Re: [PATCH v2 2/3] libiscsi, iser: Adjust data_length to include protection information

Boaz Harrosh
2014-08-06T13:25:58Z

On 08/06/2014 03:43 PM, Sagi Grimberg wrote:
> Hi Boaz,
>
<>
>>
>> I hate that you introduced this new transfer_length variable. It does
>> not exist. In BIDI supporting driver there is out_len and in_len just
>> as original code.
>
> Effectively, out_len and in_len are the same except for the bidi case.
> Moreover, the hdr->data_length is exactly the command scsi data buffer
> length, the bidi length is taken from scsi_in when we build the AHS for
> bidi (rlen_ahdr->read_length).
>
> So to me it is correct and makes sense. But I'm don't feel so strong
> about it...
>
> Mike what's your take on this?
>

I have a patch to clean all that, will send tomorrow.

What I mean is that this is on the out-path only the in path is different.
See the code this variable is only used in the if (== DMA_TO_DEVICE) case and
should be local to that scope. This is my clean up

<>
>> this particular driver puts them together in the same payload. There can be
>> other DIFF supporting drivers that put the DIFF payload on another stream / another
>> SG list, like the sata one, right?
>
> I think that DIF specification says that on the wire the data and
> protection are interleaved i.e. Block1, DIF1, Block2, DIF2...
>

No it does not. This is a per transport, and actually per device host
driver. Yes in iSCSI_tcp they are inline in HW cards they might come as
two different SGs (Like the Linux model). Even with iscsi-offload they might
want to be two SG-lists.

> So I do think that the transfer length should always include
> data_length + protection_length.
>

This is at the iscsi level. But the scsi_transfer_length() is on the scsi
level which keeps them separate. So I think the proper API should be
scsi_proto_length()

And for LLDs that want them together they can do scsi_bufflen() + scsi_proto_length()
and for other drivers they can do it separately. Don't infect iscsi level assumptions
on the generic layer API.

Again my patch fixes this.

>> And this
>>
>> This print is correct as it covers all cases. If you want to print the adjusted
>> length then OK, you'd need to store this I guess, but store it as a different
>> variable like proto_length and print it as an additional variable.
>
> But it is the transfer length that is sent in iSCSI header. Why do you
> want to print it as additional info?

I want to see what was the length the app/FS sent, then as added
info how much was added for DIFF, your way there is lost information.

> for transactions that include DIF
> the length is the data + protection.
>
> It is still one-to-one isn't it?
>

No! the original submitted length is lost from the print

> Sagi.
>

Shalom
Boaz