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

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jul 29, 2022 at 07:52:20AM +0300, Serge Semin wrote:
> 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.

Are there bindings in the tree that are broken and will be fixed by
this?

> 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.

Does the above apply to the pending Baikal-T1 driver?  If so, let's
just include this patch in that series.  Then we'll have a user of
this functionality and we'll be able to exercise and test this code.

Bjorn