Re: Bidirectional data transfers from SCSI layer

[Boaz Harrosh <bharrosh@panasas.com>]

On 08/18/2009 09:22 AM, Madhavi Manchala wrote:
> Dear All,
> 
> I have seen that SCSI supports bi-directional commands
> DMA_BIDIRECTIONAL. However, I did not get such type of commands in my
> mass storage driver. My device supports bi-directional data transfers.
> My question is, how can I get such type of requests to my mass storage
> driver? If so, how can I handle the IN and OUT buffers? Are there
> separate buffers for both? Is there any support for the bi-directional
> in the current SCSI implementation? I am currently using the 2.6.28
> kernel code.
> 
> We have only one pointer for data buffer instead of two, one for data
> OUT and one for data IN in command block. There is only one length
> variable. How can we use the OUT and IN buffers? How can I pass
> different IN length and OUT length to lower stack from SCSI layer?
> 
> Is there any thing need to be changed in the SCSI layer code to handle
> the bi-directional data transfers?
> 
> Please point to me some links which shows the bi-directional data transfers.
> 
> Thanks in advance.
> 
> With Regards,
> Madhavi M.
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Bidi is fully supported since 2.6.25

If you want example of how to issue bidi commands you can see these two files:
block/bsg.c
drivers/scsi/osd/osd_initiator.c

Basically you allocate two struct request one for OUT one for IN. You map
some memory on them and do "out_req->next_rq = in_req;" And you execute
the out_req.
These two request will go together as one bidi request.

On the scsi-driver side you do:

	if (scsi_bidi_cmnd(cmnd)) {
		in_sgl = scsi_in(cmnd)->table.sgl;
		in_nents = scsi_in(cmnd)->table.nents;
		in_bytes = scsi_in(cmnd)->length;

		/* Same thing for out */
		out_sgl = scsi_out(cmnd)->table.sgl;
		...
	}

scsi_in() and scsi_out() will return a pointer or NULL if the
cmnd does not have a buffer in this direction. So for example
scsi_in() is always used on receive regardless if it is a bidi
or a uni_read.

NOTE: in bidi_commands
	scsi_bidi_cmnd(cmnd) == true;
	but
	cmnd->sc_data_direction == DMA_TO_DEVICE;
	DMA_BIDIRECTIONAL is *never* returned from scsi
	and it means something else inside the DMA engines.

For example driver that supports bidi see:
drivers/scsi/iscsi_tcp.c
drivers/scsi/scsi_debug.c
(Search for "scsi_in" and "scsi_out")

What type of scsi-device are you using? what bidi commands?

Have a good day
Boaz