efi/earlycon: Fix write-combine mapping on x86

On x86, until PAT is initialized, WC translates into UC-. Since we
calculate and store pgprot_writecombine(PAGE_KERNEL) when earlycon is
initialized, this means we actually use UC- mappings instead of WC
mappings, which makes scrolling very slow.

Instead store a boolean flag to indicate whether we want to use
writeback or write-combine mappings, and recalculate the actual pgprot_t
we need on every mapping. Once PAT is initialized, we will start using
write-combine mappings, which speeds up the scrolling considerably.
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Fixes: 69c1f396 ("efi/x86: Convert x86 EFI earlyprintk into generic earlycon implementation")
Link: https://lkml.kernel.org/r/20191224132909.102540-2-ardb@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/efi/earlycon.c

@@ -17,7 +17,7 @@ static const struct console *earlycon_console __initdata;
 static const struct font_desc *font;
 static u32 efi_x, efi_y;
 static u64 fb_base;
-static pgprot_t fb_prot;
+static bool fb_wb;
 static void *efi_fb;
 
 /*
@@ -33,10 +33,8 @@ static int __init efi_earlycon_remap_fb(void)
 	if (!earlycon_console || !(earlycon_console->flags & CON_ENABLED))
 		return 0;
 
-	if (pgprot_val(fb_prot) == pgprot_val(PAGE_KERNEL))
-		efi_fb = memremap(fb_base, screen_info.lfb_size, MEMREMAP_WB);
-	else
-		efi_fb = memremap(fb_base, screen_info.lfb_size, MEMREMAP_WC);
+	efi_fb = memremap(fb_base, screen_info.lfb_size,
+			  fb_wb ? MEMREMAP_WB : MEMREMAP_WC);
 
 	return efi_fb ? 0 : -ENOMEM;
 }
@@ -53,9 +51,12 @@ late_initcall(efi_earlycon_unmap_fb);
 
 static __ref void *efi_earlycon_map(unsigned long start, unsigned long len)
 {
+	pgprot_t fb_prot;
+
 	if (efi_fb)
 		return efi_fb + start;
 
+	fb_prot = fb_wb ? PAGE_KERNEL : pgprot_writecombine(PAGE_KERNEL);
 	return early_memremap_prot(fb_base + start, len, pgprot_val(fb_prot));
 }
 
@@ -215,10 +216,7 @@ static int __init efi_earlycon_setup(struct earlycon_device *device,
 	if (screen_info.capabilities & VIDEO_CAPABILITY_64BIT_BASE)
 		fb_base |= (u64)screen_info.ext_lfb_base << 32;
 
-	if (opt && !strcmp(opt, "ram"))
-		fb_prot = PAGE_KERNEL;
-	else
-		fb_prot = pgprot_writecombine(PAGE_KERNEL);
+	fb_wb = opt && !strcmp(opt, "ram");
 
 	si = &screen_info;
 	xres = si->lfb_width;