Re: [PATCH] RISC-V: defconfig: Enable Generic PCIE by default

[Palmer Dabbelt]

On Fri, 21 Dec 2018 11:27:39 PST (-0800), Paul Walmsley wrote:
> On Fri, 21 Dec 2018, Alistair Francis wrote:
>
> > When the MicroSemi driver does eventually go upstream this will
> > probably have to be discussed though as either the config or device
> > tree will need to be edited to ensure only one PCIe device is present.
>
> The right way to do that is to have two separate DT files: one for the
> HiFive-U board alone; the other for the HiFive-U plus the expansion board
> combo.  There shouldn't be any problems with keeping both drivers enabled
> in the defconfig.

We actually need at least three:

* One for the HiFive Unleashed
* One for the HiFive Unleashed, when attached to the Micro Semi expansion 
 board.
* One for the HiFive Unleashed, when attached to a VC707.

We may end up needing additional device trees for the newer Xilinx boards, 
depending on if it's possible to probe for that 32-bit DMA limit on the VC707 
or not.  Then, of course, we'll need an image-specific device tree whenever we 
put an accelerator on the FPGA.

Ideally we'd have a discoverable bus, but that's going to be a bit of a way 
out.  For now we'll have to get by with different device trees, but that should 
get significantly easier when we get a proper bootloader working.