Re: [PATCH 2/2] arch: switch the default on ARCH_HAS_SG_CHAIN

From: Palmer Dabbelt
Date: Fri Nov 16 2018 - 11:52:19 EST

On Fri, 09 Nov 2018 01:00:08 PST (-0800), Christoph Hellwig wrote:
These days architectures are mostly out of the business of dealing with
struct scatterlist at all, unless they have architecture specific iommu
drivers. Replace the ARCH_HAS_SG_CHAIN symbol with a ARCH_NO_SG_CHAIN
one only enabled for architectures with horrible legacy iommu drivers
like alpha and parisc, and conditionally for arm which wants to keep it
disable for legacy platforms.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
.../features/io/sg-chain/arch-support.txt | 33 -------------------
arch/alpha/Kconfig | 1 +
arch/arc/Kconfig | 1 -
arch/arm/Kconfig | 2 +-
arch/arm64/Kconfig | 1 -
arch/ia64/Kconfig | 1 -
arch/parisc/Kconfig | 1 +
arch/powerpc/Kconfig | 1 -
arch/s390/Kconfig | 1 -
arch/sparc/Kconfig | 1 -
arch/x86/Kconfig | 1 -
arch/xtensa/Kconfig | 1 -
include/linux/scatterlist.h | 6 ++--
lib/Kconfig | 2 +-
lib/scatterlist.c | 2 +-
15 files changed, 8 insertions(+), 47 deletions(-)
delete mode 100644 Documentation/features/io/sg-chain/arch-support.txt

As far as I can tell, it looks like m68k, mips, and powerpc mention an IOMMU in their ports, don't set ARCH_HAS_SG_CHAIN, and with this patch set won't set ARCH_NO_SG_CHAIN. The issue is that I'm not sure how to determine what constitutes a horrible legacy IOMMU, at least with respect to not being able to use scatterlist chaining.

I poked through the 68k stuff (sun3 and sun3x) and the only thing I see that's odd is that DVMA_PAGE_SHIFT is 13. That matches PAGE_SHIFT for the subarch, though, so I don't think it's an issue. It looks like the mention in MIPS is trivial, so that should be fine. There's a lot in the PC port so I just glanced at it. I assume it's OK, but I did see an explicit mention of SG in their generic IOMMU. There's a lot of flavors here so I'm not sure how well they all work.

I don't really think that's enough for my reviewed-by tag, so feel free to give me some pointers if you're looking for one -- I'd be happy to lean about IOMMUs (particularly broken ones, as that might help prevent us from designing a broken one), I'm just not sure what I'm looking for.

I'm going to assume you're going to merge this and I'll drop the RISC-V patch that turns on ARCH_HAS_SG_CHAIN.

Thanks!

diff --git a/Documentation/features/io/sg-chain/arch-support.txt b/Documentation/features/io/sg-chain/arch-support.txt
deleted file mode 100644
index 6554f0372c3f..000000000000
--- a/Documentation/features/io/sg-chain/arch-support.txt
+++ /dev/null
@@ -1,33 +0,0 @@
-#
-# Feature name: sg-chain
-# Kconfig: ARCH_HAS_SG_CHAIN
-# description: arch supports chained scatter-gather lists
-#
- -----------------------
- | arch |status|
- -----------------------
- | alpha: | TODO |
- | arc: | ok |
- | arm: | ok |
- | arm64: | ok |
- | c6x: | TODO |
- | h8300: | TODO |
- | hexagon: | TODO |
- | ia64: | ok |
- | m68k: | TODO |
- | microblaze: | TODO |
- | mips: | TODO |
- | nds32: | TODO |
- | nios2: | TODO |
- | openrisc: | TODO |
- | parisc: | TODO |
- | powerpc: | ok |
- | riscv: | TODO |
- | s390: | ok |
- | sh: | TODO |
- | sparc: | ok |
- | um: | TODO |
- | unicore32: | TODO |
- | x86: | ok |
- | xtensa: | TODO |
- -----------------------
diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 5b4f88363453..a7e748a46c18 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -5,6 +5,7 @@ config ALPHA
select ARCH_MIGHT_HAVE_PC_PARPORT
select ARCH_MIGHT_HAVE_PC_SERIO
select ARCH_NO_PREEMPT
+ select ARCH_NO_SG_CHAIN
select ARCH_USE_CMPXCHG_LOCKREF
select HAVE_AOUT
select HAVE_IDE
diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index c9e2a1323536..fd48d698da29 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -13,7 +13,6 @@ config ARC
select ARCH_HAS_PTE_SPECIAL
select ARCH_HAS_SYNC_DMA_FOR_CPU
select ARCH_HAS_SYNC_DMA_FOR_DEVICE
- select ARCH_HAS_SG_CHAIN
select ARCH_SUPPORTS_ATOMIC_RMW if ARC_HAS_LLSC
select BUILDTIME_EXTABLE_SORT
select CLONE_BACKWARDS
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 91be74d8df65..199eb62230b0 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -19,6 +19,7 @@ config ARM
select ARCH_HAVE_CUSTOM_GPIO_H
select ARCH_HAS_GCOV_PROFILE_ALL
select ARCH_MIGHT_HAVE_PC_PARPORT
+ select ARCH_NO_SG_CHAIN if !ARM_HAS_SG_CHAIN
select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
select ARCH_SUPPORTS_ATOMIC_RMW
@@ -118,7 +119,6 @@ config ARM
<http://www.arm.linux.org.uk/>.

