Re: libata/PATA: GPCMD_SET_STREAMING via SG_IO does nothing

[Mark Lord <lkml@rtr.ca>]

Sebastian Kemper wrote:
> Hi Mark!
> 
> On Wed, Nov 14, 2007 at 11:41:37AM -0500, Mark Lord wrote:
>> Ahh.. got it.  The host_status returned (not checked by that code) was 7,
>> which means "host error".
>>
>> In this case, that's because the cmd_len is (16), which is too large for 
>> ATAPI.
>> It needs to be changed to (12) instead.
> 
> I don't understand. You seem to use a cmd_len of 16 in hdparm yourself.
..

Really?  Where?


> And why does it work with the "old" ATA driver and not with libata if
> 16 is too large for ATAPI in general?
..

Now that *is* the question.
And the answer appears to be that ide-cd.c ignores the passed-in cmd_len,
and replaces it with:

   cmd_len = COMMAND_SIZE(rq->cmd[0]);

That's a SCSI macro to calculate the correct cmd_len based on the SCSI opcode,
which is very appropriate here.   We should do that too (we don't) in libata.

But not exactly as IDE does it -- that's actually a bug:  the code needs to also
check that the new cmd_len is no larger than the original, or we'll get an Oops.

I'll cook up a patch for that shortly and try it before posting it here.

-ml