[PATCH v2 2/2] USB: doc: Binding document for ehci-platform driver

On Wed, 24 Oct 2012, Stephen Warren wrote:

Device tree files always need a completely specific value in the
compatible property, even when less-specific/more-generic values are
also present. So for example, the Linux driver might only care about the
following existing:

compatible = "usb-ehci".

However, any actual EHCI controller is always some specific model, so
the compatible value must define which specific model it is, e.g.:

compatible = "nvidia,tegra20-ehci", "usb-ehci".

Now lets say the Tegra30 EHCI controller is identical to the Tegra20
controller. That needs to be explicitly represented too:

compatible = "nvidia,tegra30-ehci", "nvidia,tegra20-ehci", "usb-ehci".

In that case, the driver might still only declare support for
"nvidia,tegra20-ehci", but "nvidia,tegra30-ehci" is added to be explicit
about the actual HW model.

This doesn't continue forever though; you typically only list the
specific HW model, the base specific model it's compatible with, and any
generic value needed. So, a hypothetical Tegra40 EHCI controller would be:

compatible = "nvidia,tegra40-ehci", "nvidia,tegra20-ehci", "usb-ehci".

Given that, there is always something to key any newly required quirk
off, so even if the need for a quirk is found later, the device tree
already contains the information needed to trigger it.

Okay.  It appears that quite a few .dts/.dtsi files mention "usb-ehci", 
for no apparent reason (given that the drivers don't list it):

[stern at iolanthe usb-3.6]$ find arch -name '*.dts*' | xargs grep usb-ehci
arch/mips/cavium-octeon/octeon_3xxx.dts:                                compatible = "cavium,octeon-6335-ehci","usb-ehci";
arch/mips/cavium-octeon/octeon_68xx.dts:                                compatible = "cavium,octeon-6335-ehci","usb-ehci";
arch/arm/boot/dts/tegra20.dtsi:         compatible = "nvidia,tegra20-ehci", "usb-ehci";
arch/arm/boot/dts/tegra20.dtsi:         compatible = "nvidia,tegra20-ehci", "usb-ehci";
arch/arm/boot/dts/tegra20.dtsi:         compatible = "nvidia,tegra20-ehci", "usb-ehci";
arch/arm/boot/dts/at91sam9x5.dtsi:                      compatible = "atmel,at91sam9g45-ehci", "usb-ehci";
arch/arm/boot/dts/spear13xx.dtsi:                       compatible = "st,spear600-ehci", "usb-ehci";
arch/arm/boot/dts/spear13xx.dtsi:                       compatible = "st,spear600-ehci", "usb-ehci";
arch/arm/boot/dts/spear3xx.dtsi:                        compatible = "st,spear600-ehci", "usb-ehci";
arch/arm/boot/dts/spear600.dtsi:                        compatible = "st,spear600-ehci", "usb-ehci";
arch/arm/boot/dts/spear600.dtsi:                        compatible = "st,spear600-ehci", "usb-ehci";
arch/arm/boot/dts/at91sam9g45.dtsi:                     compatible = "atmel,at91sam9g45-ehci", "usb-ehci";
arch/powerpc/boot/dts/wii.dts:                  compatible = "nintendo,hollywood-usb-ehci",
arch/powerpc/boot/dts/wii.dts:                                  "usb-ehci";
arch/powerpc/boot/dts/sequoia.dts:                      compatible = "ibm,usb-ehci-440epx", "usb-ehci";
arch/powerpc/boot/dts/canyonlands.dts:                  compatible = "ibm,usb-ehci-460ex", "usb-ehci";

Is there a real reason that I'm not aware of?  Or can all these entries
safely be removed?

Alan Stern