[PATCH] x86/setup: move trim_snb_memory() later in setup_arch to fix boot hangs

[Mike Rapoport]

Commit a799c2bd29d1 ("x86/setup: Consolidate early memory reservations")
moved reservation of the memory inaccessible by Sandy Bride integrated
graphics very early and as the result on systems with such devices the
first 1M was reserved by trim_snb_memory() which prevented the allocation
of the real mode trampoline and made the boot hang very early.

Since the purpose of trim_snb_memory() is to prevent problematic pages ever
reaching the graphics device, it is safe to reserve these pages after
memblock allocations are possible.

Move trim_snb_memory later in boot so that it will be called after
reserve_real_mode() and make comments describing trim_snb_memory()
operation more elaborate.

Fixes: a799c2bd29d1 ("x86/setup: Consolidate early memory reservations")
Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
---
 arch/x86/kernel/setup.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 59e5e0903b0c..ccdcfb19df1e 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -633,11 +633,16 @@ static void __init trim_snb_memory(void)
 	printk(KERN_DEBUG "reserving inaccessible SNB gfx pages\n");
 
 	/*
-	 * Reserve all memory below the 1 MB mark that has not
-	 * already been reserved.
+	 * SandyBridge integrated graphic devices have a bug that prevents
+	 * them from accessing certain memory ranges, namely anything below
+	 * 1M and in the pages listed in the bad_pages.
+	 *
+	 * To avoid these pages being ever accessed by SNB gfx device
+	 * reserve all memory below the 1 MB mark and bad_pages that have
+	 * not already been reserved at boot time.
 	 */
 	memblock_reserve(0, 1<<20);
-	
+
 	for (i = 0; i < ARRAY_SIZE(bad_pages); i++) {
 		if (memblock_reserve(bad_pages[i], PAGE_SIZE))
 			printk(KERN_WARNING "failed to reserve 0x%08lx\n",
@@ -746,8 +751,6 @@ static void __init early_reserve_memory(void)
 
 	reserve_ibft_region();
 	reserve_bios_regions();
-
-	trim_snb_memory();
 }
 
 /*
@@ -1083,6 +1086,13 @@ void __init setup_arch(char **cmdline_p)
 
 	reserve_real_mode();
 
+	/*
+	 * Reserving memory causing GPU hangs on Sandy Bridge integrated
+	 * graphic devices should be done after we allocated memory under
+	 * 1M for the real mode trampoline
+	 */
+	trim_snb_memory();
+
 	init_mem_mapping();
 
 	idt_setup_early_pf();
-- 
2.28.0

-- 
Sincerely yours,
Mike.