RE: [PATCHv3 5/6] ARM: imx: Add initial support for Freescale LS1021A

-----Original Message-----
From: Arnd Bergmann [mailto:arnd@arndb.de]
Sent: Wednesday, September 10, 2014 3:42 PM
To: Lu Jingchang-B35083
Cc: linux-arm-kernel@lists.infradead.org; Guo Shawn-R65073;
devicetree@vger.kernel.org
Subject: Re: [PATCHv3 5/6] ARM: imx: Add initial support for Freescale
LS1021A

On Wednesday 10 September 2014 03:31:19 Jingchang Lu wrote:

quoted

quoted

quoted

+DT_MACHINE_START(LS1021A, "Freescale LS1021A")  #ifdef
+CONFIG_ZONE_DMA
+       .dma_zone_size  = SZ_128M,
+#endif
+       .init_machine   = ls1021a_init_machine,
+       .dt_compat      = ls1021a_dt_compat,
+       .restart        = mxc_restart,
+MACHINE_END

I believe someone recently posted a patch to derive the dma_zone_size
from device tree. Can yo try to find that and see if that will work for

you?

quoted

quoted

Can you explain what the reason is for needing a DMA zone?

With LPAE enabled on our SoC, we meet the system complaint of
"coherent DMA mask 0xffffffff is smaller than system GFP_DMA mask
0xffffffffffffffff", and I notice that CONFIG_ZONE_DMA and dma_zone_size

is a common resolve for this.

Ok, I see. The actual point of dma_zone_size however is slightly different,
and I think you should not use it like this. We normally only use ZONE_DMA
if there are devices that have a limitation smaller than 4GB, and that
appear to be the case for your system.

The message you quote is only present in arch/powerpc, so I'm not sure
what symptoms you are actually seeing. Please try removing the
dma_zone_size setting for your platform and report if it works or what the
symptom is if it does not work with the latest kernel.

We definitely need to get this to work out of the box without a
dma_zone_size hack.

Can you describe what the memory layout is of your platform? Can you have
RAM installed above the 4GB physical address boundary? If you can, are
there any devices that are unable to perform DMA into that memory without
the use of an IOMMU?

Arnd

Our first LS1021A support is based on kernel-3.12, where when LPAE enabled
it will compare the device's coherent_dma_mask with arm_dma_limit, which is
64-bit 0xffffffffffffffff without CONFIG_ZONE_DMA, with CONFIG_ZONE_DMA can
limit the comparison and avoid the warning.
All device can address 32-bit address space on LS1021A, With you comment above,
I remove the dma_zone_size and only reserve the CONFIG_ZONE_DMA On kernel-3.12,
finding all works well, so I will remove the dma_zone_size setting on 3.12. Thanks.

I have a look on latest kernel, finding get_coherent_dma_mask() has limited the mask
to (u64)DMA_BIT_MASK(32), does this mean the CONFIG_ZONE_DMA is not needed for me
here? Thanks.

Best Regards,
Jingchang
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