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

From: Stathis Voukelatos <[email-masked]>
Date: Wed, 18 Feb 2015 09:40:22 +0000

On 17/02/15 17:30, Mark Rutland wrote:

>> 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.
>
> In what cases would it _not_ be the same? From your description this is
> that clock, no?
>

Counters can often have a divider applied to their input clock and
therefore run at a scaled down frequency. This is not the case in the
first SoC where the sniffer is used, so for simplicity I can modify as
you suggest and remove that field from 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.
>
> Or submit a patch exporting it, along with the rationale for doing so?
>

Will do that.

>> 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.
>
> Fundamentally, the use-case for this is monitoring an ethernet
> interface. So regardless of which kernel framework this plumbs into,
> there needs to be a way to go from ethN to whatever this is exposed as.
>
> Exposing a completely separate interface makes no sense. Singleton stuff
> like that inevitably gets broken as someone later builds a board with
> multiple instances of some similar IP block.
>
> So I would imagine that either the link between interface and monitoring
> interface would be described somewhere in the filesystem, or there'd be
> a syscall/ioctl/whatever to go from an interface to the appropriate
> monitoring interface.
>
> That all depends on exactly how this gets exposed in the end, however.
>

After the first version of the patch was submitted, the feedback from 
the netdev list was to expose it as a network interface as this would
allow it to be accessed by standard user space monitoring tools.
It definitely makes sense to link it to the associated ethernet netdev,
but I am not sure if there is a framework in the kernel to do it at
the driver level?

> Thanks,
> Mark.
> --

Thank you,
Stathis