Re: [PATCH v10 1/5] usb: xhci: Change the XHCI link order in the Makefile

[Florian Fainelli <f.fainelli@gmail.com>]

On 5/13/2020 10:39 AM, Alan Stern wrote:
> On Wed, May 13, 2020 at 07:05:05PM +0200, Greg Kroah-Hartman wrote:
>> On Wed, May 13, 2020 at 09:31:11AM -0700, Florian Fainelli wrote:
>>>
>>>
>>> On 5/13/2020 9:27 AM, Greg Kroah-Hartman wrote:
>>>> On Wed, May 13, 2020 at 08:08:07AM -0700, Florian Fainelli wrote:
>>>>>
>>>>>
>>>>> On 5/13/2020 5:26 AM, Greg Kroah-Hartman wrote:
>>>>>> On Tue, May 12, 2020 at 11:00:15AM -0400, Al Cooper wrote:
>>>>>>> Some BRCMSTB USB chips have an XHCI, EHCI and OHCI controller
>>>>>>> on the same port where XHCI handles 3.0 devices, EHCI handles 2.0
>>>>>>> devices and OHCI handles <2.0 devices. Currently the Makefile
>>>>>>> has XHCI linking at the bottom which will result in the XHIC driver
>>>>>>> initalizing after the EHCI and OHCI drivers and any installed 3.0
>>>>>>> device will be seen as a 2.0 device. Moving the XHCI linking
>>>>>>> above the EHCI and OHCI linking fixes the issue.
>>>>>>
>>>>>> What happens if all of these are modules and they are loaded in a
>>>>>> different order?  This makefile change will not help with that, you need
>>>>>> to have logic in the code in order to properly coordinate this type of
>>>>>> mess, sorry.
>>>>>
>>>>> I believe we should be using module soft dependencies to instruct the
>>>>> module loaders to load the modules in the correct order, so something
>>>>> like this would do (not tested) for xhci-plat-hcd.c:
>>>>>
>>>>> MODULE_SOFTDEP("post: ehci-hcd ohci-hcd");
>>>>>
>>>>> and I am not sure whether we need to add the opposite for ehci-hcd and
>>>>> ohci-hcd:
>>>>>
>>>>> MODULE_SOFTDEP("pre: xhci-plat-hcd");
>>>>
>>>> That's a nice start, but what happens if that isn't honored?  This
>>>> really needs to work properly for any order as you never can guarantee
>>>> module/driver loading order in a system of modules.
>>>
>>> I also suggested that device links may help, though I am not sure. What
>>> do you suggest to be done?
>>
>> No idea.  device links will help if you defer the probe properly until
>> you see the proper drivers binding correctly.
> 
> I suspect that in general there is no way to do this properly.
> 
> We can't modify ehci-hcd and ohci-hcd to make them wait.  In fact, for 
> all they know, xhci-hcd will _never_ be loaded.
> 
> One thing that might be possible (although not all platforms may support 
> it) is if xhci-hcd could somehow disconnect all devices attached to a 
> peer port when it starts up.  But that would be disruptive to any 
> devices that aren't USB-3.
> 
> We faced a very similar ordering problem between ehci-hcd and 
> [ou]hci-hcd many years ago, and we never found a good solution.  
> We did arrange the link order so that ehci-hcd precedes the others, and 
> we added a warning message to ehci-hcd which gets printed if the module 
> initialization routine runs after [ou]hci-hcd is loaded.  Also, there 
> are MODULE_SOFTDEP lines in ohci-pci.c and uhci-pci.c.

Given that these modules are used on specific SoC platforms, where we
usually provide a reference implementation of user space and kernel
space and documentation, it seems to me that the MODULE_SOFTDEP(),
despite being a hint and best effort from user space module loaders is
probably acceptable.
-- 
Florian