netdev - Re: [PATCH v2 1/3] Ethernet packet sniffer: Device tree binding and vendor prefix

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Message-ID: <54E376AF.80602@linn.co.uk>
Date:	Tue, 17 Feb 2015 17:13:19 +0000
From:	Stathis Voukelatos <stathis.voukelatos@...n.co.uk>
To:	Mark Rutland <mark.rutland@....com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"abrestic@...omium.org" <abrestic@...omium.org>
Subject: Re: [PATCH v2 1/3] Ethernet packet sniffer: Device tree binding and
 vendor prefix

Hi Mark,

On 17/02/15 16:35, Mark Rutland wrote:
>
>>>> +- tstamp-hz : frequency of the timestamp counter
>
> Is this the frequency the clock is running at, or a frequency that it
> should be programmed to in order to be used?
>
> The former can be queried from the common clock framework, and if you
> intended the latter the wording shuold be a little more explicit about
> that being the case.
>

It is the frequency of the timestamp values supplied to the sniffer
module. It is used by the driver to convert to nanoseconds.
I was trying to be somewhat generic here and not assume that it
is necessarily the same as the 'tstamp' clock below, in which case we
could indeed obtain it using the common clock framework.

>> See: include/linux/clocksource.h
>> The driver uses a cyclecounter and timecounter to convert raw timestamps
>> to nanoseconds. 'tstamp-shift' refers to the 'shift' field of the
>> cyclecounter structure, that can be used to improve the precision of
>> the conversion
>
> Sure, but the very concept of a cyclecounter is a Linux implementation
> detail. If we have the frequency of the timer we should be able to
> dynamically generate this, so there's no need for this to be in the DT.
>

Most networking driver use hard-coded values for that, but in my case
I did not want to assume a certain fixed clock frequency. I will remove
it from the DT and generate it dynamically. There is a kernel function
clocks_calc_mult_shift() to do it but unfortunately it is not exported,
so I guess I will need to replicate the code.

>>> As mentioned previously, I think the relation between this unit and the
>>> MAC and/or PHY needs to be explicitly described in the DT.
>>
>> Do you suggest a field along the lines of:
>> mac = <&eth_controller_0>;
>> The driver could check that it exists and is valid but does
>> not need to make use of it.
>
> I would expect some level of the software stack to make use of it, or
> you have no idea which ethernet interface is related to this monitoring
> interface. Perhaps current systems only have one interface, but that
> shouldn't be relied upon.

Yes, but the sniffer module is hard-wired to a certain Ethernet Mii
interface. We can add an entry to tie it to an Ethernet controller, but
apart of a sanity check I am not sure what else the S/W can do.

>
> Thanks,
> Mark.
>

Thank you,
Stathis
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