Re: [PATCH] USB: usblp: add USBLP_QUIRK_NO_SET_INTF flag

[Alan Stern <stern@rowland.harvard.edu>]

On Thu, Jan 21, 2021 at 07:06:27PM -0600, Jeremy Figgins wrote:
> On 1/21/21 5:02 PM, Pete Zaitcev wrote:
> > On Thu, 21 Jan 2021 14:29:29 -0500
> > Alan Stern <stern@rowland.harvard.edu> wrote:
> > 
> > > > I'm also concerned about regressions. This is a legacy class driver,
> > > > only used where CUPS is not applicable, mostly with truly ancient
> > > > devices. So yes, setting a zero altsetting after enumeration should
> > > > be unnecessary. But you never know with the old firmware.
> > 
> > > How about skipping the call whenever the interface has only one
> > > altsetting?
> > 
> > Do you mean when it's only one and not equal to zero?
> > 
> > BTW, one other thing bothers me. Jeremy confirmed that my patch
> > worked, which skips the call when USB_QUIRK_NO_SET_INTF is set.
> > But if we look into drivers/usb/core/message.c, the control
> > exchange to set the altsetting is skipped in that case anyway.
> > So, usblp was calling usb_set_protocol, the suspect control was
> > skipped, but something else caused a problem. Could it be the
> > attempt to clear halt that triggered the problem?
> > 
> > -- Pete
> > 
> 
> 
> In my debugging, I found that it was the calls to usb_control_msg_send() in
> both usb_set_interface() and usb_clear_halt() caused the printer to lockup.
> I suppose another way to resolve this would to have a flag to prevent
> usb_clear_halt()'s call to usb_control_msg_send(), but I'm not an expert in
> USB, so I'm not sure of the ramifications of that.

The simplest solution in all cases is just to avoid calling either 
usb_set_interface or usb_clear_halt.  Especially since in cases where 
the interface has only one altsetting, neither call should be necessary.

Alan Stern