linux-kernel - Re: [PATCH] usb: dwc2: host: Fix wMaxPacketSize handling (fix webcam regression)

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Message-ID: <20190603224417.GN40515@google.com>
Date:   Mon, 3 Jun 2019 15:44:17 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Minas Harutyunyan <hminas@...opsys.com>,
        linux-rockchip@...ts.infradead.org,
        Stefan Wahren <stefan.wahren@...e.com>, tfiga@...omium.org,
        groeck@...omium.org, Martin Schiller <ms@....tdt.de>,
        stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: dwc2: host: Fix wMaxPacketSize handling (fix webcam
 regression)

On Fri, May 31, 2019 at 01:04:12PM -0700, Douglas Anderson wrote:
> In commit abb621844f6a ("usb: ch9: make usb_endpoint_maxp() return
> only packet size") the API to usb_endpoint_maxp() changed.  It used to
> just return wMaxPacketSize but after that commit it returned
> wMaxPacketSize with the high bits (the multiplier) masked off.  If you
> wanted to get the multiplier it was now up to your code to call the
> new usb_endpoint_maxp_mult() which was introduced in
> commit 541b6fe63023 ("usb: add helper to extract bits 12:11 of
> wMaxPacketSize").
> 
> Prior to the API change most host drivers were updated, but no update
> was made to dwc2.  Presumably it was assumed that dwc2 was too
> simplistic to use the multiplier and thus just didn't support a
> certain class of USB devices.  However, it turns out that dwc2 did use
> the multiplier and many devices using it were working quite nicely.
> That means that many USB devices have been broken since the API
> change.  One such device is a Logitech HD Pro Webcam C920.
> 
> Specifically, though dwc2 didn't directly call usb_endpoint_maxp(), it
> did call usb_maxpacket() which in turn called usb_endpoint_maxp().
> 
> Let's update dwc2 to work properly with the new API.
> 
> Fixes: abb621844f6a ("usb: ch9: make usb_endpoint_maxp() return only packet size")
> Cc: stable@...r.kernel.org
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

I'm not really familiar with the dwc2 driver, but this looks
reasonable to me. FWIW:

Reviewed-by: Matthias Kaehlcke <mka@...omium.org>