config ARM_HAS_SG_CHAIN
- select ARCH_HAS_SG_CHAIN
bool

config ARM_DMA_USE_IOMMU
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 787d7850e064..4c851b3a7b7c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -23,7 +23,6 @@ config ARM64
select ARCH_HAS_MEMBARRIER_SYNC_CORE
select ARCH_HAS_PTE_SPECIAL
select ARCH_HAS_SET_MEMORY
- select ARCH_HAS_SG_CHAIN
select ARCH_HAS_STRICT_KERNEL_RWX
select ARCH_HAS_STRICT_MODULE_RWX
select ARCH_HAS_SYNC_DMA_FOR_DEVICE
diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
index 36773def6920..d6f203658994 100644
--- a/arch/ia64/Kconfig
+++ b/arch/ia64/Kconfig
@@ -29,7 +29,6 @@ config IA64
select HAVE_MEMBLOCK_NODE_MAP
select HAVE_VIRT_CPU_ACCOUNTING
select ARCH_HAS_DMA_MARK_CLEAN
- select ARCH_HAS_SG_CHAIN
select VIRT_TO_BUS
select ARCH_DISCARD_MEMBLOCK
select GENERIC_IRQ_PROBE
diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
index 92a339ee28b3..428ee50fc3db 100644
--- a/arch/parisc/Kconfig
+++ b/arch/parisc/Kconfig
@@ -11,6 +11,7 @@ config PARISC
select ARCH_HAS_ELF_RANDOMIZE
select ARCH_HAS_STRICT_KERNEL_RWX
select ARCH_HAS_UBSAN_SANITIZE_ALL
+ select ARCH_NO_SG_CHAIN
select ARCH_SUPPORTS_MEMORY_FAILURE
select RTC_CLASS
select RTC_DRV_GENERIC
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 8be31261aec8..4bc8edd83cee 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -138,7 +138,6 @@ config PPC
select ARCH_HAS_PTE_SPECIAL
select ARCH_HAS_MEMBARRIER_CALLBACKS
select ARCH_HAS_SCALED_CPUTIME if VIRT_CPU_ACCOUNTING_NATIVE && PPC64
- select ARCH_HAS_SG_CHAIN
select ARCH_HAS_STRICT_KERNEL_RWX if ((PPC_BOOK3S_64 || PPC32) && !RELOCATABLE && !HIBERNATION)
select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
select ARCH_HAS_UACCESS_FLUSHCACHE if PPC64
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 5173366af8f3..5624e8607054 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -73,7 +73,6 @@ config S390
select ARCH_HAS_KCOV
select ARCH_HAS_PTE_SPECIAL
select ARCH_HAS_SET_MEMORY
- select ARCH_HAS_SG_CHAIN
select ARCH_HAS_STRICT_KERNEL_RWX
select ARCH_HAS_STRICT_MODULE_RWX
select ARCH_HAS_UBSAN_SANITIZE_ALL
diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index 490b2c95c212..8853b6ceae17 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -40,7 +40,6 @@ config SPARC
select MODULES_USE_ELF_RELA
select ODD_RT_SIGACTION
select OLD_SIGSUSPEND
- select ARCH_HAS_SG_CHAIN
select CPU_NO_EFFICIENT_FFS
select LOCKDEP_SMALL if LOCKDEP
select NEED_DMA_MAP_STATE
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index ba7e3464ee92..6faab6f9b57e 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -66,7 +66,6 @@ config X86
select ARCH_HAS_UACCESS_FLUSHCACHE if X86_64
select ARCH_HAS_UACCESS_MCSAFE if X86_64 && X86_MCE
select ARCH_HAS_SET_MEMORY
- select ARCH_HAS_SG_CHAIN
select ARCH_HAS_STRICT_KERNEL_RWX
select ARCH_HAS_STRICT_MODULE_RWX
select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
index d29b7365da8d..e3387919eb22 100644
--- a/arch/xtensa/Kconfig
+++ b/arch/xtensa/Kconfig
@@ -1,7 +1,6 @@
# SPDX-License-Identifier: GPL-2.0
config XTENSA
def_bool y
- select ARCH_HAS_SG_CHAIN
select ARCH_HAS_SYNC_DMA_FOR_CPU
select ARCH_HAS_SYNC_DMA_FOR_DEVICE
select ARCH_NO_COHERENT_DMA_MMAP if !MMU
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 093aa57120b0..b96f0d0b5b8f 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -324,10 +324,10 @@ size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
* Like SG_CHUNK_SIZE, but for archs that have sg chaining. This limit
* is totally arbitrary, a setting of 2048 will get you at least 8mb ios.
*/
-#ifdef CONFIG_ARCH_HAS_SG_CHAIN
-#define SG_MAX_SEGMENTS 2048
-#else
+#ifdef CONFIG_ARCH_NO_SG_CHAIN
#define SG_MAX_SEGMENTS SG_CHUNK_SIZE
+#else
+#define SG_MAX_SEGMENTS 2048
#endif

#ifdef CONFIG_SG_POOL
diff --git a/lib/Kconfig b/lib/Kconfig
index a9965f4af4dd..d5a5e2ebf286 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -577,7 +577,7 @@ config SG_POOL
# sg chaining option
#

-config ARCH_HAS_SG_CHAIN
+config ARCH_NO_SG_CHAIN
def_bool n

config ARCH_HAS_PMEM_API
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 7c6096a71704..9ba349e775ef 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -271,7 +271,7 @@ int __sg_alloc_table(struct sg_table *table, unsigned int nents,

if (nents == 0)
return -EINVAL;
-#ifndef CONFIG_ARCH_HAS_SG_CHAIN
+#ifdef CONFIG_ARCH_NO_SG_CHAIN
if (WARN_ON_ONCE(nents > max_ents))
return -EINVAL;
#endif