Re: Xen on RP4

[Elliott Mitchell <ehem+xen@xxxxxxx>]

On Mon, Oct 26, 2020 at 01:31:42PM +0000, Julien Grall wrote:
> On 24/10/2020 06:35, Elliott Mitchell wrote:
> > ACPI has a distinct
> > means of specifying a limited DMA-width; the above fails, because it
> > assumes a *device-tree*.
> 
> Do you know if it would be possible to infer from the ACPI static table 
> the DMA-width?

Yes, and it is.  Due to not knowing much about ACPI tables I don't know
what the C code would look like though (problem is which documentation
should I be looking at first?).

Handy bit of information is in the RP4 Tianocore table source:
https://github.com/tianocore/edk2-platforms/blob/d492639638eee331ac3389e6cf53ea266c3c84b3/Platform/RaspberryPi/AcpiTables/Dsdt.asl

      Name (_DMA, ResourceTemplate() {
        //
        // Only the first GB is available.
        // Bus 0xC0000000 -> CPU 0x00000000.
        //
        QWordMemory (ResourceConsumer,
          ,
          MinFixed,
          MaxFixed,
          NonCacheable,
          ReadWrite,
          0x0,
          0x00000000C0000000, // MIN
          0x00000000FFFFFFFF, // MAX
          0xFFFFFFFF40000000, // TRA
          0x0000000040000000, // LEN
          ,
          ,
          )
      })

There should be some corresponding code in the Linux 5.9 kernels.  From
the look of that, it might even be possible to specify a memory range
which didn't start at address 0.


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