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Date:   Mon, 11 Jul 2022 11:42:43 +0100
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Subject: Re: [PATCH] swiotlb: ensure io_tlb_default_mem spinlock always
 initialised
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To:     Christoph Hellwig <hch@lst.de>
Cc:     Robin Murphy <robin.murphy@arm.com>, linux-kernel@vger.kernel.org,
        iommu@lists.linux.dev, iommu@lists.linux-foundation.org,
        Sudip Mukherjee <sudip.mukherjee@sifive.com>,
        Jude Onyenegecha <jude.onyenegecha@sifive.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>
References: <20220708170811.270589-1-ben.dooks@sifive.com>
 <fdf5d34e-7668-c05c-9098-30e34939c88a@arm.com>
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 <e6c43ef0-870b-5fe6-141e-0a3d566b030e@arm.com> <20220711102134.GB4639@lst.de>
 <4fa8b709-c883-54dc-c302-20c9e55ae93a@sifive.com>
 <20220711103921.GA6542@lst.de>
From:   Ben Dooks <ben.dooks@sifive.com>
In-Reply-To: <20220711103921.GA6542@lst.de>
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On 11/07/2022 11:39, Christoph Hellwig wrote:
> 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.

Ok, is there any examples currently in the kernel that have no memory
in the DMA32 zone that do use swiotlb?