Re: [PATCH] libata: add support for READ/WRITE LONG

[Jeff Garzik <jgarzik@pobox.com>]

Alan Cox wrote:
>>     I think it only requires the *host* to drop to PIO0 timings.  In which 
>> case it should be achievable w/o libata modification -- if the driver has to 
>> "snoop" command and turn off prefetch, why not switch to PIO0 temporarily?
> 
> This isn't a big issue. Eventually we have to support sending speed
> change commands and once we do that the caller will be able to switch to
> PIO0 and back again.

We should be pretty close to that.  One of the bigger blockages is 
needing to stop (freeze) all ports, before performing an out-of-band SET 
FEATURES and related tuning.

For controllers that don't need such synchronization (though I argue it 
is wise practice for all controllers) -- generally the ones that do not 
snoop SET FEATURES -- you could probably get it going with a few lines 
of code change:  removing the current SET FEATURES - XFER MODE filter, 
and calling the requisite callbacks.

Even in the current EH we don't get this quite right.  The main culprit 
is Promise controllers.  SET FEATURES is snooped on these controllers, 
and registers are programmed.  It is known to corrupt data if a mode 
change occurs while data is transferring on /other/ ports.  If new-EH 
decides to change the mode on a Promise controller, we might hit this 
scenario, because the apparatus does not know to stop all ports on that 
controller, rather than just the single port that is current frozen and 
inside EH.

	Jeff