Bidirectional support

Bidirectional support

[Liran Schour]

The OSD standard requires support for bidirectional commands (write and get
obj attribute at the same command for example). The SAM-3 defines that in
order to execute a SCSI command the caller should supply an input an output
buffer. It seems that in the current Linux SCSI implementation the API
accept only one buffer, input our output buffer depends on the
data_direction flag.
Is there some work going on trying to implement bidirectional support?
Is it sound reasonable to extend the scatter-gather structure to include
data direction flag and by that bypass the interface problem and allow
support for bidirectional commands?

Thanks,
- Liran

Re: Bidirectional support

[Christoph Hellwig]

On Tue, Nov 18, 2003 at 07:41:05PM +0200, Liran Schour wrote:
> The OSD standard requires support for bidirectional commands (write and get
> obj attribute at the same command for example). The SAM-3 defines that in
> order to execute a SCSI command the caller should supply an input an output
> buffer. It seems that in the current Linux SCSI implementation the API
> accept only one buffer, input our output buffer depends on the
> data_direction flag.
> Is there some work going on trying to implement bidirectional support?

I haven't heard about such work beeing done.

> Is it sound reasonable to extend the scatter-gather structure to include
> data direction flag and by that bypass the interface problem and allow
> support for bidirectional commands?

Why do you need the flag in the scatterlist?  We already have a dma_direction
enum in the scsi_cmnd that's pased to the dma mapping routines, it may
be DMA_BIDRECTIONAL although that's currently not used by any code.

Re: Bidirectional support

[Liran Schour]

Christoph Hellwig wrote:
>
> > Is it sound reasonable to extend the scatter-gather structure to
include
> > data direction flag and by that bypass the interface problem and allow
> > support for bidirectional commands?
>
> Why do you need the flag in the scatterlist?  We already have a
dma_direction
> enum in the scsi_cmnd that's pased to the dma mapping routines, it may
> be DMA_BIDRECTIONAL although that's currently not used by any code.

I can use this DMA_BIDIRECTIONAL flag. But I still do need somthing like a
scatterlist to overcome the fact that we have only one pointer for data
buffer instead of two, one for data out and one for data in.

- Liran