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

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

On Sun, Jan 17, 2021 at 11:44:16PM -0600, Pete Zaitcev wrote:
> On Sun, 17 Jan 2021 15:36:39 -0600
> Jeremy Figgins <kernel@jeremyfiggins.com> wrote:
> 
> > The naming is designed to mirror the existing
> > USB_QUIRK_NO_SET_INTF flag, but that flag is
> > not sufficient to make these devices work.
> > +	{ 0x0416, 0x5011, USBLP_QUIRK_NO_SET_INTF }, /* Winbond Electronics Corp. Virtual Com Port */
> 
> Jeremy, thanks for the patch. It looks mostly fine code-wise (quirk is
> out of numerical order), but I have a question: did you consider keying
> off usblp->dev->quirks instead?
> 
> How about this:
> 
> diff --git a/drivers/usb/class/usblp.c b/drivers/usb/class/usblp.c
> index 37062130a03c..0c4a98f00797 100644
> --- a/drivers/usb/class/usblp.c
> +++ b/drivers/usb/class/usblp.c
> @@ -1315,7 +1315,11 @@ static int usblp_set_protocol(struct usblp *usblp, int protocol)
>  	alts = usblp->protocol[protocol].alt_setting;
>  	if (alts < 0)
>  		return -EINVAL;
> -	r = usb_set_interface(usblp->dev, usblp->ifnum, alts);
> +	if (usblp->dev->quirks & USB_QUIRK_NO_SET_INTF) {
> +		r = 0;
> +	} else {
> +		r = usb_set_interface(usblp->dev, usblp->ifnum, alts);
> +	}
>  	if (r < 0) {
>  		printk(KERN_ERR "usblp: can't set desired altsetting %d on interface %d\n",
>  			alts, usblp->ifnum);

Would it be practical simply to skip the usb_set_interface() call 
whenever alts is 0?  After all, devices use altsetting 0 by default; it 
shouldn't be necessary to tell them to do so.

Alan Stern