Randy Dunlap <rdunlap@infradead.org> reported this problem
on i386:

> drivers/built-in.o: In function `ci_hdrc_host_init':
> (.text+0x2ce75c): undefined reference to `ehci_init_driver'
>
> When USB_EHCI_HCD=m and USB_CHIPIDEA=y.

In fact, this problem is existed on all platforms which are using
chipidea driver. The root cause of this problem is the chipidea host
uses symbol exported from ehci-hcd, but chipidea core
does not depends on USB_EHCI_HCD. So, chipidea driver
will not be compiled as module if USB_EHCI_HCD=m.

It is very hard to give a perfect solution since chipidea core
depends on USB || USB_GADGET, and chipdiea host depends on
both USB_EHCI_HCD and USB_CHIPIDEA, the same problem exists for
gadget.

To fix this problem, we had to have below assumptions:

- If USB_EHCI_HCD=y && USB_GADGET=y, USB_CHIPIDEA can be 'y'.

- If USB_EHCI_HCD=m && USB_GADGET=y, USB_CHIPIDEA=m
or USB_CHIPIDEA_HOST can't be seen if USB_CHIPIDEA=y.
It will cause compile error due to no glue layer for ehci:

> error: #error "missing bus glue for ehci-hcd"

So, we had to compile USB_CHIPIDEA=m if USB_EHCI_HCD=m,
current ehci hcd core guarantee it.

- If USB_EHCI_HCD=y && USB_GADGET=m, USB_CHIPIDEA=m
or USB_CHIPIDEA_UDC can't be seen if USB_CHIPIDEA=y.
Of cos, the gadget will out of working at this situation,
so the user had to compile USB_CHIPIDEA=m.

- USB_EHCI_HCD=m && USB_GADGET=m, we can't see
USB_CHIPIDEA_HOST and USB_CHIPIDEA_UDC unless
USB_CHIPIDEA=m.

The reason why it has above assumptions:
- If both ehci core and gadget core build as module,
the chipidea has to build as module.
- If one of ehci core or gadget core is built in, another
is built as module, we can only enable the function which
is built in, or enable both roles as modules (USB_CHIPIDEA=m),
since chipidea core driver takes care of both host and device roles.
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>