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

[Chen-Yu Tsai]

On Tue, Jun 2, 2026 at 5:48 AM Rob Herring <robh@xxxxxxxxxx> wrote:
>
> On Tue, May 19, 2026 at 12:51:49PM +0530, Manivannan Sadhasivam wrote:
> > On Mon, May 18, 2026 at 05:02:11PM +0800, Chen-Yu Tsai wrote:
> > > On Fri, May 15, 2026 at 8:34 PM Manivannan Sadhasivam <mani@xxxxxxxxxx> wrote:
> > > >
> > > > On Fri, May 15, 2026 at 05:16:19PM +0800, Chen-Yu Tsai wrote:
> > > > > On Thu, May 14, 2026 at 7:48 PM Manivannan Sadhasivam <mani@xxxxxxxxxx> 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@xxxxxxxxxx> 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.
> > > >
> > > >
> > > > Hmm. Can you print the DMA address programmed to the device? i.e., the address
> > > > returned by dma_map_single() in the driver.
> > >
> > > On a working system still using the restricted-dma-pool memory region,
> > > it gives something like 0x00000000c0009000, so indeed it is 1:1 mapping?
> >
> > It has to be 1:1 mapping.
> >
> > > These are for the RX/TX descriptors [1][2].
> > >
> > > When using dma-ranges, the failure is from dma_alloc_coherent() [3][4],
> > > which is the descriptor ring. On a working system, this is something
> > > like 0x00000000c0c9d000, so again 1:1.
> > >
> > > [1] https://elixir.bootlin.com/linux/v7.0.8/source/drivers/net/wireless/realtek/rtw88/pci.c#L221
> > > [2] https://elixir.bootlin.com/linux/v7.0.8/source/drivers/net/wireless/realtek/rtw88/pci.c#L829
> > > [3] https://elixir.bootlin.com/linux/v7.0.8/source/drivers/net/wireless/realtek/rtw88/pci.c#L192
> > > [4] https://elixir.bootlin.com/linux/v7.0.8/source/drivers/net/wireless/realtek/rtw88/pci.c#L265
> > >
> > > > Also, using prefetchable flag is not correct for DMA memory. You should use:
> > > >
> > > >         dma-ranges = <0x02000000 0 0xc0000000 0 0xc0000000 0 0x4000000>;
> > >
> > > This didn't work either. What exactly is supposed to handle dma-ranges?
> > > I see some code parsing it in the PCI core, but it just saves it to a list.
> > >
> >
> > I think the failure is due to marking the memory as 'reserved' in DT. With
> > 'dma-ranges', the allocator will only ensure that the allocated memory stays
> > within this limit. But the allocator itself will not use this property to
> > allocate from the reserved region.
>
> So the region should not be reserved. Reserved generally means the OS
> shouldn't use the region (though maybe a specific driver/device can). It
> should just be a CMA area I think.

No. The whole reason for this is that we want an isolated piece of memory
for PCIe so that PCIe transfers cannot corrupt memory used by other parts
of the system. Normally the IOMMU provides the isolation. On this platform
there is no IOMMU for the PCIe controller.

System memory isolation as setup by the firmware already restricts the
PCIe controller to only be able to use this region. And we absolutely do
not want any other part of the system, be it kernel, DMA buffers, or
userspace, to use it.

And also, if it is not reserved, the kernel is free to put "other" reserved
regions, such as the default CMA region or the software IO TLB region in
this piece of memory.

So I believe using memory-region is the way to go. The memory needs to be
reserved, and assigned to just the PCIe controller.


Thanks
ChenYu