[PATCH v2] efi/earlycon: Remap entire framebuffer after page initialization

[Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>]

When the commit 69c1f396f25b ("efi/x86: Convert x86 EFI earlyprintk
into generic earlycon implementation") moved x86 specific EFI earlyprintk
implementation to shared location it also tweaked the behaviour. In particular
it dropped a trick with full framebuffer remapping after page initialization.
This led to two regressions:
1) very slow scrolling after page initialization;
2) kernel hang when keep_bootcon parameter is being provided.

Returning the trick back fixes #2 and mitigates, i.e. reduces the window when
slowness appears, #1 presumably due to eliminating heavy map()/unmap()
operations per each pixel line on the screen.

Fixes: 69c1f396f25b ("efi/x86: Convert x86 EFI earlyprintk into generic earlycon implementation")
Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: Alexander Graf <agraf@xxxxxxx>
Cc: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
---
v2:
- add __exitcall()
- swap type and attribute for __init / __exit functions as most used in the kernel
 drivers/firmware/efi/earlycon.c | 34 +++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/drivers/firmware/efi/earlycon.c b/drivers/firmware/efi/earlycon.c
index c9a0efca17b0..8c6a3c67b7f2 100644
--- a/drivers/firmware/efi/earlycon.c
+++ b/drivers/firmware/efi/earlycon.c
@@ -17,14 +17,48 @@ static const struct font_desc *font;
 static u32 efi_x, efi_y;
 static u64 fb_base;
 static pgprot_t fb_prot;
+static void *efi_fb;
+
+/*
+ * efi earlycon needs to use early_memremap() to map the framebuffer.
+ * But early_memremap() is not usable for 'earlycon=efifb keep_bootcon',
+ * memremap() should be used instead. memremap() will be available after
+ * paging_init() which is earlier than initcall callbacks. Thus adding this
+ * early initcall function early_efi_map_fb() to map the whole efi framebuffer.
+ */
+static int __init early_efi_map_fb(void)
+{
+	u32 size;
+
+	size = screen_info.lfb_size;
+	if (pgprot_val(fb_prot) == pgprot_val(PAGE_KERNEL))
+		efi_fb = memremap(fb_base, size, MEMREMAP_WB);
+	else
+		efi_fb = memremap(fb_base, size, MEMREMAP_WC);
+
+	return efi_fb ? 0 : -ENOMEM;
+}
+early_initcall(early_efi_map_fb);
+
+static void __exit early_efi_unmap_fb(void)
+{
+	memunmap(efi_fb);
+}
+__exitcall(early_efi_unmap_fb);
 
 static __ref void *efi_earlycon_map(unsigned long start, unsigned long len)
 {
+	if (efi_fb)
+		return efi_fb + start;
+
 	return early_memremap_prot(fb_base + start, len, pgprot_val(fb_prot));
 }
 
 static __ref void efi_earlycon_unmap(void *addr, unsigned long len)
 {
+	if (efi_fb)
+		return;
+
 	early_memunmap(addr, len);
 }
 
-- 
2.24.0