linux-kernel - Re: [resend/standalone PATCH v4] Add auxiliary bus support

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Date:   Mon, 4 Jan 2021 19:12:47 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        alsa-devel@...a-project.org, Kiran Patil <kiran.patil@...el.com>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Shiraz Saleem <shiraz.saleem@...el.com>,
        Martin Habets <mhabets@...arflare.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Fred Oh <fred.oh@...ux.intel.com>,
        Dave Ertman <david.m.ertman@...el.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Netdev <netdev@...r.kernel.org>,
        Leon Romanovsky <leonro@...dia.com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Parav Pandit <parav@...lanox.com>,
        Lee Jones <lee.jones@...aro.org>
Subject: Re: [resend/standalone PATCH v4] Add auxiliary bus support

On Mon, Jan 4, 2021 at 5:53 PM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Mon, Jan 04, 2021 at 04:51:51PM -0800, Dan Williams wrote:
> > On Mon, Jan 4, 2021 at 4:14 PM Jason Gunthorpe <jgg@...dia.com> wrote:
> > >
> > > On Mon, Jan 04, 2021 at 09:19:30PM +0000, Mark Brown wrote:
> > >
> > >
> > > > > Regardless of the shortcut to make everything a struct
> > > > > platform_device, I think it was a mistake to put OF devices on
> > > > > platform_bus. Those should have remained on some of_bus even if they
> > > >
> > > > Like I keep saying the same thing applies to all non-enumerable buses -
> > > > exactly the same considerations exist for all the other buses like I2C
> > > > (including the ACPI naming issue you mention below), and for that matter
> > > > with enumerable buses which can have firmware info.
> > >
> > > And most busses do already have their own bus type. ACPI, I2C, PCI,
> > > etc. It is just a few that have been squished into platform, notably
> > > OF.
> > >
> >
> > I'll note that ACPI is an outlier that places devices on 2 buses,
> > where new acpi_driver instances are discouraged [1] in favor of
> > platform_drivers. ACPI scan handlers are awkwardly integrated into the
> > Linux device model.
> >
> > So while I agree with sentiment that an "ACPI bus" should
> > theoretically stand on its own there is legacy to unwind.
> >
> > I only bring that up to keep the focus on how to organize drivers
> > going forward, because trying to map some of these arguments backwards
> > runs into difficulties.
> >
> > [1]: http://lore.kernel.org/r/CAJZ5v0j_ReK3AGDdw7fLvmw_7knECCg2U_huKgJzQeLCy8smug@mail.gmail.com
>
> Well, this is the exact kind of thing I think we are talking about
> here..
>
> > > It should be split up based on the unique naming scheme and any bus
> > > specific API elements - like raw access to ACPI or OF data or what
> > > have you for other FW bus types.
> >
> > I agree that the pendulum may have swung too far towards "reuse
> > existing bus_type", and auxiliary-bus unwinds some of that, but does
> > the bus_type really want to be an indirection for driver apis outside
> > of bus-specific operations?
>
> If the bus is the "enumeration entity" and we define that things like
> name, resources, gpio's, regulators, etc are a generic part of what is
> enumerated, then it makes sense that the bus would have methods
> to handle those things too.
>
> In other words, the only way to learn what GPIO 'resource' is to ask
> the enumeration mechnism that is providing the bus. If the enumeration
> and bus are 1:1 then you can use a function pointer on the bus type
> instead of open coding a dispatch based on an indirect indication.
>

I get that, but I'm fearing a gigantic bus_ops structure that has
narrow helpers like ->gpio_count() that mean nothing to the many other
clients of the bus. Maybe I'm overestimating the pressure there will
be to widen the ops structure at the bus level.