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.