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

From: Stathis Voukelatos
Date: Wed Feb 18 2015 - 04:40:39 EST

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?


Thank you,
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