[RFC v1 4/4] swiotlb: Add an option to allow dynamic bounce buffers

[Petr Tesarik]

From: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>

Dynamic allocation of bounce buffers may introduce regression for
some workloads. The expected outcomes are bigger worst-case I/O
latency reduced performance for some workloads. Unfortunately,
real-world testing has been too unstable to draw any conclusion.

To stay on the safe side, make the feature disabled by default and
let people turn it on with "swiotlb=dynamic" if needed. Since this
option can be combined with "force", the parser must be modified to
allow multiple options separated by commas.

A new bool field is added to struct io_tlb_mem to tell whether
dynamic allocations are allowed. This field is always false for
DMA restricted pools. It is also false for other software IO
TLBs unless "swiotlb=dynamic" was specified.

Signed-off-by: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |  6 +++++-
 include/linux/swiotlb.h                       |  3 ++-
 kernel/dma/swiotlb.c                          | 19 ++++++++++++++-----
 3 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6cfa6e3996cf..6240a463631b 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6081,14 +6081,18 @@
 			Execution Facility on pSeries.
 
 	swiotlb=	[ARM,IA-64,PPC,MIPS,X86]
-			Format: { <int> [,<int>] | force | noforce }
+			Format: { <int> [,<int>] [,option-list] | option-list }
 			<int> -- Number of I/O TLB slabs
 			<int> -- Second integer after comma. Number of swiotlb
 				 areas with their own lock. Will be rounded up
 				 to a power of 2.
+			<option-list> -- Comma-separated list of options.
+
+			Available options:
 			force -- force using of bounce buffers even if they
 			         wouldn't be automatically used by the kernel
 			noforce -- Never use bounce buffers (for debugging)
+			dynamic -- allow dynamic allocation of bounce buffers
 
 	switches=	[HW,M68k]
 
diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index 0ef27d6491b9..628e25ad7db7 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -101,6 +101,7 @@ struct io_tlb_mem {
 	bool late_alloc;
 	bool force_bounce;
 	bool for_alloc;
+	bool allow_dyn;
 	unsigned int nareas;
 	unsigned int area_nslabs;
 	struct io_tlb_area *areas;
@@ -123,7 +124,7 @@ static inline bool is_swiotlb_buffer(struct device *dev, phys_addr_t paddr)
 
 	return mem &&
 		(is_swiotlb_fixed(mem, paddr) ||
-		 is_swiotlb_dyn(mem, paddr));
+		 (mem->allow_dyn && is_swiotlb_dyn(mem, paddr)));
 }
 
 static inline bool is_swiotlb_force_bounce(struct device *dev)
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index c6a0b8f2aa6f..3efaefebb6af 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -78,6 +78,7 @@ struct io_tlb_dyn_slot {
 
 static bool swiotlb_force_bounce;
 static bool swiotlb_force_disable;
+static bool swiotlb_dynamic;
 
 struct io_tlb_mem io_tlb_default_mem;
 
@@ -159,10 +160,17 @@ setup_io_tlb_npages(char *str)
 		swiotlb_adjust_nareas(simple_strtoul(str, &str, 0));
 	if (*str == ',')
 		++str;
-	if (!strcmp(str, "force"))
-		swiotlb_force_bounce = true;
-	else if (!strcmp(str, "noforce"))
-		swiotlb_force_disable = true;
+	while (str && *str) {
+		char *opt = strsep(&str, ",");
+		if (!strcmp(opt, "force"))
+			swiotlb_force_bounce = true;
+		else if (!strcmp(opt, "noforce"))
+			swiotlb_force_disable = true;
+		else if (!strcmp(opt, "dynamic"))
+			swiotlb_dynamic = true;
+		else
+			pr_warn("Invalid swiotlb option: %s", opt);
+	}
 
 	return 0;
 }
@@ -287,6 +295,7 @@ static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,
 	mem->area_nslabs = nslabs / mem->nareas;
 
 	mem->force_bounce = swiotlb_force_bounce || (flags & SWIOTLB_FORCE);
+	mem->allow_dyn = swiotlb_dynamic;
 
 	for (i = 0; i < mem->nareas; i++) {
 		spin_lock_init(&mem->areas[i].lock);
@@ -930,7 +939,7 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
 	}
 
 	tlb_addr = (phys_addr_t)DMA_MAPPING_ERROR;
-	if (!is_swiotlb_for_alloc(dev))
+	if (mem->allow_dyn)
 		tlb_addr = swiotlb_dyn_map(dev, orig_addr, alloc_size,
 					   alloc_align_mask, dir, attrs);
 	if (tlb_addr == (phys_addr_t)DMA_MAPPING_ERROR)
-- 
2.25.1