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Message-ID: <6b2fa7af-0fef-4c18-a6ff-1bf4ea16810f@arm.com>
Date: Wed, 13 Mar 2024 12:43:52 +0000
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Subject: Re: [PATCH] direct-dma: WARN_ON_ONCE when the page is not addressable
 by device's coherent_dma_mask
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To: John Hsu <john.hsu@mediatek.com>, Christoph Hellwig <hch@lst.de>,
 Marek Szyprowski <m.szyprowski@samsung.com>,
 Matthias Brugger <matthias.bgg@gmail.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Cc: surenb@google.com, Chinwen Chang <chinwen.change@mediatek.com>,
 Casper Li <casper.li@mediatek.com>,
 Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>, iommu@lists.linux.dev,
 linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
 linux-mediatek@lists.infradead.org
References: <20240313121933.15839-1-john.hsu@mediatek.com>
From: Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <20240313121933.15839-1-john.hsu@mediatek.com>
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On 2024-03-13 12:19 pm, John Hsu wrote:
> From: JohnHsu <john.hsu@mediatek.com>
> 
> The dma_direct_alloc() may return null in some cases. For example, the
> allocated page is not addressable for the device's coherent_dma_mask,
> and the allocated page will be assigned to null.
> 
> This patch can WARN_ON_ONCE() when the returned page is null in
> dma_direct_alloc. It helps the developers position the root cause of
> allocation failure rapidly.

No. Failure to allocate a buffer can happen for any number of reasons, 
it is not specific to dma-direct, and in some cases it is even expected, 
hence why DMA_ATTR_NO_WARN exists. And either way it's still not a 
condition worthy of panicking when panic_on_warn is in use.

Sure, this may well be a handy development hack for debugging a 
particular driver which isn't handling failure correctly, but it is not 
suitable for mainline.

Thanks,
Robin.

> Signed-off-by: JohnHsu <john.hsu@mediatek.com>
> ---
>   kernel/dma/direct.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 9596ae1aa0da..a73b8ad1ef9e 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -156,6 +156,8 @@ static struct page *__dma_direct_alloc_pages(struct device *dev, size_t size,
>   		}
>   	}
>   
> +	WARN_ON_ONCE(!page);
> +
>   	return page;
>   }
>