[PATCH] dt-bindings: PCI: mediatek-gen3: Allow memory-region for restricted DMA buffer

[Chen-Yu Tsai]

On Thu, May 14, 2026 at 7:48 PM Manivannan Sadhasivam <mani at kernel.org> wrote:
>
> On Thu, May 14, 2026 at 03:54:29PM +0800, Chen-Yu Tsai wrote:
> > On Thu, May 14, 2026 at 1:23 PM Manivannan Sadhasivam <mani at kernel.org> wrote:
> > >
> > > On Fri, May 08, 2026 at 02:36:32PM +0800, Chen-Yu Tsai wrote:
> > > > On some SoCs without an IOMMU behind the PCIe controller, the PCIe
> > > > controller memory access could be limited to a small region by the
> > > > firmware configuring a memory protection unit. This memory region
> > > > must be assigned to the PCIe controller so that the OS knows to
> > > > use that region. Otherwise PCIe devices would not work properly.
> > > >
> > >
> > > So this means, the PCIe devices can only access a specific carveout memory
> > > configured by MPU for DMA? If so, you should use 'dma-ranges' as suggested by
> > > Rob.
> > >
> > > 'memory-region' also serves the purpose, but for PCI, we have the dedicated
> > > 'dma-ranges' property.
> >
> > I think I need some sort of guide on writing the 'dma-ranges' property,
> > because it is not working for me.
> >
> > I'm adding
> >
> >     dma-ranges = <0x42000000 0 0x00000000 0 0xc0000000 0 0x4000000>;
> >
>
> So the device DMA address start from 0x0? Isn't it a 1:1 mapping?

I actually don't know. But

>         dma-ranges = <0x42000000 0 0xc0000000 0 0xc0000000 0 0x4000000>;

this didn't work either.