LoongArch: Make WriteCombine configurable for ioremap()

LoongArch maintains cache coherency in hardware, but when paired with
LS7A chipsets the WUC attribute (Weak-ordered UnCached, which is similar
to WriteCombine) is out of the scope of cache coherency machanism for
PCIe devices (this is a PCIe protocol violation, which may be fixed in
newer chipsets).

This means WUC can only used for write-only memory regions now, so this
option is disabled by default, making WUC silently fallback to SUC for
ioremap(). You can enable this option if the kernel is ensured to run on
hardware without this bug.

Kernel parameter writecombine=on/off can be used to override the Kconfig
option.

Cc: [e-mail]
Suggested-by: WANG Xuerui <[e-mail]>
Reviewed-by: WANG Xuerui <[e-mail]>
Signed-off-by: Huacai Chen <[e-mail]>

Documentation/admin-guide/kernel-parameters.rst

···
 	KVM	Kernel Virtual Machine support is enabled.
 	LIBATA  Libata driver is enabled
 	LP	Printer support is enabled.
+	LOONGARCH LoongArch architecture is enabled.
 	LOOP	Loopback device support is enabled.
 	M68k	M68k architecture is enabled.
 			These options have more detailed description inside of

Documentation/admin-guide/kernel-parameters.txt

···
 			When enabled, memory and cache locality will be
 			impacted.
 
+	writecombine=	[LOONGARCH] Control the MAT (Memory Access Type) of
+			ioremap_wc().
+
+			on   - Enable writecombine, use WUC for ioremap_wc()
+			off  - Disable writecombine, use SUC for ioremap_wc()
+
 	x2apic_phys	[X86-64,APIC] Use x2apic physical mode instead of
 			default x2apic cluster mode on platforms
 			supporting x2apic.

arch/loongarch/Kconfig

···
 	  protection support. However, you can enable LoongArch DMW-based
 	  ioremap() for better performance.
 
+config ARCH_WRITECOMBINE
+	bool "Enable WriteCombine (WUC) for ioremap()"
+	help
+	  LoongArch maintains cache coherency in hardware, but when paired
+	  with LS7A chipsets the WUC attribute (Weak-ordered UnCached, which
+	  is similar to WriteCombine) is out of the scope of cache coherency
+	  machanism for PCIe devices (this is a PCIe protocol violation, which
+	  may be fixed in newer chipsets).
+
+	  This means WUC can only used for write-only memory regions now, so
+	  this option is disabled by default, making WUC silently fallback to
+	  SUC for ioremap(). You can enable this option if the kernel is ensured
+	  to run on hardware without this bug.
+
+	  You can override this setting via writecombine=on/off boot parameter.
+
 config ARCH_STRICT_ALIGN
 	bool "Enable -mstrict-align to prevent unaligned accesses" if EXPERT
 	default y

arch/loongarch/include/asm/io.h

···
  * @offset:    bus address of the memory
  * @size:      size of the resource to map
  */
+extern pgprot_t pgprot_wc;
+
 #define ioremap_wc(offset, size)	\
-	ioremap_prot((offset), (size), pgprot_val(PAGE_KERNEL_WUC))
+	ioremap_prot((offset), (size), pgprot_val(pgprot_wc))
 
 #define ioremap_cache(offset, size)	\
 	ioremap_prot((offset), (size), pgprot_val(PAGE_KERNEL))

arch/loongarch/kernel/setup.c

···
 	dmi_walk(find_tokens, NULL);
 }
 
+#ifdef CONFIG_ARCH_WRITECOMBINE
+pgprot_t pgprot_wc = PAGE_KERNEL_WUC;
+#else
+pgprot_t pgprot_wc = PAGE_KERNEL_SUC;
+#endif
+
+EXPORT_SYMBOL(pgprot_wc);
+
+static int __init setup_writecombine(char *p)
+{
+	if (!strcmp(p, "on"))
+		pgprot_wc = PAGE_KERNEL_WUC;
+	else if (!strcmp(p, "off"))
+		pgprot_wc = PAGE_KERNEL_SUC;
+	else
+		pr_warn("Unknown writecombine setting \"%s\".\n", p);
+
+	return 0;
+}
+early_param("writecombine", setup_writecombine);
+
 static int usermem __initdata;
 
 static int __init early_parse_mem(char *p)