Re: [PATCH] usb: core: improve handling of hubs with no ports

From: Jack Pham <quic_jackp@quicinc.com>
To: Alan Stern <stern@rowland.harvard.edu>,
	Heiner Kallweit <hkallweit1@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Linux USB Mailing List <linux-usb@vger.kernel.org>,
	"open list:ARM/Amlogic Meson..."
	<linux-amlogic@lists.infradead.org>,
	Tung Nguyen <tunguyen@apm.com>,
	"Mathias Nyman" <mathias.nyman@intel.com>
Subject: Re: [PATCH] usb: core: improve handling of hubs with no ports
Date: Thu, 24 Feb 2022 12:06:38 -0800	[thread overview]
Message-ID: <20220224200637.GE13801@jackp-linux.qualcomm.com> (raw)
In-Reply-To: <Yhaxb0OvMwLtd6vr@rowland.harvard.edu>

On Wed, Feb 23, 2022 at 05:13:03PM -0500, Alan Stern wrote:
> On Wed, Feb 23, 2022 at 09:58:56PM +0100, Heiner Kallweit wrote:
> > On 23.02.2022 15:17, Alan Stern wrote:
> > > Don't change register_root_hub().  Just change xhci_plat_probe(); make 
> > > it skip the second call to usb_add_hcd() if there are no USB-3 ports.

I believe this had been attempted in the past, but it does not appear
that patch was ever accepted:

https://lore.kernel.org/linux-usb/1517221474-19627-1-git-send-email-tqnguyen@apm.com/

Jack

> > This works on my system. However a consequence is that xhci->shared_hcd
> > is NULL.
> 
> Why is that?  xhci->shared_hcd doesn't get set in usb_add_hcd(), so 
> skipping that call shouldn't cause it to be NULL.
> 
> Note: If you skip calling usb_add_hcd(), you will also have to skip the 
> corresponding call to usb_remove_hcd().  There may be a few more 
> subtleties involved as well; like I said before, I'm not an expert on 
> this driver.  You should ask the xhci-hcd maintainer for advice.
> 
> Alan Stern
> 
> >  There are a few places like the following in xhci.c where
> > this may result in a NPE. Not knowing the USB subsystem in detail
> > I can't say whether these places are in any relevant path.
> > 
> > static int xhci_run_finished(struct xhci_hcd *xhci)
> > {
> >         if (xhci_start(xhci)) {
> >                 xhci_halt(xhci);
> >                 return -ENODEV;
> >         }
> >         xhci->shared_hcd->state = HC_STATE_RUNNING;
> > 
> > 
> > 
> > > Alan Stern
> > > 
> > >> What I can do: submit my patches as RFC, then there's a better basis
> > >> for a discussion.
> > >>
> > >>> Alan Stern
> > >>
> > >> Heiner
> > 
> > Heiner

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