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Re: Xen on RP4

Hi Elliott,

On 26/10/2020 16:03, Elliott Mitchell wrote:
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?).

What you provided below is an excerpt of the DSDT. AFAIK, DSDT content is written in AML. So far the shortest implementation I have seen for the AML parser is around 5000 lines (see [1]). It might be possible to strip some the code, although I think this will still probably too big for a single workaround.

What I meant by "static table" is a table that looks like a structure and can be parsed in a few lines. If we can't find on contain the DMA window, then the next best solution is to find a way to identity the platform.

I don't know enough ACPI to know if this solution is possible. A good starter would probably be the ACPI spec [2].

Handy bit of information is in the RP4 Tianocore table source:

       Name (_DMA, ResourceTemplate() {
         // Only the first GB is available.
         // Bus 0xC0000000 -> CPU 0x00000000.
         QWordMemory (ResourceConsumer,
           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.


[1] https://github.com/openbsd/src/blob/master/sys/dev/acpi/dsdt.c
[2] https://www.uefi.org/sites/default/files/resources/ACPI_6.0.pdf

Julien Grall



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