Re: Xen on RP4

[Julien Grall <julien@xen.org>]

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:
> 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.
> 
> 

Cheers,

[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