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

[Heiner Kallweit]

On 24.02.2022 21:06, Jack Pham wrote:
> 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/
> 
I also found that xhci at several places relies on a proper shared_hcd,
even if there are no USB3 ports. Therefore maybe go with the less invasive
original version of my patch?

https://www.spinics.net/lists/linux-usb/msg222998.html


> 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