[PATCH] efi/earlycon: Fix write-combine mapping on x86

Arvind Sankar
2019-12-10T23:24:13Z

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.

Fixes: 69c1f396f25b ("efi/x86: Convert x86 EFI earlyprintk into generic earlycon implementation")
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
This applies on top of
efi/earlycon: Remap entire framebuffer after page initialization
https://git.kernel.org/tip/b418d660bb9798d2249ac6a46c844389ef50b6a5

drivers/firmware/efi/earlycon.c | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/drivers/firmware/efi/earlycon.c b/drivers/firmware/efi/earlycon.c
index ee5a6431fb9c..18f6a61f3e17 100644
--- a/drivers/firmware/efi/earlycon.c
+++ b/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);

pr_info("Mapped earlycon framebuffer\n");

@@ -58,9 +56,12 @@ 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));
}

@@ -220,10 +221,7 @@ 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;
--
2.23.0