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SubjectRe: [External] Re: [PATCH v5 5/8] usb: mausb_host: Introduce PAL processing
On Sun, 26 Apr 2020, Vladimir Stankovic wrote:

> On 26.4.20. 16:31, Alan Stern wrote:
> > On Sun, 26 Apr 2020, Vladimir Stankovic wrote:
> >
> >> On 26.4.20. 02:32, Alan Stern wrote:
> >>> On Sat, 25 Apr 2020 vladimir.stankovic@displaylink.com wrote:
> >>>
> >>>> Protocol adaptation layer (PAL) implementation has been added to
> >>>> introduce MA-USB structures and logic.
> >>>>
> >>>> Signed-off-by: Vladimir Stankovic <vladimir.stankovic@displaylink.com>
> >>>
> >>> ...
> >>>
> >>>> + /*
> >>>> + * Masking URB_SHORT_NOT_OK flag as SCSI driver is adding it where it
> >>>> + * should not, so it is breaking the USB drive on the linux
> >>>> + */
> >>>> + urb->transfer_flags &= ~URB_SHORT_NOT_OK;
> >>>
> >>> Removing the SHORT_NOT_OK flag is _not_ a valid thing to do. It will
> >>> cause drivers to malfunction.
> >>>
> >>> Can you please explain this comment?
> >>>
> >>> What SCSI driver?
> >>>
> >>> When is the flag being added?
> >>>
> >>> How does it break USB drives?
> >>>
> >>> Why haven't you already reported this problem to the
> >>> appropriate maintainers?
> >>>
> >>> Alan Stern
> >>>
> >>
> >> Hi,
> >>
> >> Issue that removal of SHORT_NOT_OK flag addressed is linked to particular
> >> set of Kingston USB 3.0 flash drives (super speed) - other USB flash drives
> >> haven't had this flag set. Without this "fix", those Kingston flash drives
> >> are not being enumerated properly.
> >
> > Please explain in detail how the enumeration of these Kingston flash
> > drives fails. Or if such an explanation has already been posted,
> > please provide a link to it.
>
> Will reproduce the issue once again (w/o the fix) and run through the events.
> Issue has been noticed during early development, and addressed right away.
> >
> >> This particular line was added in the early stage of development, during
> >> enumeration process implementation. The reason why it remained in the code
> >> since is because we haven't noticed any side-effects, even with various
> >> USB devices being attached to remote MA-USB device, including flash drives,
> >> cameras, wireless mice, etc.
> >
> > Come to think of it, the SHORT_NOT_OK flag is mainly used with HCDs
> > that don't have scatter-gather support. Since your mausb driver does
> > support scatter-gather, you most likely won't encounter any problems
> > unless you go looking for them specifically.
> >
> >> The problem has been reported, and is actively being investigated.
> >
> > Where was the problem reported (URL to a mailing list archive)? Who is
> > investigating it?
>
> Ticket has been submitted to DisplayLink's internal issue-tracking system
> and is being investigated by mausb-host-devel team.

Okay. What SCSI driver does the comment refer to? Is it something
internal to DisplayLink or is it part of the regular Linux kernel?

Alan Stern

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