Draft patches for your review/advice, thanks.

1/9: remove irq_on from ata_bus_reset() and ata_std_postreset()
2/9: add ->irq_off() for symmetry
3/9: implement ->irq_off() in LLDDs
4/9: call irq_off from bmdma_freeze()
5/9: use freeze()/thaw() for polling
6/9: add freeze()/thaw() to old EH LLDDs
7/9: pdc_freeze() semantic change
8/9: remove writing of tf->ctl from ata_tf_load()
9/9: Remove irq_on/off. Rename freeze()/thaw() to irq_on/off.