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

Andy Shevchenko
2019-12-11T11:04:38Z

On Wed, Dec 11, 2019 at 08:22:56AM +0000, Ard Biesheuvel wrote:
> (+ 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>

Make sense.
One comment below.

> > - 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);

I would really like to keep the style with if-else.

--
With Best Regards,
Andy Shevchenko