mips: mt76xx: Implement new d-cache fix in last_stage_init()

With commit 06985289d452 ("watchdog: Implement generic watchdog_reset()
version") the init sequence has changed in arch_misc_init(), resulting
in a re-appearance of the d-cache issue on MT7688 boards (e.g. gardena).
When this happens, the first (or sometimes later ones as well) TFTP
command hangs and does not complete correctly. This leads to the
assumption that the d-cache is not in a clean state once the ethernet
driver is called (d-cache is used here for the buffers). The old work-
around with the cache flush somehow does not work any more now with
the new code change.

Unfortunately adding CONFIG_SYS_MALLOC_CLEAR_ON_INIT also did not fix
this issue. With v2019.07-rc3 it shows again. The time of accessing
the data seems to be very important here. It needs to be "very late"
in the boot process.

Testing has shown, that copying a 64KiB area in DDR at a very late
bootup time, directly before calling into the prompt, fixes this issue.
Flushing of the complete d-cache does not seem to necessary, as this
copy alone seems to fix this problem.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

2 files changed, 27 insertions, 1 deletions

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 5cb9bdf2ee7..e3e79455679 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -84,13 +84,13 @@ config ARCH_MTMIPS
 	select DM_SERIAL
 	imply DM_SPI
 	imply DM_SPI_FLASH
+	select LAST_STAGE_INIT
 	select MIPS_TUNE_24KC
 	select OF_CONTROL
 	select ROM_EXCEPTION_VECTORS
 	select SUPPORTS_CPU_MIPS32_R1
 	select SUPPORTS_CPU_MIPS32_R2
 	select SUPPORTS_LITTLE_ENDIAN
-	select SYS_MALLOC_CLEAR_ON_INIT
 	select SYSRESET
 
 config ARCH_JZ47XX
diff --git a/arch/mips/mach-mtmips/cpu.c b/arch/mips/mach-mtmips/cpu.c
index b0a6397d68f..7afc2c5940d 100644
--- a/arch/mips/mach-mtmips/cpu.c
+++ b/arch/mips/mach-mtmips/cpu.c
@@ -68,3 +68,29 @@ int print_cpuinfo(void)
 
 	return 0;
 }
+
+int last_stage_init(void)
+{
+	void *src, *dst;
+
+	src = malloc(SZ_64K);
+	dst = malloc(SZ_64K);
+	if (!src || !dst) {
+		printf("Can't allocate buffer for cache cleanup copy!\n");
+		return 0;
+	}
+
+	/*
+	 * It has been noticed, that sometimes the d-cache is not in a
+	 * "clean-state" when U-Boot is running on MT7688. This was
+	 * detected when using the ethernet driver (which uses d-cache)
+	 * and a TFTP command does not complete. Copying an area of 64KiB
+	 * in DDR at a very late bootup time in U-Boot, directly before
+	 * calling into the prompt, seems to fix this issue.
+	 */
+	memcpy(dst, src, SZ_64K);
+	free(src);
+	free(dst);
+
+	return 0;
+}