Re: [PATCH v1 1/2] dt-binding: ptp: add bindings document for dte based ptp clock

From: Florian Fainelli
Date: Thu Jun 22 2017 - 21:04:54 EST

On 06/22/2017 05:42 PM, Scott Branden wrote:
> On 17-06-21 08:19 PM, Rob Herring wrote:
>> On Tue, Jun 20, 2017 at 3:48 PM, Scott Branden
>> <scott.branden@xxxxxxxxxxxx> wrote:
>>> Hi Rob,
>>> On 17-06-18 07:04 AM, Rob Herring wrote:
>>>> On Mon, Jun 12, 2017 at 01:26:00PM -0700, Arun Parameswaran wrote:
>>>>> Add device tree binding documentation for the Broadcom DTE
>>>>> PTP clock driver.
>>>>> Signed-off-by: Arun Parameswaran <arun.parameswaran@xxxxxxxxxxxx>
>>>>> ---
>>>>> Documentation/devicetree/bindings/ptp/brcm,ptp-dte.txt | 13
>>>>> +++++++++++++
>>>>> 1 file changed, 13 insertions(+)
>>>>> create mode 100644
>>>>> Documentation/devicetree/bindings/ptp/brcm,ptp-dte.txt
>>>>> diff --git a/Documentation/devicetree/bindings/ptp/brcm,ptp-dte.txt
>>>>> b/Documentation/devicetree/bindings/ptp/brcm,ptp-dte.txt
>>>>> new file mode 100644
>>>>> index 0000000..07590bc
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/ptp/brcm,ptp-dte.txt
>>>>> @@ -0,0 +1,13 @@
>>>>> +* Broadcom Digital Timing Engine(DTE) based PTP clock driver
>>>> Bindings describe h/w, not drivers.
>>>>> +
>>>>> +Required properties:
>>>>> +- compatible: should be "brcm,ptp-dte"
>>>> Looks too generic. You need SoC specific compatible strings.
>>> Rob, could you please help me understand the use of adding SoC specific
>>> compatible strings.
>>> I still don't get it.
>>> It's my understanding that the SoC compatibility string is to future
>>> proof
>>> against bugs/incompatibilities
>>> between different versions of the hardware block due to integration
>>> issues
>>> or any other reason.
>>> You can then compare in your driver because the strings were already
>>> used in
>>> the dtb.
>>> That would make sense if you can't already differentiate what SoC you
>>> are
>>> running on.
>>> But the SoC is already specified in the root of the device tree in the
>>> compatible string?
>>> Why can't you just use of_machine_is_compatible inside your driver when
>>> needed?
>> Use of of_machine_is_compatible in drivers will result in the same
>> mess we had with machine_is_X defines pre-DT. It practically
>> guarantees that you must update the driver for a new SoC (with
>> fallback compatibles you don't). Plus the matching logic for
>> of_machine_is_compatible is open coded logic in every driver which is
>> worse IMO than having a standard match table.
> I don't understand what you mean by fallback compatible then.
> Let's say I have 3 SoCs that each contain the same ip block.
> You want us to add a fallback compatibility per SoC, is that correct?

I think Rob meant a fallback compatibility for the particular block.
E.g: brcm,iproc-ptp is the fallback compatible string, but in your
SoC-specific DTS, you would have at least:

compatible = "brcm,cygnus-ptp", "brcm,iproc-ptp";

Where cygnus-ptp is more specific than iproc-ptp

> Then, if there is a workaround discovered in a particular SoC the driver
> can be updated in the future without changing the dtb.
> Then, the block gets added to a 4th SoC.
> You want us to another new compatibility string for the new SoC?
> If the new SoC has a bug then the driver has to be updated whether it is
> in uses the fallback compatible or machine_is_compatible string.
> There is no difference in amount of code added to a driver when a new
> SoC is introduced into the system that has bugs that need to be handled
> by the driver.
> The difference is in your recommendation we need to go through all the
> drivers used by the new SoC and add fallback compatibility strings.

Not really, the fallback is what the driver should be matching by
default (hence the name fallback) and if and only if you need to have a
SoC-specific behavior in your driver (because of bugs, or slight
differences) would you be matching this SoC-specific compatible string
to capture that and key the driver behavior based off that.

> Then, we have to modify all the devicetree documentation for all the
> drivers. Then, we have to ensure that all dts files populate this new
> fallback string (even if it is unused). We don't see the benefit in
> doing any of that. Using machine_is_compatible and having less
> compatibility strings to deal appears much cleaner and more foolproof
> for all situations.

When you introduce a new SoC, you would update all the bindings for the
devices (not drivers) that are susceptible to be used by this SoC. If
all goes well, your SoC has little bugs, and your drivers don't even
need to see an update because they are already matching the fallback
compatible string.

That's what I understand from the suggestion but I may be totally off
rails here.