[PATCH v3 07/12] usb: chipidea: add a usb2 driver for ci13xxx

[Peter Chen]

> > > >
> > > > Some people wanted the possibility to set the DMA mask as this
> > > > USB2 CI driver does not do specific Berlin operation and can be
> reused later.
> > > > I don't particularly need to call dma_coerce_mask_and_coherent()
> > > > in my case, as far as I know.
> > >
> > > Ok, just remove the call then and rely on the default mask.
> > >
> > > > They can maybe give the restrictions they might want to put on the
> > > > DMA mask.
> > >
> > > If the restriction is from the bus, it should get handled
> > > automatically by the device probe as long as the correct dma-ranges
> > > property is there (though we have a small bug there at the moment).
> > > If there is a variation of ci13xxx that can't do 32-bit DMA, that
> > > should use a different compatible string and pass a fixed mask into
> > > dma_set_mask_and_coherent() based on the device.
> > >
> >
> > Correct me if my below understanding is wrong please.
> >
> > For three chipidea users:
> > user_a: don't need dma mask
> > user_b: dma mask value is dma_mask_b
> > user_c: dma mask value is dma_mask_c
> >
> > We don't need to call dma_coerce_mask_and_coherent() at generic
> > chipidea glue driver, and set dma-ranges as dma_mask_b at user_b's
> > dts, and set dma-ranges as dma_mask_c at user_c's dts.
> 
> The dma-ranges property doesn't just encode a dma-mask for a device, but
> rather how the children of a bus see the memory address space of the
> parent.
> 
> For traditional reasons we default to assuming that a 32-bit dma_mask is
> correct, but there may be multiple reasons why this is not true:
> 
> - you have a device connected to a bus that is smaller than 32-bit wide
>   (and that would have a dma-ranges property describing that)
> - you have a device that has fewer than 32 address lines but is connected
>   to a 32-bit upstream bus (hence the dma-ranges describe all 32 bit,
>   but the driver must set a smaller mask)
> - you have a 64-bit capable device connected to a 32-bit bus: the driver
>   will ask for a 64-bit mask, but the dma_set_mask() call refuses this
>   because of the bus capabilities.
> - you have a 64-bit capable device on a 64-bit capable bus, and the
>   dma_set_mask call should succeed.
> 
> The mask is definitely never "user" specific, but is a combination of the
> device you have and the bus it is connected to.
> 

Thanks, arnd.

For chipidea generic glue layer case, if there are three devices who use this
driver, and all devices have 32-bit bus, some devices have less 32 address lines.
For example:

- the device_a doesn't need to use dma_mask
- the device_b needs dma_mask as 0xfffffffff
- the device_c needs dma_mask as 0xfffffff0, assume it has only 28 address lines

My questions are:
- Can we not set dma_mask at driver, and only set dma-ranges at dts for device_b
and device_c as a solution to cover this different dma mask use case?

- If we can't use this solution, would you suggest one?

- If we can use this solution, for device_b and device_c, how can we write dma-ranges?
I can't find any arm platforms use it, only some powerpc platform use it.
According to the definition from Power_ePAPR_APPROVED_v1.1.pdf, it is
dma-ranges = <child-bus-address, parent-bus-address, length>
but I find the powerpc has different way for using dma-ranges. 

Peter