linux-kernel - Re: [PATCH] usb: host: ehci-tegra: fix Kconfig depencies

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Date:   Mon, 25 Jan 2021 22:10:24 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Chen <peter.chen@...nel.org>,
        Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <treding@...dia.com>,
        Arnd Bergmann <arnd@...db.de>,
        USB list <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: host: ehci-tegra: fix Kconfig depencies

On Mon, Jan 25, 2021 at 5:22 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Mon, Jan 25, 2021 at 12:32:30PM +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > Selecting the chipidea driver from the old Kconfig symbol
> > can lead to a missing dependency:
>
> Arnd:
>
> I found this whole patch a little confusing.  For example, in the
> sentence above, what does "the old Kconfig symbol" refer to?
>
> Comparing the various Kconfig files, I see what the problem is.  The
> commit which this one fixes made CONFIG_EHCI_TEGRA select
> CONFIG_USB_CHIPIDEA, but it didn't make EHCI_TEGRA depend on the things
> that USB_CHIPIDEA depends on.  Can you please state this more explicitly
> in the patch description?

Sure, I'll resend in a bit.

> > WARNING: unmet direct dependencies detected for USB_CHIPIDEA
> >   Depends on [m]: USB_SUPPORT [=y] && (USB_EHCI_HCD [=y] && USB_GADGET [=m] || USB_EHCI_HCD [=y] && !USB_GADGET [=m] || !USB_EHCI_HCD [=y] && USB_GADGET [=m]) && HAS_DMA [=y]
> >   Selected by [y]:
> >   - USB_EHCI_TEGRA [=y] && USB_SUPPORT [=y] && USB [=y] && USB_EHCI_HCD [=y] && ARCH_TEGRA [=y]
> > aarch64-linux-ld: drivers/usb/chipidea/otg.o: in function `ci_handle_vbus_change':
> > otg.c:(.text+0x3c8): undefined reference to `usb_gadget_vbus_connect'
> > aarch64-linux-ld: otg.c:(.text+0x42c): undefined reference to `usb_gadget_vbus_disconnect'
> > aarch64-linux-ld: drivers/usb/chipidea/otg.o: in function `ci_otg_work':
> > otg.c:(.text+0x5d4): undefined reference to `usb_gadget_vbus_disconnect'
> > ...
> >
> > Duplicate the dependency to ensure that this driver can
> > only be a loadable module if one of its dependencies is.
> >
> > Fixes: c3590c7656fb ("usb: host: ehci-tegra: Remove the driver")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  drivers/usb/host/Kconfig | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/usb/host/Kconfig b/drivers/usb/host/Kconfig
> > index 160e5d3927e2..66b01b619ecd 100644
> > --- a/drivers/usb/host/Kconfig
> > +++ b/drivers/usb/host/Kconfig
> > @@ -269,6 +269,7 @@ config USB_EHCI_HCD_AT91
> >  config USB_EHCI_TEGRA
> >       tristate "NVIDIA Tegra HCD support"
> >       depends on ARCH_TEGRA
> > +     depends on ((USB_EHCI_HCD && USB_GADGET) || (USB_EHCI_HCD && !USB_GADGET) || (!USB_EHCI_HCD && USB_GADGET)) && HAS_DMA
> >       select USB_CHIPIDEA
> >       select USB_CHIPIDEA_HOST
> >       select USB_CHIPIDEA_TEGRA
>
> Isn't there at least one other missing dependency?  This entry selects
> USB_CHIPIDEA_TEGRA, which depends on OF.  So shouldn't this entry also
> depend on OF?  Or does the Kconfig system detect that for us?

Yes, there is a hard dependency on ARCH_TEGRA, which implies OF.

> Also, while I'm no expert on the Kconfig language, it seems that the new
> "depends" line could be a lot easier to understand if it was refactored
> with some comments added.  Yes, I realize you just copied the existing
> dependency from the USB_CHIPIDEA entry -- that one could stand to be
> cleaned up as well.
>
> For instance, how about putting the HAS_DMA part into a separate line,
> since it's unrelated to the other stuff?

Actually it's probably best to just drop the HAS_DMA here, as it is also
implied by ARCH_TEGRA.

>  And the rest looks like it
> could be changed to:
>
>         depends on USB_EHCI_HCD || USB_GADGET
>
> although that probably isn't quite valid.  Still, can't it be changed to
> something simpler than
>
>         (USB_EHCI_HCD && USB_GADGET) || (USB_EHCI_HCD && !USB_GADGET) ||
>                         (!USB_EHCI_HCD && USB_GADGET)

The problem is that if either USB_EHCI_HCD or USB_GADGET are loadable
modules, then USB_CHIPIDEA must not be built-in, but if one of the two is
disabled, we must still have a dependency on the other. I guess it could be
rewritten into

      depends on USB_EHCI_HCD || USB_GADGET  # needs at least one of the two
      depends on m ||  USB_EHCI_HCD!=m # must be =m if USB_EHCI_HCD=m
      depends on m ||  USB_GADGET!=m # must be =m if USB_GADGET=m

I see that USB_EHCI_TEGRA already depends on USB_EHCI_HCD,
so I think this collapses into a much simpler

      depends on USB_GADGET || USB_GADGET=n # for USB_CHIPIDEA

         Arnd