linux-kernel - Re: [PATCH] swiotlb: ensure io_tlb_default

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Date:   Mon, 11 Jul 2022 12:39:21 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ben Dooks <ben.dooks@...ive.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
        iommu@...ts.linux-foundation.org,
        Sudip Mukherjee <sudip.mukherjee@...ive.com>,
        Jude Onyenegecha <jude.onyenegecha@...ive.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH] swiotlb: ensure io_tlb_default_mem spinlock always
 initialised

On Mon, Jul 11, 2022 at 11:24:51AM +0100, Ben Dooks wrote:
> On 11/07/2022 11:21, Christoph Hellwig wrote:
>> On Mon, Jul 11, 2022 at 11:07:17AM +0100, Robin Murphy wrote:
>>> If none of your peripherals should need SWIOTLB, then the fact that
>>> you're ending up in swiotlb_map() at all is a clear sign that
>>> something's wrong. Most likely someone's forgotten to set their DMA
>>> masks correctly.
>>
>> Yes.
>
> Possibly, we had at least one driver which attempted to set a 32 bit
> DMA mask which had to be removed as the DMA layer accepts this but
> since there is no DMA32 memory the allocator then just fails.
>
> I expect the above may need to be a separate discussion(s) of how to
> default the DMA mask and how to stop the implicit acceptance of setting
> a 32-bit DMA mask.

No.  Linux simply assumes you can do 32-bit DMA and this won't
change.  So we'll need to fix your platform to support swiotlb
eventually.