linux-kernel - Looking for help with Kconfig dependencies

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Date:   Fri, 18 Jun 2021 10:05:48 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Douglas Anderson <dianders@...omium.org>
Subject: Looking for help with Kconfig dependencies

Hi,

I'm adding a new driver and have an issue with Kconfig dependencies
that I coulnd't sort out so far.

Patch https://lore.kernel.org/patchwork/patch/1444212/ adds the new
onboard_usb_hub driver which exports two functions,
onboard_hub_create_pdevs() and onboard_hub_destroy_pdevs(). It also
provides stubs for these functions which are used when the driver
is not selected (CONFIG_USB_ONBOARD_HUB=n).

The new exported functions are called by the xhci-plat driver
(https://lore.kernel.org/patchwork/patch/1444215/). Since xhci-plat
now depends on symbols from the onboard_hub_driver the following
dependency was added to its Kconfig entry:

  config USB_XHCI_PLATFORM
    tristate "Generic xHCI driver for a platform device"
    select USB_XHCI_RCAR if ARCH_RENESAS
 +  depends on USB_ONBOARD_HUB || !USB_ONBOARD_HUB

This generally seems to work, however when USB_XHCI_PLATFORM is
forced to be builtin by another driver that depends on it (e.g.
USB_DWC3) it is still possible to build the onboard_hub driver
as a module, which results in unresolved symbols:

aarch64-linux-gnu-ld: drivers/usb/host/xhci-plat.o: in function
`xhci_plat_remove':
drivers/usb/host/xhci-plat.c:427: undefined reference to
`onboard_hub_destroy_pdevs'
drivers/usb/host/xhci-plat.c:427:(.text+0x82c): relocation truncated
to fit: R_AARCH64_CALL26 against undefined symbol
`onboard_hub_destroy_pdevs'
aarch64-linux-gnu-ld: drivers/usb/host/xhci-plat.o: in function
`xhci_plat_probe':
drivers/usb/host/xhci-plat.c:379: undefined reference to
`onboard_hub_create_pdevs'
drivers/usb/host/xhci-plat.c:379:(.text+0x131c): relocation truncated
to fit: R_AARCH64_CALL26 against undefined symbol
`onboard_hub_create_pdevs'

Kconfig generates the following warning with this configuration:

WARNING: unmet direct dependencies detected for USB_XHCI_PLATFORM
  Depends on [m]: USB_SUPPORT [=y] && USB [=y] && USB_XHCI_HCD [=y] && (USB_ONBOARD_HUB [=m] || !USB_ONBOARD_HUB [=m])
  Selected by [y]:
  - USB_DWC3 [=y] && USB_SUPPORT [=y] && (USB [=y] || USB_GADGET [=y]) && HAS_DMA [=y] && USB_XHCI_HCD [=y]
  Selected by [m]:
  - USB_CDNS_SUPPORT [=m] && USB_SUPPORT [=y] && (USB [=y] || USB_GADGET [=y]) && HAS_DMA [=y] && USB_XHCI_HCD [=y]
  - USB_BRCMSTB [=m] && USB_SUPPORT [=y] && USB [=y] && (ARCH_BRCMSTB [=y] && PHY_BRCM_USB [=m] || COMPILE_TEST [=y]) && USB_XHCI_HCD [=y]
  - USB_XHCI_MVEBU [=m] && USB_SUPPORT [=y] && USB [=y] && USB_XHCI_HCD [=y] && HAS_IOMEM [=y] && (ARCH_MVEBU [=y] || COMPILE_TEST [=y])

I read through kconfig-language.rst and experimented a fair bit,
but haven't found a working solution. Any advice would be
appreciated.

Thanks

Matthias