Re: [patch ide-dev 8/9] make ide_task_ioctl() use REQ_DRIVE_TASKFILE

[Mark Lord <liml@rtr.ca>]

SATA, PATA, or anything else:  if it has to cross the PCI bus,
a simple readX()/writeX() can stall the CPU for the equivalent
of hundreds of instructions.  I agree with Jeff, it is always
worth even moderately complex logic to avoid I/O.

Note that an isolated write{bwl}() *may* be almost free in most
cases, due to write buffers between the CPU and the bus.
But those buffers are of limited depth (typically 3/4 entries),
and a stall there often causes a 0.5us (or more) delay.

When measuring PATA hardware, I found the delay was often between
600ns and 1200ns (0.6us to 1.2us), per readX()/writeX().
With SATA, it will likely be around 11 PCI clocks, or say 363ns
on most current platforms.

Cheers