Alan Stern wrote:
The SCSI code has no means of knowing the actual length transferred,
so has no choice but to believe the length byte in the reply.
But the USB code does the transferring itself, and knows precisely
how many bytes were transferred. If 36 bytes were transferred and
the additional length byte is 0, indicating a length of 5, then the
USB code can fix the response and change the additional length byte
to 31, indicating a length of 36. That way the SCSI code knows that
not 5 but 36 bytes are valid, and it gets actual vendor and model strings.
I'm not familiar with the details of the SCSI code you are referring to,
but usb-storage does make available the actual transfer length. All the
transport routine paths set the resid field of the Scsi_Cmnd structure
properly. With this information, there should be no difficulty in
determining how many bytes were transferred. (Maybe the setting doesn't
percolate up to the particular code you mention -- and maybe other host
adapter drivers don't set resid correctly so you cannot rely on its value.
I don't know what other problems might crop up.)
In this most recent case reported, this looks very similar to
overflow residual, but not quite the same. I.e. *more* data is
actually immediately available (in the buffer) than *we requested* or can
find out by other means (i.e. checking the ADDITIONAL LENGTH field).
But if we requested 36 and the transport provided 36 then the
residual should be 0 (set by the transport). Had we requested 5 and the
transport provided 36 then the residual should be 31, but there's no way
of reporting this residual, since it is an *overflow* residual, and from
the comments therein, cmd->resid is *only underflow* residual.
Thus, *if* the cmd->resid field is set properly, then something like this,
in SCSI Core, might suffice:
</Disclaimer: this is NOT actual C code, but should be descriptive enough/>
int bytes_requested = <whatever>;
int bytes_got, additional_len;
...
</ok, we got response from INQUIRY/>
additional_len = buffer[4];
bytes_got = max(bytes_requested - cmd->resid, 0);
</now use bytes_got exclusively and ignore additional_len/>
...
</do assignments and whatever, and just before we're done print
a nice warning message if bytes_got != additional_len+5 />
--
Luben
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