[U-Boot] [PATCH] Fix unreliable detection of DRAM size on Orange Pi 3

[megous at megous.com <megous@megous.com>]

From: Ondrej Jirman <megous@megous.com>

Orange Pi 3 has 2 GiB of DRAM, that sometime get misdetected
as 4 GiB, due to false negative result from mctl_mem_matches()
when detecting number of column address bits. This leads to
u-boot detecting more address bits than there are and the
boot process hangs shortly after.

In mctl_mem_matches() we need to wait for each write to finish,
separately. Without this, the check is not reliable for some
unknown reason, probably having to do with unpredictable memory
access ordering.

Patch was made with help from André Przywara, who noticed that
my original idea about detection failing due to read-back from
cache without involving DRAM was false, because data cache is
still of at the time of the DRAM size autodetection.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Cc: André Przywara <andre.przywara@arm.com>
---
 arch/arm/mach-sunxi/dram_helpers.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/mach-sunxi/dram_helpers.c b/arch/arm/mach-sunxi/dram_helpers.c
index 239ab421a8..6dba448638 100644
--- a/arch/arm/mach-sunxi/dram_helpers.c
+++ b/arch/arm/mach-sunxi/dram_helpers.c
@@ -30,6 +30,7 @@ bool mctl_mem_matches(u32 offset)
 {
 	/* Try to write different values to RAM at two addresses */
 	writel(0, CONFIG_SYS_SDRAM_BASE);
+	dsb();
 	writel(0xaa55aa55, (ulong)CONFIG_SYS_SDRAM_BASE + offset);
 	dsb();
 	/* Check if the same value is actually observed when reading back */
-- 
2.22.0