Re: what's a platform device?

[Kumar Gala <galak@kernel.crashing.org>]

On Thu, 23 Feb 2006, Kumar Gala wrote:

> >> 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'm not sure I follow this. How is PCI different?  How would "kumar"  
> bus be different?
> 
> >> 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?
> 
> This makes sense, but you seem to be talking about hierarchy more the  
> functionality.  I agree in your description of hierarchy.
> 
> I was looking at it from a functional point of view, maybe more from  
> the device view then from the bus.  I need a struct device type that  
> contains resources, a name, an id.  I'll do matching based on name.   
>  From a functional point of view platform does all this.
> 
> Based on your description would you say that a platform_device's  
> parent device should always be platform_bus? [I'm getting at the fact  
> that we allow pdev->dev.parent to be set by the caller of  
> platform_device_add].
> 
> Hmm, as I think about this further, I think that its more coincidence  
> that the functionality for the "kumar" bus is equivalent to that of  
> the "platform" bus.
> 

What about a new bus_type that uses all the sematics of the platform_bus.  
Doing someting like the following which would allow the caller to specify 
their own bus_type.

I'm just trying to avoid duplicating alot of code that already exists in 
base/platform.c

- kumar

diff --git a/drivers/base/platform.c b/drivers/base/platform.c
index 461554a..fb320e9 100644
--- a/drivers/base/platform.c
+++ b/drivers/base/platform.c
@@ -242,7 +242,8 @@ int platform_device_add(struct platform_
        if (!pdev->dev.parent)
                pdev->dev.parent = &platform_bus;
 
-       pdev->dev.bus = &platform_bus_type;
+       if (!pdev->dev.bus)
+               pdev->dev.bus = &platform_bus_type;
 
        if (pdev->id != -1)
                snprintf(pdev->dev.bus_id, BUS_ID_SIZE, "%s.%u", pdev->name, pdev->id);
@@ -426,7 +427,8 @@ static int platform_drv_resume(struct de
  */
 int platform_driver_register(struct platform_driver *drv)
 {
-       drv->driver.bus = &platform_bus_type;
+       if (!drv->driver.bus)
+               drv->driver.bus = &platform_bus_type;
        if (drv->probe)
                drv->driver.probe = platform_drv_probe;
        if (drv->remove)


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