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Subject: Re: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB
To:     linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>
Cc:     opendmb@gmail.com, Jonathan Corbet <corbet@lwn.net>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        "open list:SWIOTLB SUBSYSTEM" <iommu@lists.linux-foundation.org>
References: <20210318191816.4185226-1-f.fainelli@gmail.com>
From:   Florian Fainelli <f.fainelli@gmail.com>
Message-ID: <bbd44c42-cedc-7bd6-a443-c991fd080298@gmail.com>
Date:   Thu, 18 Mar 2021 12:22:13 -0700
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On 3/18/2021 12:18 PM, Florian Fainelli wrote:
> It may be useful to disable the SWIOTLB completely for testing or when a
> platform is known not to have any DRAM addressing limitations what so
> ever.
> 
> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Christoph, in addition to this change, how would you feel if we
qualified the swiotlb_init() in arch/arm/mm/init.c with a:


if (memblock_end_of_DRAM() >= SZ_4G)
	swiotlb_init(1)

right now this is made unconditional whenever ARM_LPAE is enabled which
is the case for the platforms I maintain (ARCH_BRCMSTB) however we do
not really need a SWIOTLB so long as the largest DRAM physical address
does not exceed 4GB AFAICT.

Thanks!

> ---
>  Documentation/admin-guide/kernel-parameters.txt | 1 +
>  include/linux/swiotlb.h                         | 1 +
>  kernel/dma/swiotlb.c                            | 9 +++++++++
>  3 files changed, 11 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 04545725f187..b0223e48921e 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5278,6 +5278,7 @@
>  			force -- force using of bounce buffers even if they
>  			         wouldn't be automatically used by the kernel
>  			noforce -- Never use bounce buffers (for debugging)
> +			off -- Completely disable SWIOTLB
>  
>  	switches=	[HW,M68k]
>  
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 5857a937c637..23f86243defe 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -15,6 +15,7 @@ enum swiotlb_force {
>  	SWIOTLB_NORMAL,		/* Default - depending on HW DMA mask etc. */
>  	SWIOTLB_FORCE,		/* swiotlb=force */
>  	SWIOTLB_NO_FORCE,	/* swiotlb=noforce */
> +	SWIOTLB_OFF,		/* swiotlb=off */
>  };
>  
>  /*
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index c10e855a03bc..d7a4a789c7d3 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -126,6 +126,8 @@ setup_io_tlb_npages(char *str)
>  	} else if (!strcmp(str, "noforce")) {
>  		swiotlb_force = SWIOTLB_NO_FORCE;
>  		io_tlb_nslabs = 1;
> +	} else if (!strcmp(str, "off")) {
> +		swiotlb_force = SWIOTLB_OFF;
>  	}
>  
>  	return 0;
> @@ -229,6 +231,9 @@ int __init swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose)
>  	unsigned long i, bytes;
>  	size_t alloc_size;
>  
> +	if (swiotlb_force == SWIOTLB_OFF)
> +		return 0;
> +
>  	bytes = nslabs << IO_TLB_SHIFT;
>  
>  	io_tlb_nslabs = nslabs;
> @@ -284,6 +289,9 @@ swiotlb_init(int verbose)
>  	unsigned char *vstart;
>  	unsigned long bytes;
>  
> +	if (swiotlb_force == SWIOTLB_OFF)
> +		goto out;
> +
>  	if (!io_tlb_nslabs) {
>  		io_tlb_nslabs = (default_size >> IO_TLB_SHIFT);
>  		io_tlb_nslabs = ALIGN(io_tlb_nslabs, IO_TLB_SEGSIZE);
> @@ -302,6 +310,7 @@ swiotlb_init(int verbose)
>  		io_tlb_start = 0;
>  	}
>  	pr_warn("Cannot allocate buffer");
> +out:
>  	no_iotlb_memory = true;
>  }
>  
> 

-- 
Florian