Subject: [PATCH V2 1/3] scsi: mptxsas: try 64 bit DMA when 32 bit DMA fails

From: [email-masked] (Timur Tabi)
Date: Tue, 10 Nov 2015 15:03:59 -0600

On 11/10/2015 01:13 PM, Arnd Bergmann wrote:
> If the mask is 64-bit by default on ARM64, that is a bug that we need
> to fix urgently. Can you verify this?

I think the mask is 0 by default, because there's no code in ARM64 that 
actually sets the mask.

Take a look at arch_setup_pdev_archdata() in 
arch/powerpc/kernel/setup-common.c.

void arch_setup_pdev_archdata(struct platform_device *pdev)
{
         pdev->archdata.dma_mask = DMA_BIT_MASK(32);
         pdev->dev.dma_mask = &pdev->archdata.dma_mask;
         set_dma_ops(&pdev->dev, &dma_direct_ops);
}

I don't see anything equivalent in arch/arm64

> A lot of PCI devices can only do 32-bit DMA, and we have plenty
> of drivers that don't bother setting a mask at all because the 32-bit
> mask is the default on all other architectures.

In our drivers for 32-bit devices, we have to explicitly set the DMA 
mask to 32-bits in order to get any DMA working.

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