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

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Message-ID: <20201218233608.GA552508@nvidia.com>
Date:   Fri, 18 Dec 2020 19:36:08 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
CC:     Mark Brown <broonie@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Dan Williams" <dan.j.williams@...el.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@...aro.org>
Subject: Re: [resend/standalone PATCH v4] Add auxiliary bus support

On Fri, Dec 18, 2020 at 10:16:58PM +0100, Alexandre Belloni wrote:

> But then again, what about non-enumerable devices on the PCI device? I
> feel this would exactly fit MFD. This is a collection of IPs that exist
> as standalone but in this case are grouped in a single device.

So, if mfd had a mfd_device and a mfd bus_type then drivers would need
to have both a mfd_driver and a platform_driver to bind. Look at
something like drivers/char/tpm/tpm_tis.c to see how a multi-probe
driver is structured

See Mark's remarks about the old of_platform_device, to explain why we
don't have a 'dt_device' today

> Note that I then have another issue because the kernel doesn't support
> irq controllers on PCI and this is exactly what my SoC has. But for now,
> I can just duplicate the irqchip driver in the MFD driver.

I think Thomas fixed that recently on x86 at least.. 

Having to put dummy irq chip drivers in MFD anything sounds scary :|

> Let me point to drivers/net/ethernet/cadence/macb_pci.c which is a
> fairly recent example. It does exactly that and I'm not sure you could
> do it otherwise while still not having to duplicate most of macb_probe.

Creating a platform_device to avoid restructuring the driver's probe
and device logic to be generic is a *really* horrible reason to use a
platform device.

Jason