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    SubjectRe: Worrisome IDE PIO transfers...
    Date
    On Sunday 29 of February 2004 09:50, Geert Uytterhoeven wrote:
    > On Sun, 29 Feb 2004, Bartlomiej Zolnierkiewicz wrote:
    > > [ Geert added to cc: ]
    > >
    > > On Sunday 29 of February 2004 00:24, Jeff Garzik wrote:
    > > > Looking at the function that is used to transfer data when in PIO
    > > > mode...
    > > >
    > > > void taskfile_output_data (ide_drive_t *drive, void *buffer, u32
    > > > wcount) {
    > > > if (drive->bswap) {
    > > > ata_bswap_data(buffer, wcount);
    > > > HWIF(drive)->ata_output_data(drive, buffer, wcount);
    > > > ata_bswap_data(buffer, wcount);
    > > > } else {
    > > > HWIF(drive)->ata_output_data(drive, buffer, wcount);
    > > > }
    > > > }
    > > >
    > > > Swapping the data in-place is very, very wrong... you don't want to
    > > > be touching the data that userspace might have mmap'd ...
    > > > Additionally, byteswapping back and forth for each PIO sector chews
    > > > unnecessary CPU.
    > >
    > > This is used for accessing "normal" disks on beasts with byte-swapped IDE
    > > bus (Atari/Q40/TiVo) and "byteswapped" disks on normal machines.
    > >
    > > [ Hm. actually I don't see how it can be used for accessing "normal"
    > > disks, as data is byteswapped by IDE bus and then swapped back by IDE
    > > driver. ]
    >
    > Why not? The only difference between `normal' and `byteswapped' disks is
    > that the bytes in a 16-bit word are swapped (sic :-), so you can convert in
    > both directions by swapping the bytes. Normal disks have been used on Atari
    > before, so it should (still) work.

    Oh yes, IDE driver fixes byteorder on Atari/Q40 only when "bswap" is used.

    > BTW, the generic tree misses this patch, which was deemed inappropriate
    > before, but is needed to make sure the drive identification block is
    > correct on those machines:

    Thanks. I can't see a smarter way to fix this now. :/

    Bartlomiej

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