Re: libata passthru: support PIO multi commands

[Sergei Shtylyov <sshtylyov@ru.mvista.com>]

Albert Lee wrote:
> 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.

    This is generally a bad practice to guess protocol based on opcode. What 
if the code will have to handle a vendor unique command (or some other command 
not yet known to it but known to issuer)?

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

    I'd agree here.

> e.g. t_dir:
>      libata could possible infer the direction from the command opcode

    Bad idea again.

>      or from the protocol number (e.g. 4: PIO_IN / 5: PIO_OUT).

    This is reasonable if DMA direction can also be inferred from the protocol 
number.

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

    I think it's better to allow inconsistency then to limit functionality.
There's another option though -- let the caller specify the default protocol 
for the command to be issued or override it if it *knows* what it's doing.

> 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

- let the caller use defaults based on command code or override them.

> --
> albert

MBR, Sergei