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

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Date:   Tue, 20 Jun 2017 13:48:18 -0700
From:   Scott Branden <scott.branden@...adcom.com>
To:     Rob Herring <robh@...nel.org>,
        Arun Parameswaran <arun.parameswaran@...adcom.com>
Cc:     Richard Cochran <richardcochran@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH v1 1/2] dt-binding: ptp: add bindings document for dte
 based ptp clock

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@...adcom.com>
>> ---
>>   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?

Please explain what I'm missing.  I see other drivers already following 
the of_machine_is_compatible
approach and it makes more sense to me than adding SoC specific 
compatible strings into every
driver.

Regards,
  Scott