linux-kernel - Re: For the problem when using swiotlb

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Message-ID: <1650523.IrjSXzxF5O@wuerfel>
Date:	Mon, 17 Nov 2014 13:18:42 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Ding Tianhong <dingtianhong@...wei.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: For the problem when using swiotlb

On Monday 17 November 2014 19:56:27 Ding Tianhong wrote:
> Hi Catalin:
>         The commit 3690951fc6d42f3a0903987677d0e592c49dd8db(arm64: Use swiotlb late initialisation)
> switches the DMA mapping code to swiotlb_tlb_late_init_with_default_size(), this will occur a problem
> when I run the scsi stress tests, the message as below:
> 
>         sas_controller b1000000.sas: swiotlb buffer is full (sz: 65536 bytes)..
>         DMA: Out of SW-IOMMU space for 65536 bytes at device b1000000.sas
> 
> The reason is that the swiotlb_tlb_late_init_with_default_size() could only alloc 16M memory for DMA-mapping,
> and the param in cmdline "swiotlb=xxx" is useless because the get_free_pages() only use the buddy to assigned a
> maximum memory of 16M(The MAX_ORDER is 13 for 4k pages), obviously 16M is too small in many scenes, but
> the swiotlb_init() which could reserved a bigger memory as wished could work well for most drivers.
> 
> I could not get a better way to fix this problem except to revert this patch, so could you please give me some
> advise and help me, thanks very much.

In general, you should not need to use swiotlb for most devices, in
particular for high-performance devices like network or block.

Please make sure that you have set up the dma-ranges properties in
your DT properly to allow 64-bit DMA if the device supports it.

	Arnd
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