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SubjectRe: allow larger than require DMA masks

On Fri, Feb 15, 2019 at 03:45:54PM +0100, Christoph Hellwig wrote:
> Hi all,
> this series finishes off converting our dma mask model to split between
> device capabilities (dev->dma_mask and dev->coherent_dma_mask) and system
> limitations (dev->bus_dma_mask). We already accept larger than required
> masks in most dma_map_ops implementation, in case of x86 and
> implementations based on it since the dawn of time. Only one parisc
> and two sparc64 instances failed larger than required DMA masks, and
> this series fixes that up and updates the documentation that devices
> don't need to handle DMA mask fallbacks.

I just tried latest linux-5.4 git on my hp c8000 (parisc), and got the following

[ 27.246866] sata_sil24 0000:00:01.0: Applying completion IRQ loss on PCI-X errata fix
[ 27.336968] sata_sil24 0000:00:01.0: DMA enable failed
[ 27.476922] sata_sil24: probe of 0000:00:01.0 failed with error -5

This is caused by commit dcc02c19cc06bd7bc1b6db0aa0087a2b6eb05b94:
Author: Christoph Hellwig <>
Date: Mon Aug 26 12:57:24 2019 +0200

sata_sil24: use dma_set_mask_and_coherent

Use the dma_set_mask_and_coherent helper to set the DMA mask. Rely
on the relatively recent change that setting a larger than required
mask will never fail to avoid the need for the boilerplate 32-bit
fallback code.

Signed-off-by: Christoph Hellwig <>
Signed-off-by: Jens Axboe <>

However, the real problem seems to be in sba_dma_supported():

» /* Documentation/DMA-API-HOWTO.txt tells drivers to try 64-bit
» * first, then fall back to 32-bit if that fails.
» * We are just "encouraging" 32-bit DMA masks here since we can
» * never allow IOMMU bypass unless we add special support for ZX1.
» */
if (mask > ~0U)
» » return 0;

Removing the if() makes the DMA mapping work. It's almost midnight here,
so i won't look into that any further today. Does anyone have an opinion
on this behaviour? Otherwise i will look a bit more into this in the
next days.


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