netdev - Re: [PATCH net-next] misc: Add Renesas Synchronization Management Unit (SMU) support

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Date:   Tue, 16 Feb 2021 21:44:45 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Min Li <min.li.xe@...esas.com>
Cc:     Derek Kiernan <derek.kiernan@...inx.com>,
        Dragan Cvetic <dragan.cvetic@...inx.com>,
        Arnd Bergmann <arnd@...db.de>,
        gregkh <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH net-next] misc: Add Renesas Synchronization Management
 Unit (SMU) support

On Tue, Feb 16, 2021 at 6:10 PM Min Li <min.li.xe@...esas.com> wrote:
> > > If I come up with a new file and move all the abstraction code there,
> > > does that work?
> >
> > I think so, but it's more important to figure out a good user space interface
> > first. The ioctl interfaces should be written on a higher-level abstraction, to
> > ensure they can work with any hardware implementation and are not
> > specific to Renesas devices.
> >
> > Can you describe on an abstract level how a user would use the character
> > device, and what they achieve by that?
>
> This driver is meant to be used by Renesas PTP Clock Manager for
> Linux (pcm4l) software for Renesas device only.
>
> About how pcm4l uses the char device, pcm4l will open the device
> and do the supported ioctl cmds on the device, simple like that.
>
> At the same time, pcm4l will also open ptp hardware clock device,
> which is /dev/ptp[x], to do clock adjustments.

I can't help but think you are evading my question I asked. If there is no
specific action that this pcm4l tool needs to perform, then I'd think
we should better not provide any interface for it at all.

I also found a reference to only closed source software at
https://www.renesas.com/us/en/software-tool/ptp-clock-manager-linux
We don't add low-level interfaces to the kernel that are only
usable by closed-source software.

Once you are able to describe the requirements for what pcm4l
actually needs from the hardware, we can start discussing what
a high-level interface would look like that can be used to replace
the your current interface, in a way that would work across vendors
and with both pcm4l and open-source tools that do the same job.

      Arnd