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Date:   Tue, 30 Jun 2020 15:16:04 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Greg KH <gregkh@...uxfoundation.org>, Takashi Iwai <tiwai@...e.de>,
        Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        davem@...emloft.net, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, nhorman@...hat.com,
        sassmann@...hat.com, Fred Oh <fred.oh@...ux.intel.com>,
        lee.jones@...aro.org
Subject: Re: [net-next v4 10/12] ASoC: SOF: Introduce descriptors for SOF
 client

On Tue, Jun 30, 2020 at 08:32:45AM -0300, Jason Gunthorpe wrote:
> On Tue, Jun 30, 2020 at 11:31:41AM +0100, Mark Brown wrote:
> > On Mon, Jun 29, 2020 at 07:59:59PM -0300, Jason Gunthorpe wrote:

> > > > What are we supposed to do with things like PCI attached FPGAs and ASICs
> > > > in that case?  They can have host visible devices with physical
> > > > resources like MMIO ranges and interrupts without those being split up
> > > > neatly as PCI subfunctions - the original use case for MFD was such
> > > > ASICs, there's a few PCI drivers in there now. 

> > > Greg has been pretty clear that MFD shouldn't have been used on top of
> > > PCI drivers.

> > The proposed bus lacks resource handling, an equivalent of
> > platform_get_resource() and friends for example, which would be needed
> > for use with physical devices.  Both that and the name suggest that it's
> > for virtual devices.

> Resource handling is only useful if the HW has a hard distinction
> between it's functional blocks. This scheme is intended for devices
> where that doesn't exist. The driver that attaches to the PCI device
> and creates the virtual devices is supposed to provide SW abstractions
> for the other drivers to sit on.

> I'm not sure why we are calling it virtual bus.

The abstraction that the PCI based MFDs (and FPGAs will be similar,
they're just dynamic MFDs to a good approximation) need is to pass
through MMIO regions, interrupts and so on which is exactly what the
platform bus offers.  The hardware is basically someone taking a bunch
of IPs and shoving them behind the MMIO/interrupt regions of a PCI
device.

> > The reason the MFDs use platform devices is that they end up having to
> > have all the features of platform devices - originally people were
> > making virtual buses for them but the code duplication is real so
> > everyone (including Greg) decided to just use what was there already.

> Maybe Greg will explain why he didn't like the earlier version of that
> stuff that used MFD

AFAICT Greg is mostly concerned about the MFDs that aren't memory
mapped, though some of them do use the resource API to pass interrupts
through.

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