Question about a quirky (DesignWare) PCIe RC in Allwinner H6

[Bjorn Helgaas]

On Mon, Mar 05, 2018 at 10:50:58PM +0800, Icenowy Zheng wrote:
> Hi everyone,
> 
> I'm trying to implement a driver for the quirky (DW) PCIe RC in the
> Allwinner H6 SoC.
> 
> The quirk is that only the "dbi" space is always mapped, but at the
> same time only 64KiB of other spaces (config, downstream IO and non-
> prefetchable memory) are accessible. To access a certain address the
> high 16-bit of the address (all bus addresses in H6 SoC are 32-bit
> despite the CPU is 64-bit) needs to be written into the
> PCIE_ADDR_PAGE_CFG register (a vendor-defined register in DBI space).
> So the access to these spaces cannot be processed correctly with just
> readl/writel, as the existing code does.
> 
> Is it possible to workaround this in the PCI subsystem of Linux?
> 
> (I have thought a workaround that only maps the current accessible
> 64KiB with the MMU, and when accessing the non-accessible part, catch
> the page fault and re-setup the map to the new 64KiB page. But surely
> it will kill the performance.)

If you only need to write to PCIE_ADDR_PAGE_CFG for *config* accesses,
this should be easy.  All config accesses go through wrappers
(pci_read_config_word(), etc) and several host bridge drivers have to
update a register with parts of the config address, e.g.,
advk_pcie_rd_conf(), mvebu_pcie_hw_rd_conf().

It sounds like you also need to update PCIE_ADDR_PAGE_CFG for I/O port
accesses.  I/O port accesses do go through a wrapper (inb(), etc), so
it might be possible in principle to intercept these, although much
more difficult than config accesses, because the config accesses are
already set up with per-host bridge hooks, while the I/O port
accessors are mostly per-arch.

But if you have to update PCIE_ADDR_PAGE_CFG for memory accesses,
that's really a problem because drivers are allowed to essentially
read a memory BAR, convert that bus address to a CPU physical memory
address, ioremap() it, then directly access the PCIe memory with loads
and stores.

Bjorn