Re: [RFC v2 4/5] DT bindings documentation for Synopsys UDC platform driver

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

On 01/19/2017 11:30 AM, Scott Branden wrote:
> Hi Rob,
> 
> On 17-01-19 09:36 AM, Rob Herring wrote:
>> On Tue, Jan 17, 2017 at 01:35:07PM +0530, Raviteja Garimella wrote:
>>> This patch adds device tree bindings documentation for Synopsys
>>> USB device controller platform driver.
>>
>> Bindings describe h/w, not drivers.
>>>
>>> Signed-off-by: Raviteja Garimella <raviteja.garimella@broadcom.com>
>>> ---
>>>  .../devicetree/bindings/usb/snps,dw-ahb-udc.txt    | 27
>>> ++++++++++++++++++++++
>>>  1 file changed, 27 insertions(+)
>>>  create mode 100644
>>> Documentation/devicetree/bindings/usb/snps,dw-ahb-udc.txt
>>>
>>> diff --git
>>> a/Documentation/devicetree/bindings/usb/snps,dw-ahb-udc.txt
>>> b/Documentation/devicetree/bindings/usb/snps,dw-ahb-udc.txt
>>> new file mode 100644
>>> index 0000000..0c18327
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/usb/snps,dw-ahb-udc.txt
>>> @@ -0,0 +1,27 @@
>>> +Synopsys USB Device controller.
>>> +
>>> +The device node is used for Synopsys Designware Cores AHB
>>> +Subsystem Device Controller (UDC).
>>> +
>>> +This device node is used by UDCs integrated it Broadcom's
>>> +Northstar2 and Cygnus SoC's.
>>
>> You need compatible strings for these in addition.
>>
> We don't need compatibility strings when an IP block is integrated into
> an SoC.  Otherwise each time we add the IP block to a new SoC we would
> need to update ever linux driver that supports that SoC.  That doesn't
> make sense?

You probably do need such a thing, here is how the compatible strings
for IP blocks integrated into SoCs could be used:

- provide a compatible strings which describes exactly the integration
of this peripheral into a given SoC, e.g: brcm,udc-ns2, the reason for
that is that you want to be able to capture the specific IP block
integration into a specific SoC and all its quirks

- if the block has its own revision scheme (and it can be relied on),
provide it: brcm,udc-v1.2 and that is probably the most meaningful
compatible string for a client program here

- have a some kind of fallback/catchall compatible string that describes
the block: brcm,udc which may also work just fine, although is not preferred

Defining compatible strings is meant to avoid making (possibly
incompatible) Device Tree binding changes in the future, and you always
have the liberty as a client program (OS, bootloader) to match only the
compatible strings you care about, from the most specific (which
includes the exact SoC) to the least specific.

The key thing is that, if the full set of compatible strings are present
and available, you can retroactively fix your driver to be more
specific, very much less so your Device Tree blob (although there is
disagreement).
-- 
Florian