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From: Mathias Nyman <>
To: Alan Stern <>
Cc: Jim Lin <>,,,,,,,,,,,,,,
Subject: Re: [PATCH v4 1/1] usb: xhci: Add Clear_TT_Buffer
Date: Wed, 8 May 2019 13:21:03 +0300	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 7.5.2019 17.29, Alan Stern wrote:
> 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.

Agree, adding address to struct usb_device sounds better.


      reply	other threads:[~2019-05-08 10:18 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-06 14:34 Jim Lin
2019-05-06 14:57 ` Alan Stern
2019-05-07  8:00   ` Mathias Nyman
2019-05-07 14:29     ` Alan Stern
2019-05-08 10:21       ` Mathias Nyman [this message]

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