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

Ard Biesheuvel
2019-12-11T08:23:01Z

(+ Andy)

On Wed, 11 Dec 2019 at 00:24, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> 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
>