linux-kernel - Re: [PATCH V3 2/5] misc: mlx5ctl: Add mlx5ctl misc driver

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Message-ID: <20231128065321.53d4d5bb@kernel.org>
Date:   Tue, 28 Nov 2023 06:53:21 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     David Ahern <dsahern@...nel.org>
Cc:     Saeed Mahameed <saeed@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Arnd Bergmann <arnd@...db.de>,
        Leon Romanovsky <leonro@...dia.com>,
        Jiri Pirko <jiri@...dia.com>, Leonid Bloch <lbloch@...dia.com>,
        Itay Avraham <itayavr@...dia.com>,
        linux-kernel@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>
Subject: Re: [PATCH V3 2/5] misc: mlx5ctl: Add mlx5ctl misc driver

On Mon, 27 Nov 2023 21:46:28 -0700 David Ahern wrote:
> > You keep saying "debug information" which is really underselling this
> > driver. Are you not going to support mstreg?
> > 
> > The common development flow as far as netdev is concerned is:
> >  - some customer is interested in a new feature of a chip
> >  - vendor hacks the support out of tree, using oot module and/or
> >    user space tooling
> >  - customer does a PoC with that hacked up, non-upstream solution
> >    - if it works, vendor has to find out a proper upstream API,
> >      hopefully agreed on with other vendors
> >    - if it doesn't match customer needs the whole thing lands in the bin
> > 
> > If the vendor specific PoC can be achieved with fully upstream software
> > we lose leverage to force vendors to agree on common APIs.  
> 
> Please elaborate on what "common" API there is to create here?

Damn, am I so bad at explaining basic things or y'all are spending
5 seconds reading this and are not really paying attention? :|

> Do you agree that each ASIC in the device is unique and hence will
> have made different trade offs - both features and nerd knobs to tune
> and affect the performance and runtime capabilities? If you do not
> agree, then we need to have a different discussion ...
> If you do, please elaborate on the outline of some common API that
> could possibly be done here.

We have devlink params. If that doesn't scale we can look for other
solutions but let's see them not scale _in practice_ first.

> You said no to the devlink parameters as a way to tune an ASIC.

What? When?

> This is a common, established tool, using a common, established message
> channel but in an open, free form way of letting a customer see what
> tunables there are for a specific H/W version and firmware version
> and then set them. That is about as common as can be for different
> vendors creating different ASICs with different goals and design
> objectives. Yet, you somehow expect all of them to have nerd knob X
> and tunable Y. That is not realistic.

I don't know what you're talking about.

> > This should all be self-evident for people familiar with netdev, but
> > I thought I'd spell it out.
> > 
> > I understand that the device has other uses, but every modern NIC has
> > other uses. I don't see how netdev can agree to this driver as long as
> > there is potential for configuring random networking things thru it.
> > All major netdev vendors have a set of out of tree tools / "expose
> > everything misc drivers", "for debug". They will soon follow your
> > example if we let this in.  
> 
> Out of tree drivers are already ingrained into customers now. Mellanox
> (in this case) has tried many different angles at getting access to H/W
> unique tunables (i.e., the devlink comment) and now dumping huge amounts
> of data. Your response to the devlink parameters attempt is to basically
> abuse the firmware upgrade command as a way to push a binary blob that
> can contain said settings (and I still have not fully wrapped my head
> around the fact that you suggested that option).
> 
> What specifically are you looking for? There are different vendors and
> different h/w options for specific market based reasons. Your hard line
> stance against needs like this is what is pushing out of tree drivers
> and tools to continue.

Sounds like you'd like a similar open-ended interface for your device.