linux-kernel - Re: [PATCH v7 1/1] usb: xhci: Add Clear_TT

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Date:   Thu, 9 May 2019 10:31:59 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Greg KH <gregkh@...uxfoundation.org>
cc:     Jim Lin <jilin@...dia.com>, <mathias.nyman@...el.com>,
        <kai.heng.feng@...onical.com>, <drinkcat@...omium.org>,
        <keescook@...omium.org>, <nsaenzjulienne@...e.de>,
        <jflat@...omium.org>, <malat@...ian.org>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 1/1] usb: xhci: Add Clear_TT_Buffer

On Thu, 9 May 2019, Greg KH wrote:

> On Thu, May 09, 2019 at 08:03:15PM +0800, Jim Lin wrote:
> > --- a/include/linux/usb.h
> > +++ b/include/linux/usb.h
> > @@ -625,6 +625,7 @@ struct usb3_lpm_parameters {
> >   *		parent->hub_delay + wHubDelay + tTPTransmissionDelay (40ns)
> >   *
> >   *	Will be used as wValue for SetIsochDelay requests.
> > + * @devaddr: address on a USB bus, assigned by controller like XHCI
> >   *
> >   * Notes:
> >   * Usbcore drivers should not set usbdev->state directly.  Instead use
> > @@ -709,6 +710,7 @@ struct usb_device {
> >  	unsigned lpm_disable_count;
> >  
> >  	u16 hub_delay;
> > +	int devaddr;
> 
> Shouldn't this be u32?

In fact the device address is an unsigned 7-bit value.  The size of the 
variable we store it in doesn't matter much.

BUT!  If it's going to be stored in a regular int then it's foolish to 
leave a 16-bit gap between it and the preceding field in the structure.  
It should be added at some appropriate spot in the structure, not at 
the end.

Overall I think this should be broken up into two patches: one to
introduce the new field and one to implement Clear-TT-Buffer for xHCI.

Furthermore, update_devnum() in hub.c should do:

	if (udev->devaddr == 0)
		udev->devaddr = devnum;

Then the code usb_hub_clear_tt_buffer() can just use devaddr without
needing to check the HCD type.

Alan Stern