linux-kernel - Re: [RFC] dma/mapping.c: WARN_ONCE on dma_addressing

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Message-ID: <Z_yjDsmjjCAMHUrh@infradead.org>
Date: Sun, 13 Apr 2025 22:54:22 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Balbir Singh <balbirs@...dia.com>
Cc: iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Christian König <christian.koenig@....com>,
	Ingo Molnar <mingo@...nel.org>, Kees Cook <kees@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Alex Deucher <alexander.deucher@....com>,
	Bert Karwatzki <spasswolf@....de>
Subject: Re: [RFC] dma/mapping.c: WARN_ONCE on dma_addressing_limited() being
 true

On Sat, Apr 12, 2025 at 07:41:10PM +1000, Balbir Singh wrote:
> In the debug and resolution of an issue involving forced use of bounce
> buffers, 7170130e4c72 ("x86/mm/init: Handle the special case of device
> private pages in add_pages(), to not increase max_pfn and trigger
> dma_addressing_limited() bounce buffers"). It would have been easier
> to debug the issue if dma_addressing_limited() had a warning about a
> device not being able to address all of memory and thus forcing all
> accesses through a bounce buffer. Please see[2].
> 
> A warning would have let the user of the system know that in their
> particular case, use_dma32 is set due to the addressing limitation
> and this would impact performance of the driver in use.
> 
> Implement a WARN_ONCE() to point to the potential use of bounce buffers
> when we hit the condition. When swiotlb is used,
> dma_addressing_limited() is used to determine the size of maximum dma
> buffer size in dma_direct_max_mapping_size(). The warning could be
> triggered in that check as well.

dma_addressing_limited is a perfectly expected condition, and returns
true for many devices and still plenty system configuation.  A kernel
warning with a stacktrace is not acceptable for that.  A simple one-line
dev_info might be ok, but could still be rather spammy on many systems.