linux-kernel - Re: Having Linux handle different "types" of memory

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Date:	Wed, 22 Jul 2015 22:20:46 +0200
From:	Mason <slash.tmp@...e.fr>
To:	Laura Abbott <labbott@...hat.com>,
	Linux ARM <linux-arm-kernel@...ts.infradead.org>
CC:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Having Linux handle different "types" of memory

On 22/07/2015 18:21, Laura Abbott wrote:

> On 07/22/2015 08:19 AM, Mason wrote:
>
>> 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.

I have control of the address ranges where memory is interleaved,
and where it is not. Is it possible to force (some) memory allocations
to only come from the latter pool?

I'm thinking another possibility is to not even give control of
the non-interleaved zone to Linux, and just ioremap it as needed.
(But then, it's not available when memory is tight.)

Regards.

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