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SubjectRe: Worrisome IDE PIO transfers...
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. ]

Yeah, just byteswapped disks are affected.


> Manfred noticed the same issue some time ago:
> http://www.ussg.iu.edu/hypermail/linux/kernel/0201.0/0768.html
> but discussion ended without final conclusion.
>
> I like Alan's idea to use loopback instead of "bswap".

Neat but no more zerocopy that way. I much prefer a swap-as-you-go...


>>Seems to me the architecture's OUTS[WL] hook (or a new, similar hook)
>>that swaps as it writes would be _much_ preferred, and eliminate this
>>possible data corruption issue.
>
> I think something similar has been already done
> (grep for insw_swapw/outsw_swapw in ide-iops.c and asm-m68k/ide.h).

Yeah, but this would need to be per-device... I agree all the other
pieces are already present.

Jeff


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