arch: switch the default on ARCH_HAS_SG_CHAIN

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: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NPalmer Dabbelt <palmer@sifive.com>

Documentation/features/io/sg-chain/arch-support.txt

-#
-# 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 |
-    -----------------------

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

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

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
@@ -119,7 +120,6 @@ config ARM
 	  <http://www.arm.linux.org.uk/>.
 
 config ARM_HAS_SG_CHAIN
-	select ARCH_HAS_SG_CHAIN
 	bool
 
 config ARM_DMA_USE_IOMMU

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

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

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

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

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

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

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

arch/xtensa/Kconfig

 # 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

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

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

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