Re: [PATCH RESEND v4 15/15] PCI: dwc: Introduce dma-ranges property support for RC-host

[Serge Semin]

On Thu, Jul 28, 2022 at 05:11:20PM -0500, Bjorn Helgaas wrote:
> On Fri, Jun 24, 2022 at 05:39:47PM +0300, Serge Semin wrote:
> > In accordance with the generic PCIe Root Port DT-bindings the "dma-ranges"
> > property has the same format as the "ranges" property. The only difference
> > is in their semantics. The "dma-ranges" property describes the PCIe-to-CPU
> > memory mapping in opposite to the CPU-to-PCIe mapping of the "ranges"
> > property. Even though the DW PCIe controllers are normally equipped with
> > the internal Address Translation Unit which inbound and outbound tables
> > can be used to implement both properties semantics, it was surprising for
> > me to discover that the host-related part of the DW PCIe driver currently
> > supports the "ranges" property only while the "dma-ranges" windows are
> > just ignored. Having the "dma-ranges" supported in the driver would be
> > very handy for the platforms, that don't tolerate the 1:1 CPU-PCIe memory
> > mapping and require a customized PCIe memory layout. So let's fix that by
> > introducing the "dma-ranges" property support.
> 

> Do we have a platform that requires this yet?  Or does this fix a bug?
> 
> I see that dw_pcie_host_init() calls devm_pci_alloc_host_bridge(),
> which eventually parses "dma-ranges", but I don't see any DWC DT
> bindings that use it yet.
> 
> I'm not clear on what value this adds today.

There are several points of having this supported.
First of all, generic PCIe DT-bindings permit having the dma-ranges
specified for the PCIe RCs. If so having it unsupported by the driver
just breaks the bindings or at least makes it incomplete.
Second, the main point of this patchset is to add the dma-ranges
support.) Especially seeing some other PCIe RC drivers do have it
supported too.
Finally. It is required for our platform (and for all the platforms
with similar issues). The problem is that the outbound source window
base address (on CPU-side) is size-unaligned. It resides at the 128MB
base address (size is somewhat about ~335MB). In case of the
one-on-one CPU->PCI mapping the peripherals with relatively big BARs
(at least of 256MB) and which need the BARs having size-aligned memory
won't be supported. So we had to remap the PCIe space to the
size-aligned base address. But in its turn that caused the PCIe-CPU
memory overlap. So PCIe DMA stopped working for the overlapped memory
due to the implicit P2P transactions. In order to fix that we had to
add the dma-ranges support to the DW PCIe driver and use it to remap
the overlapped memory. So please add this patch to the repo. We really
need it.

-Sergey

> 
> Bjorn