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

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Message-ID: <b679c1ec-be21-458e-a650-1aa14e2366a1@oss.qualcomm.com>
Date: Wed, 27 Aug 2025 09:55:20 +0800
From: Baochen Qiang <baochen.qiang@....qualcomm.com>
To: Marek Szyprowski <m.szyprowski@...sung.com>,
        Baochen Qiang <baochen.qiang@....qualcomm.com>,
        Robin Murphy <robin.murphy@....com>,
        Jeff Johnson <jjohnson@...nel.org>
Cc: 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 8/26/2025 10:09 PM, Marek Szyprowski wrote:
> On 26.08.2025 11:44, Baochen Qiang wrote:
>> Hi guys,
>>
>> I have a driver which allocate noncoherent DMA buffer and get the returned CPU addr tested:
>>
>> 	vaddr_unaligned = dma_alloc_noncoherent(ab->dev, rx_tid->unaligned_size, &paddr,
>> 						DMA_BIDIRECTIONAL, GFP_ATOMIC);
>> 	if (!vaddr_unaligned) {
>> 		spin_unlock_bh(&ab->base_lock);
>> 		return -ENOMEM;
>> 	}
>>
>> while free the buffer
>>
>> 	dma_free_noncoherent(ab->dev, rx_tid->unaligned_size,
>> 			     rx_tid->vaddr_unaligned,
>> 			     rx_tid->paddr_unaligned, DMA_BIDIRECTIONAL);
>>
>> I get below warnings:
>>
>>   DMA-API: ath11k_pci 0000:03:00.0: device driver failed to check map error[device
>> address=0x00000000f3ad7000] [size=639 bytes] [mapped as single]
>>   WARNING: CPU: 15 PID: 64303 at kernel/dma/debug.c:1036 check_unmap+0x7e2/0x950
>>   RIP: 0010:check_unmap+0x7e2/0x950
>>   Call Trace:
>>    <TASK>
>>    ? free_to_partial_list+0x9d/0x350
>>    debug_dma_unmap_page+0xac/0xc0
>>    ? debug_smp_processor_id+0x17/0x20
>>    ? rcu_is_watching+0x13/0x70
>>    dma_free_pages+0x56/0x180
>>    [...]
>>    </TASK>
>>   ---[ end trace 0000000000000000 ]---
>>   DMA-API: Mapped at:
>>    debug_dma_map_page+0x7c/0x140
>>    dma_alloc_pages+0x74/0x220
>>    [...]
>>
>> 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?
>>
>> If I was right, should we remove invocation to debug_dma_map_page() in dma_alloc_pages()?
> 
> Simply replace "if (!vaddr_unaligned)" with "if 
> dma_mapping_error(ab->dev, paddr)" and the debug code will be happy.

Thanks, much better!

But I am wondering if the debug code is worrying too much? isn't the CPU addr test already
good enough?

> 
> Best regards