Re: libata passthru: support PIO multi commands

[Albert Lee <albertcc@tw.ibm.com>]

Alan Cox wrote:
>>ata_scsi_pass_thru() is not executed at ioctl submission time (block 
>>queue submission time), but rather immediately before it is issued to 
>>the drive.  At that point you know the bus is idle, all other commands 
>>have finished executing, and dev->multi_count is fresh not stale.  The 
>>code path goes from ata_scsi_pass_thru() to ata_qc_issue() without 
>>releasing the spinlock, even.
> 
> 
> Think up to user space
> 
> Poorusersapp			set multicount to 4
> 
> Evilproprietarytuningdaemon	set multicount to 8
> 
> Poorusersapp			issue I/O
> 
> at which point an error is indeed best.
> 
> 
>>But the last point is true -- we should error rather than just warn 
>>there, AFAICS.
> 
> 
> Definitely. We've been asked "please do something stupid" and not even in
> a case where the requester may know better.
> 

It looks like the ATA passthru commands contain more information than
what libata needs to execute a command.

e.g. protocol number:
     libata could possibly infer the protocol from the command opcode.

e.g. multi_count:
     libata caches dev->multi_count. Passing multi_count along with
     each passthru command looks useless for libata.     

e.g. t_dir:
     libata could possible infer the direction from the command opcode
     or from the protocol number (e.g. 4: PIO_IN / 5: PIO_OUT).

Due to the redundant info, there is possiblely inconsistency between
the parameters. e.g. t_dir vs protocol. e.g. command vs protocol.

It seems the "redundant" parameters are designed to allow stateless SATL
implementation: The application/passthru command tells the stateless SATL
implementation the protocol and the multi_count, etc. Then SATL just
follows the instruction blindly, even if asked to do something stupid.

Currently libata
 - uses the passthru protocol number blindly
   (even if the application issues a DMA command with wrong PIO protocol.)
 - checks and warns about multi_count
 - ignores t_dir, byte_block and so on.

Maybe we need a strategy to deal with incorrect passed-thru commands?
say,
 - check and reject if something wrong
 - mimimal check and warn/ignore, if it doesn't hurt command execution

--
albert