Re: [PATCH v9 05/11] scsi: scsi_debug: Report the residual of a write

[Damien Le Moal <dlemoal@kernel.org>]

On 2026/09/28 9:15, Niklas Cassel wrote:
> On Mon, Sep 28, 2026 at 08:45:30AM +0200, Damien Le Moal wrote:
>> On 2026/09/27 7:26, Niklas Cassel wrote:
>>> A write whose data buffer is larger than its transfer length leaves a
>>> residual that the initiator is entitled to be told about. resp_read_dt0()
>>> and resp_write_tape() report it, but the write paths for a disk do not,
>>> so such a write completes with GOOD status and a residual of zero while
>>> a READ of the same length into the same buffer reports it correctly.
>>
>> I am not convinced this is correct. resid (residual count) is supposed to
>> indicate the amount of data that was *not* transferred. So if the buffer size is
>> larger that the command cdb transfer count, all data can be transferred and
>> resid should be 0. Which would mean that for this case, the write processing is
>> correct but the read side is not.
>>
>> The SBC and SPC specs are very silent about this though, I do not think that the
>> resid is actually standardized. And I do not see a clear definition for it in
>> the documentation/code comments.
> 
> The definition is in Documentation/scsi/scsi_mid_low_api.rst,
> under resid_len: "an LLD should set this unsigned integer to the requested
> transfer length (i.e. 'request_bufflen') less the number of bytes that are
> actually transferred".
> 
> Also in include/scsi/sg.h: "resid; /* [o] dxfer_len - actual_transferred */".
> 
> Both measure it against the buffer, not the CDB.
> 
> That is also what iSCSI reports: RFC 7143 11.4.5.1 measures the residual
> against the Expected Data Transfer Length, which the initiator sets from the
> buffer.
> 
> And it is what scsi_debug has always done for reads, in fill_from_dev_buffer().
> This patch just does the same for writes.

Hmm. OK. But then that really should be set automatically by the sd/st/sr
drivers and sg driver. scsi_debug emulates a device, which does not give the
residual. So scsi_debug has no business setting that command residual at all,
for any command.

> 
> 
> Let me clarify the commit message in v10 to actually reference
> Documentation/scsi/scsi_mid_low_api.rst.
> 
> 
> Kind regards,
> Niklas


-- 
Damien Le Moal
Western Digital Research