Re: [PATCH 2/2] arm: use swiotlb for bounce buffer on LPAE configs

[Peter Ujfalusi]

Hi,

On 09/07/2019 17.20, Christoph Hellwig wrote:
> The DMA API requires that 32-bit DMA masks are always supported, but on
> arm LPAE configs they do not currently work when memory is present
> above 4GB.  Wire up the swiotlb code like for all other architectures
> to provide the bounce buffering in that case.

bisect pointed me to this commit as the reason why EDMA fails to probe and sdhci is falling back to PIO mode (not using it's built in DMA).

In both cases the reason is that
dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
fails, because dma_direct_supported() is returning false.


Prints inside dma_direct_supported():
sdhci-omap 23000000.mmc: max_pfn: 880000
sdhci-omap 23000000.mmc: min_mask #1: ffffff
sdhci-omap 23000000.mmc: min_mask #2: ffffff
sdhci-omap 23000000.mmc: dev->dma_pfn_offset: 780000
sdhci-omap 23000000.mmc: PAGE_SHIFT: 12
sdhci-omap 23000000.mmc: __phys_to_dma(dev, min_mask): ff880ffffff
sdhci-omap 23000000.mmc: mask: ffffffff

Print in dma_supported() after returning from dma_direct_supported():
sdhci-omap 23000000.mmc: dma_is_direct, ret = 0

sdhci-omap 23100000.mmc: DMA is not supported for the device

keystone-k2g have this in soc0 node:
dma-ranges = <0x80000000 0x8 0x00000000 0x80000000>;

DDR starts at 0x8 0000 0000 (8G) and 2G is aliased at 0x8000 0000.

This gives the 0x780000 for dma_pfn_offset for all devices underneath it.

The DMA_BIT_MASK(24) is passed to __phys_to_dma() because CONFIG_ZONE_DMA is enabled.

SWIOTLB is enabled in kconfig.

I'm not sure how to correctly fix it, but the following patch makes things working: