linux-kernel - [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

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Date:   Thu, 18 Mar 2021 12:18:14 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     opendmb@...il.com, Florian Fainelli <f.fainelli@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-doc@...r.kernel.org (open list:DOCUMENTATION),
        iommu@...ts.linux-foundation.org (open list:SWIOTLB SUBSYSTEM)
Subject: [PATCH] swiotlb: Add swiotlb=off to disable SWIOTLB

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@...il.com>
---
 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;
 }
 
-- 
2.25.1