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Having Linux handle different "types" of memory

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  Having Linux handle different "types" of memory Mason <slash.tmp@free.fr> - 2015-07-22 17:20 +0200
    Re: Having Linux handle different "types" of memory Laura Abbott <labbott@redhat.com> - 2015-07-22 18:30 +0200

#1190049 — Having Linux handle different "types" of memory

Hello everyone,

I'm using an ARMv7 platform (Cortex A9) on Linux 3.14

The system supports two memory modules.

For performance reasons, memory is "transparently" interleaved
(with a 128-byte grain). That is, when the CPU accesses addresses
0-127, it hits DRAM0; addresses 128-255, it hits DRAM1, and so on.

The problem is that other devices in the system, mainly the
Ethernet controller, didn't get the "transparent interleaving"
treatment. They just see DRAM0 and DRAM1. And I'm guessing this
will generate all kinds of "interesting" problems when I try to
DMA from the Ethernet controller's memory to DRAM...

Is there a way to tell Linux:

1) this 1GB memory chunk here is for you and your private allocations,
but don't use it for talking to devices/peripherals.

2) this 1GB memory chunk there is for talking to devices/peripherals,
but it has lower performance, so try not to use it for your own
private memory pools, but you can if memory is /really/ tight.

Is there something like this?

Maybe one of the NUMA policies?
https://www.kernel.org/doc/Documentation/vm/numa_memory_policy.txt
(I don't see any arch/arm/mm/numa.c however)

Maybe I can pretend that there is some kind of IOMMU?
https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt
arch/arm/include/asm/dma-iommu.h

Or maybe there is an obvious solution that I'm missing?

Regards.
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#1190091

On 07/22/2015 08:19 AM, Mason wrote:
> Hello everyone,
>
> I'm using an ARMv7 platform (Cortex A9) on Linux 3.14
>
> The system supports two memory modules.
>
> For performance reasons, memory is "transparently" interleaved
> (with a 128-byte grain). That is, when the CPU accesses addresses
> 0-127, it hits DRAM0; addresses 128-255, it hits DRAM1, and so on.
>
> The problem is that other devices in the system, mainly the
> Ethernet controller, didn't get the "transparent interleaving"
> treatment. They just see DRAM0 and DRAM1. And I'm guessing this
> will generate all kinds of "interesting" problems when I try to
> DMA from the Ethernet controller's memory to DRAM...
>
> Is there a way to tell Linux:
>
> 1) this 1GB memory chunk here is for you and your private allocations,
> but don't use it for talking to devices/peripherals.
>
> 2) this 1GB memory chunk there is for talking to devices/peripherals,
> but it has lower performance, so try not to use it for your own
> private memory pools, but you can if memory is /really/ tight.
>
> Is there something like this?
>
> Maybe one of the NUMA policies?
> https://www.kernel.org/doc/Documentation/vm/numa_memory_policy.txt
> (I don't see any arch/arm/mm/numa.c however)
>
> Maybe I can pretend that there is some kind of IOMMU?
> https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt
> arch/arm/include/asm/dma-iommu.h
>
> Or maybe there is an obvious solution that I'm missing?
>

I don't think there is an easy solution right now. This is still
an open problem as far as I know. You might look into whether
marking one of the regions as a CMA region would allow you the
control you need.

Thanks,
Laura

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