linux-kernel - Re: is dma_mapping_error() check necessary for dma_alloc

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Message-ID: <aK60Py3r25U43k1E@infradead.org>
Date: Wed, 27 Aug 2025 00:31:11 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Baochen Qiang <baochen.qiang@....qualcomm.com>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Jeff Johnson <jjohnson@...nel.org>, iommu@...ts.linux.dev,
	"linux-kernel@...r.kernel.org >> linux-kernel" <linux-kernel@...r.kernel.org>,
	"ath11k@...ts.infradead.org" <ath11k@...ts.infradead.org>
Subject: Re: is dma_mapping_error() check necessary for
 dma_alloc_noncoherent()?

On Tue, Aug 26, 2025 at 05:44:42PM +0800, Baochen Qiang wrote:
> Checking code gives me the impression that I should do dma_mapping_error() check as well.
> And indeed with below diff the warning is gone:
> 
> +       dma_mapping_error(ab->dev, paddr);
> 
> However this does not make sense to me since IMO testing the CPU address is good enough, I
> can not imagine a valid case where DMA alloc/map fails while returning a valid CPU
> address, no?

Yes, this doesn't make sense.  dma_mapping_error exists to provide a
error handling path for dma_map_*, which return the dma address only.

For the dma_alloc_* interfaces that return a pointer and can signal
with a NULL return it should not be needed and dma-debug needs to
be fixed.

> If I was right, should we remove invocation to debug_dma_map_page() in dma_alloc_pages()?
> 

That allocation still needs to be tracked, so it can't just be removed
but needs to be changed to record the kind of allocation.