Re: allow larger than require DMA masks

[Sven Schnelle <svens@stackframe.org>]

Hi,

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
error:

[   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 <hch@lst.de>
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 <hch@lst.de>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

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.

Regards
Sven