netdev - Re: [PATCH net-next 10/10] wwan: core: add WWAN common private data for netdev

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Message-ID: <CAHNKnsQdWWJ1tAHt4LPS=3jWNSCDcUdQDSrkZ9aakYp-4iaKVw@mail.gmail.com>
Date:   Sun, 20 Jun 2021 17:39:07 +0300
From:   Sergey Ryazanov <ryazanov.s.a@...il.com>
To:     Loic Poulain <loic.poulain@...aro.org>
Cc:     Johannes Berg <johannes@...solutions.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        M Chetan Kumar <m.chetan.kumar@...el.com>,
        Intel Corporation <linuxwwan@...el.com>
Subject: Re: [PATCH net-next 10/10] wwan: core: add WWAN common private data
 for netdev

On Tue, Jun 15, 2021 at 10:24 AM Loic Poulain <loic.poulain@...aro.org> wrote:
> On Tue, 15 Jun 2021 at 02:30, Sergey Ryazanov <ryazanov.s.a@...il.com> wrote:
>> The WWAN core not only multiplex the netdev configuration data, but
>> process it too, and needs some space to store its private data
>> associated with the netdev. Add a structure to keep common WWAN core
>> data. The structure will be stored inside the netdev private data before
>> WWAN driver private data and have a field to make it easier to access
>> the driver data. Also add a helper function that simplifies drivers
>> access to their data.
>
> Would it be possible to store wwan_netdev_priv at the end of priv data instead?
> That would allow drivers to use the standard netdev_priv without any change.
> And would also simplify forwarding to rmnet (in mhi_net) since rmnet
> uses netdev_priv.

I do not think that mimicking something by one subsystem for another
is generally a good idea. This could look good in a short term, but
finally it will become a headache due to involvement of too many
entities.

IMHO, a suitable approach to share the rmnet library and data
structures among drivers is to make the rmnet interface more generic.

E.g. consider such netdev/rtnl specific function:

static int rmnet_foo_action(struct net_device *dev, ...)
{
    struct rmnet_priv *rmdev = netdev_priv(dev);
    <do a foo action here>
}

It could be split into a wrapper and an actual handler:

int __rmnet_foo_action(struct rmnet_priv *rmdev, ...)
{
    <do a foo action here>
}
EXPORT_GPL(__rmnet_foo_action)

static int rmnet_foo_action(struct net_device *dev, ...)
{
    struct rmnet_priv *rmdev = netdev_priv(dev);
    return __rmnet_foo_action(rmdev, ...)
}

So a call from mhi_net to rmnet could looks like this:

static int mhi_net_foo_action(struct net_device *dev, ...)
{
    struct rmnet_priv *rmdev = wwan_netdev_drvpriv(dev);
    return __rmnet_foo_action(rmdev, ...)
}

In such a way, only the rmnet users know something special, while
other wwan core users and the core itself behave without any
surprises. E.g. any regular wwan core minidriver can access the
link_id field of the wwan common data by calling netdev_priv() without
further calculating the common data offset.

>> At the moment we use the common WWAN private data to store the WWAN data
>> link (channel) id at the time the link is created, and report it back to
>> user using the .fill_info() RTNL callback. This should help the user to
>> be aware which network interface is binded to which WWAN device data
>> channel.
>>
>> Signed-off-by: Sergey Ryazanov <ryazanov.s.a@...il.com>
>> CC: M Chetan Kumar <m.chetan.kumar@...el.com>
>> CC: Intel Corporation <linuxwwan@...el.com>
>> ---
>>  drivers/net/mhi/net.c                 | 12 +++++------
>>  drivers/net/mhi/proto_mbim.c          |  5 +++--
>>  drivers/net/wwan/iosm/iosm_ipc_wwan.c | 12 +++++------
>>  drivers/net/wwan/wwan_core.c          | 29 ++++++++++++++++++++++++++-
>>  include/linux/wwan.h                  | 18 +++++++++++++++++
>>  5 files changed, 61 insertions(+), 15 deletions(-)

-- 
Sergey