Re: [PATCH] swiotlb: ensure io_tlb_default_mem spinlock always initialised

From: Ben Dooks <ben.dooks@sifive.com>
To: Christoph Hellwig <hch@lst.de>, Robin Murphy <robin.murphy@arm.com>
Cc: 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>
Subject: Re: [PATCH] swiotlb: ensure io_tlb_default_mem spinlock always initialised
Date: Mon, 11 Jul 2022 11:24:51 +0100	[thread overview]
Message-ID: <4fa8b709-c883-54dc-c302-20c9e55ae93a@sifive.com> (raw)
In-Reply-To: <20220711102134.GB4639@lst.de>

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.
>>
>> However, by inspection it seems we do have a bug here as well, for which
>> the correct fix should be as below. The fireworks you're *supposed* to
>> get in that situation are considerably louder and more obvious than a
>> DEBUG_SPINLOCK complaint ;)
> 
> This looks sensible, I'll pick it up.