linux-kernel - Re: [PATCH v4 1/1] usb: xhci: Add Clear_TT

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Date:   Tue, 7 May 2019 10:29:33 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Mathias Nyman <mathias.nyman@...ux.intel.com>
cc:     Jim Lin <jilin@...dia.com>, <gregkh@...uxfoundation.org>,
        <mathias.nyman@...el.com>, <hminas@...opsys.com>,
        <kai.heng.feng@...onical.com>, <drinkcat@...omium.org>,
        <prime.zeng@...ilicon.com>, <malat@...ian.org>,
        <nsaenzjulienne@...e.de>, <jflat@...omium.org>,
        <linus.walleij@...aro.org>, <clabbe@...libre.com>,
        <colin.king@...onical.com>, <linux-usb@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/1] usb: xhci: Add Clear_TT_Buffer

On Tue, 7 May 2019, Mathias Nyman wrote:

> On 6.5.2019 17.57, Alan Stern wrote:
> > On Mon, 6 May 2019, Jim Lin wrote:
> > 
> >> USB 2.0 specification chapter 11.17.5 says "as part of endpoint halt
> >> processing for full-/low-speed endpoints connected via a TT, the host
> >> software must use the Clear_TT_Buffer request to the TT to ensure
> >> that the buffer is not in the busy state".
> >>
> >> In our case, a full-speed speaker (ConferenceCam) is behind a high-
> >> speed hub (ConferenceCam Connect), sometimes once we get STALL on a
> >> request we may continue to get STALL with the folllowing requests,
> >> like Set_Interface.
> >>
> >> Here we add Clear_TT_Buffer for the following Set_Interface requests
> >> to get ACK successfully.
> >>
> >> Originally usb_hub_clear_tt_buffer uses urb->dev->devnum as device
> >> address while sending Clear_TT_Buffer command, but this doesn't work
> >> for XHCI.
> > 
> > Why doesn't it work for xHCI?  Clear-TT-Buffer is part of the USB 2.0
> > spec; it should work exactly the same for xHCI as for a USB-2.0 host
> > controller.
> > 
> > Alan Stern
> > 
> 
> For other host controllers udev->devnum is the same as the address of the
> usb device, chosen and set by usb core.
> 
> With xHC the controller hardware assigns the address, and won't be the same as
> devnum.
> 
> The Clear-TT-Buffer request sent to the hub includes the address of the LS/FS
> child device in wValue field. usb_hub_clear_tt_buffer() uses udev->devnum to set the
> address wValue. This won't work for devices connected to xHC

I see.  Thanks for the explanation; it makes sense now.  The patch
description should explain this too.

Wouldn't it be better to add a field containing the device address to
struct usb_device?  And also export it, either in sysfs or debugfs?  
It seems like the kind of thing that might be important for debugging.  
If we did this then the usb_hub_clear_tt_buffer API wouldn't need to be
changed.

Alan Stern