Date: Wed, 22 Feb 2006 21:13:12 -0800
From: Greg KH <greg@kroah.com>
To: Kumar Gala <galak@kernel.crashing.org>
Cc: Russell King <rmk+lkml@arm.linux.org.uk>, linux-kernel@vger.kernel.org
Subject: Re: what's a platform device?
Message-ID: <20060223051312.GA14534@kroah.com>
References: <Pine.LNX.4.44.0602221517370.21264-100000@gate.crashing.org> <20060223043937.GA7204@kroah.com> <C29A9C12-5A2E-4609-8A74-8C3E63891C1F@kernel.crashing.org>
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On Wed, Feb 22, 2006 at 10:55:16PM -0600, Kumar Gala wrote:
> 
> On Feb 22, 2006, at 10:39 PM, Greg KH wrote:
> 
> >On Wed, Feb 22, 2006 at 03:47:40PM -0600, Kumar Gala wrote:
> >>Guys,
> >>
> >>I was hoping to get your opinion on a question I had.  The  
> >>question comes
> >>down to what we think a "platform device" is.
> >>
> >>The situation I have is an FPGA connected over PCI.  The FPGA  
> >>implements
> >>various device functionality (serial ports, I2C controller, IR,  
> >>etc.) as a
> >>single PCI device/function.  The FPGA breaks any notion of a true PCI
> >>device, it uses PCI as a device interconnect more than anything else.
> >>
> >>In talking to Greg about this, he suggested I just create a new  
> >>bus_type
> >>for this similar to what is being done for usb-serial.  As I  
> >>started to
> >>think about what I wanted ended up being a platform_device plus a  
> >>sysfs
> >>entry for the MMIO region.
> >>
> >>So, it seems that a "platform device" is a pretty generic concept  
> >>now.  Do
> >>you guys thing its acceptable to use a platform device for my  
> >>needs or
> >>should I create some new bus_type?  Do we have a better definition  
> >>of what
> >>a platform device is or might be?
> >
> >Well, your FPGA is a pci device, right?  It's only the devices that  
> >hang
> >off of it that you are concerned about.  I really think you should  
> >make
> >your own bus type, as it's not much work, and it would not disturb the
> >existing platform devices, which do not know about mmio regions like
> >PCI.
> 
> Yes, the FPGA is a pci device.
> 
> Not sure I follow exactly what you mean by the fact that platform  
> devices dont know about mmio regions.  They know about struct  
> resource and iomem_resource & ioport_resource.

Yes, as they have no "bus" to attach too.  That's why they are there,
they are for devices with no bus, but are merely "raw" memory mapped
devices.

> I think I might be missing something fundamental here.  In  
> implementing my own bus_type, I'll end up introducing my own struct  
> foobar_device which looked pretty much like struct platform_device.
> Then I'll need a set of functions to assign resources, etc.
> 
> I got no issue implementing my own bus_type, but I clearly feel like  
> I'm missing something here (just not sure what it is :)

I guess I look at your FPGA as a PCI "bridge" chip, that bridges between
the PCI bus, and your "kumar" bus (for lack of a better name).  Your
devices hang off of that bus, which is attached to the FPGA, which is
attached to the pci bridge, and so on.  If you use the platform bus, you
break that link.

Does that make sense?

Russell probably has other thoughts about this.

thanks,

greg k-h
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