i.MX28 - Use OCRAM (iRAM) for DMA buffer allocation

[jörg_krause]

Hi,

ALSA supports using internal RAM (iRAM) for DMA buffer allocation: http://mailman.alsa-project.org/pipermail/alsa-devel/2013-October/067628.html.

As i.MX28 uses the SoC generic DMA engine, this module tries to preallocate buffer from the iRAM: http://lxr.free-electrons.com/source/sound/soc/soc-generic-dmaengine-pcm.c#L294. However, this allocation fails, because it is has not been defined by default in the i.MX28 device tree.

I wonder, if it might have some benefits to use the OCRAM for ALSA DMA buffer allocation instead of SDRAM? At least, the OCRAM runs at 200 MHz (connected to the AHB bus). SDRAM can, depending on the DDR chip, run at 133 MHz or 200 MHz, right?

I attached two patches to enable iRAM support for ALSA DMA buffer allocation.

Any feedback is welcome!

Original Attachment has been moved to: 0001-ARM-dts-imx28-Add-OCRAM-support.patch.zip

Original Attachment has been moved to: 0002-ARM-dts-imx28-Use-iRAM-for-DMA-buffer-allocation.patch.zip

[Yuri]

Hello,

IRAM has two advantages :

1) it always present and ready - so, may be used for boot ;

2) it may be used in parallel with DRAM for external memory

    bandwidth reduction.

Regards,

Yuri.

[Yuri]

Hello,

  Internal memory of i.MX devices mainly is intended for boot

purposes and here memory performance is not very critical.

Usually DRAM is more preferable because of high throughput,

when using burst accesses.

Have a great day,
Yuri

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[jörg_krause]

Hello Yuri,

is it true only for i.MX28 as for example i.MX27, i.MX53 and i.MX6QDL kernel device trees are using iRAM for their VPU (video processing unit) which does really sound like a lot of burst accesses?

Best regards

Jörg Krause

[Yuri]

Hello,

  You may look at the following discussion regarding OCRAM performance,

which can clarify the issue in general.

i.MX53 i.RAM (OCRAM) seems very slow, test code included. 

Regards,

Yuri.

[jörg_krause]

I've tested the memio tool and I get a result which is about a fifths of the values the user has measured for i.MX53: ~27k us. This leads to several questions:

1) Why is i.MX53 iRAM is so slow compared to i.MX28? Is it buggy?

2) If iRAM has such a bad performance, why is it still used on i.MX cpus for the VPU in the Linux kernel?

Best regards

Jörg Krause