netdev - Re: [PATCH net-next] net: wwan: iosm: device trace collection using relayfs

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Date:   Mon, 15 Nov 2021 16:20:29 +0300
From:   Sergey Ryazanov <ryazanov.s.a@...il.com>
To:     M Chetan Kumar <m.chetan.kumar@...ux.intel.com>,
        Loic Poulain <loic.poulain@...aro.org>,
        Johannes Berg <johannes@...solutions.net>
Cc:     netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>, krishna.c.sudi@...el.com,
        M Chetan Kumar <m.chetan.kumar@...el.com>,
        Intel Corporation <linuxwwan@...el.com>
Subject: Re: [PATCH net-next] net: wwan: iosm: device trace collection using relayfs

On Mon, Nov 15, 2021 at 1:49 PM M Chetan Kumar
<m.chetan.kumar@...ux.intel.com> wrote:
> This patch brings in support for device trace collection.
> It implements relayfs interface for pushing device trace
> from kernel space to user space.
>
> On driver load iosm debugfs entry is created in below path
> /sys/kernel/debug/

This code will break as soon as the user attaches a second modem to
the host. The debugfs directory will not be created due to the name
collision, so the user will only be able to trace the second modem
when he detaches the first one.

In general, I like the idea of exporting debug information via
debugfs. But the direct export of a debug interface of a driver that
intensively uses some subsystem (bypassing the subsyste) looks odd.

This pollutes debugfs root with unrelated directories and makes it
harder for an end user to match control devices from /dev with debugfs
entries.

Maybe we should add a common debugfs infrastructure to the wwan
subsystem to facilitate the driver development and end user
operations. So the driver should care only about a particular debug
knob without caring too much about the base directory for a device
instance.

E.g. for the IOSM driver this will mean that the trace file will be moved from

/sys/kernel/debugfs/iosm/trace

to the

/sys/kernel/debugfs/wwan/wwan0/trace

-- 
Sergey