arm64: acpi: Map EFI_MEMORY_WT memory as Normal-NC

The only user we have of Normal Write-Through memory is in the ACPI code
when mapping memory regions advertised as EFI_MEMORY_WT. Since most (all?)
CPUs treat write-through as non-cacheable under the hood, don't bother
with the extra memory type here and just treat EFI_MEMORY_WT the same way
as EFI_MEMORY_WC by mapping it to the Normal-NC memory type instead and
emitting a warning if we have failed to find an alternative EFI memory
type.

Cc: Ard Biesheuvel <[e-mail]>
Cc: Lorenzo Pieralisi <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Acked-by: Catalin Marinas <[e-mail]>
Acked-by: Ard Biesheuvel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/kernel/acpi.c

···
 	}
 }
 
+static pgprot_t __acpi_get_writethrough_mem_attribute(void)
+{
+	/*
+	 * Although UEFI specifies the use of Normal Write-through for
+	 * EFI_MEMORY_WT, it is seldom used in practice and not implemented
+	 * by most (all?) CPUs. Rather than allocate a MAIR just for this
+	 * purpose, emit a warning and use Normal Non-cacheable instead.
+	 */
+	pr_warn_once("No MAIR allocation for EFI_MEMORY_WT; treating as Normal Non-cacheable\n");
+	return __pgprot(PROT_NORMAL_NC);
+}
+
 pgprot_t __acpi_get_mem_attribute(phys_addr_t addr)
 {
 	/*
···
 	 * types" of UEFI 2.5 section 2.3.6.1, each EFI memory type is
 	 * mapped to a corresponding MAIR attribute encoding.
 	 * The EFI memory attribute advises all possible capabilities
-	 * of a memory region. We use the most efficient capability.
+	 * of a memory region.
 	 */
 
 	u64 attr;
···
 	attr = efi_mem_attributes(addr);
 	if (attr & EFI_MEMORY_WB)
 		return PAGE_KERNEL;
-	if (attr & EFI_MEMORY_WT)
-		return __pgprot(PROT_NORMAL_WT);
 	if (attr & EFI_MEMORY_WC)
 		return __pgprot(PROT_NORMAL_NC);
+	if (attr & EFI_MEMORY_WT)
+		return __acpi_get_writethrough_mem_attribute();
 	return __pgprot(PROT_DEVICE_nGnRnE);
 }
 
···
 		default:
 			if (region->attribute & EFI_MEMORY_WB)
 				prot = PAGE_KERNEL;
-			else if (region->attribute & EFI_MEMORY_WT)
-				prot = __pgprot(PROT_NORMAL_WT);
 			else if (region->attribute & EFI_MEMORY_WC)
 				prot = __pgprot(PROT_NORMAL_NC);
+			else if (region->attribute & EFI_MEMORY_WT)
+				prot = __acpi_get_writethrough_mem_attribute();
 		}
 	}
 	return __ioremap(phys, size, prot);