Re: [PATCH 4/4] pata_platform: Use 16-bit wide data transfer

[One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>]

On Mon, 25 Aug 2014 15:34:54 -0400
Tejun Heo <tj@kernel.org> wrote:

> Hello,
> 
> On Sat, Aug 23, 2014 at 09:29:15PM +0400, Alexander Shiyan wrote:
> > > Shouldn't we distinguish them instead of forcing 16bit transfer on
> > > all?
> > 
> > I do not see any reason to distinguish. This embodiment can be used with
> > 16-bit and 32-bit bus, and even for 64-bit ;)
> 
> Hmmm... we do care about doing 32bit when possible as there are cases
> where PIO is the only data tarnsfer mode available 

Because some environments you get faster data transfers if you throw data
at the controller in 32bit chunks. In some cases that's because the
controller is clever about it and pipelining (eg some VLbus devices) in
others for the simple reason that the external bus cycles are either
16bit or converted that way and the CPU executes a 32bit move followed by
an add faster than 16bit/add/16bit/add.

(There are legacy memory mapped devices around where the fastest way
to read them on an older x86 is to use the FPU to do 64bit reads, but some
things should not be encouraged ;-) )


Alan