Re: when limited to a single DRQ block per disk transaction

[Pat LaVarre <p.lavarre@ieee.org>]

Jeff G:

> This limit does not apply to DMA.
> Only to PIO, PIO multi, and ATAPI PIO.

Understood.

> > Do we mean to be discussing a limit of one PATA DRQ INTRQ per CDB?
> 
> Correct.
> 
> If DRQ block size is 16 sectors, then each CDB is limited to a maximum 
> of 16 sectors.

To me, the limit looks worse than that.  [If I still don't get this in
your next reply, I'll now volunteer to wait a day before I reply again
in this thread.]

In PATA ATA PIO, distinct from ATAPI PIO, to guarantee at most one PATA
DRQ INTRQ per CDB when reading or writing more than one LBA/CDB, we have
to resort to ops x C4 C5 C6 = "READ MULTIPLE", "WRITE MULTIPLE", "SET
MULTIPLE MODE".  linux-2.6.6-bk1/include/linux/* gives these ops the
names WIN_SETMULT WIN_MULTREAD WIN_MULTWRITE.

In PATA ATAPI PIO, distinct from ATA PIO, we cannot guarantee at most
one PATA DRQ INTRQ per CDB, unless we limit the host to reading at most
one LBA/CDB, because the ATAPI device may issue more than the minimum
number of INTRQ that a chosen byte count limit guarantees.

Therefore the AHCI limit of at most one PATA DRQ INTRQ per CDB is in
effect an abolition of ATAPI PIO block i/o, except for the mostly
theoretical possibility of hosts and devices that together tolerate the
overhead of shattering a stream into single blocks.

Else I've misunderstood something vital.

Pat LaVarre