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

[Hannes Reinecke <hare@suse.de>]

On 10/30/19 4:12 PM, Bart Van Assche wrote:
> 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.
> 
But that is an error on submission, and should be aborted before it even 
got send to the drive.

However, this does not relate to the residual, which is handled after 
the command completes (and which sparked this entire thread ...).

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
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