Re: [PATCH] libata: Add MMIO support to pata_sil680

[Jeff Garzik <jeff@garzik.org>]

Alan Cox wrote:
> On Thu, 24 May 2007 06:52:13 -0400
> Jeff Garzik <jgarzik@pobox.com> wrote:
> 
>> Alan Cox wrote:
>>>> Currently libata does the 400ns thingy in the inline ata_pause() which
>>>> unconditionally does ata_altstatus(); ndelay(400); (and thus does it on
>>>> PIO, MMIO, anything ...).
>>> It might actually improve performance for PATA if we did as we'll get rid
>>> of an excess (expensive) PCI read access for non MMIO cases.
>> The spec says you should read AltStatus.  I am definitely not inclined 
>> to change that part.
> 
> Which spec says that and where ?

Found it!  It was in the spec I consider more authoritative than any 
official ATA spec:  Hale Landis' ATADRVR (http://www.ata-atapi.com/):


// This macro provides a small delay that is used in several
// places in the ATA command protocols:
// 1) It is recommended that the host delay 400ns after
//    writing the command register.
// 2) ATA-4 has added a new requirement that the host delay
//    400ns if the DEV bit in the Device/Head register is
//    changed.  This was not recommended or required in ATA-1,
//    ATA-2 or ATA-3.  This is the easy way to do that since it
//    works in all PIO modes.
// 3) ATA-4 has added another new requirement that the host delay
//    after the last word of a data transfer before checking the
//    status register.  This was not recommended or required in
//    ATA-1, ATA-2 or ATA-3.  This is the easy to do that since it
//    works in all PIO modes.

#define DELAY400NS  { pio_inbyte( CB_ASTAT ); pio_inbyte( CB_ASTAT );  \
                       pio_inbyte( CB_ASTAT ); pio_inbyte( CB_ASTAT ); }


libata was originally written by following ATADRVR IO-for-IO.  You can 
still see some of the mostly-unmodified bitbang sequences in 
libata-core.c in places like dev-select or dev-chk.  Sometimes you have 
to look pretty hard, since hooks make the bitbang sequences much harder 
to follow.

Obviously libata's "pause" differs from Hale's version a bit, but the 
basic version is sound and works for everybody from ancient PATA PIO to 
modern SATA MMIO.

	Jeff