Re: [PATCH] scsi: Fix scsi_get/set_resid() interface

[Bart Van Assche <bvanassche@acm.org>]

On 10/30/19 1:30 AM, Hannes Reinecke wrote:
> On 10/28/19 9:38 PM, Bart Van Assche wrote:
>> If the residual is changed from signed into unsigned, how is a SCSI 
>> LLD expected to report the difference between residual overflow and 
>> residual underflow to the SCSI core?
>
> You don't have to. To quote RFC 3720 page 122:
> 
>       bit 5 - (O) set for Residual Overflow.  In this case, the Residual
>         Count indicates the number of bytes that were not transferred
>         because the initiator's Expected Data Transfer Length was not
>         sufficient.  For a bidirectional operation, the Residual Count
>         contains the residual for the write operation.
> 
> IE the 'overflow' setting in the iSCSI command response is an indicator 
> that there _would_ be more data if the command request _would_ have 
> specified a larger buffer.
> But as it didn't, the entire buffer was filled, and the overflow counter 
> is set.
> Which, of course, is then ignored by the linux SCSI stack as the request 
> got all data, and the residual is set to zero.
> Then it's left to the caller to re-send with a larger buffer if 
> required. But it's nothing the SCSI stack can nor should be attempting 
> on its own.

Hi Hannes,

I do not agree that reporting a residual overflow by calling 
scsi_set_resid(..., 0) is acceptable. For reads a residual overflow 
means that the length specified in the CDB (scsi_bufflen()) exceeds the 
data buffer size (length of scsi_sglist()). I think it's dangerous to 
report to the block layer that such requests completed successfully and 
with residual zero.

Bart.