Re: [PATCH 14/14] libata: use PIO for misc ATAPI commands

[Tejun Heo <htejun@gmail.com>]

Mark Lord wrote:
> I'm split on this one.  For fast systems (typical notebook/desktop) it's
> almost a non-issue either way.
> 
> But a lot of media boxes will be using this code, and they tend to have
> very low-power, slow-clockrate CPUs in the 200-800Mhz range, and so the
> real-time hit there (from PIO) will have a much more significant impact.
> 
> Using DMA as much as possible on those slower platforms is definitely
> a plus towards avoiding non-jerky, skipped frames, or start-stoppy DVD
> recording.
> 
> Now the most intensive commands are still DMA under the proposed scheme,
> so it's not those that one would be concerned with.  But dropping to PIO
> even for a few uncommon commands will still peg a real-time hit or two
> on a slower media processor.
> 
> So..  ????

One thing I don't understand about this argument is that PIO cycle time
is not determined by CPU power.  It's bound by PCI and ATA bus speed.
If you put a faster CPU on the job, it just ends up wasting the same
amount of time burning up more CPU cycles or am I misjudging power of
those embedded CPUs?

-- 
tejun