ARM: i.MX6: boot: detect USB serial downloader more reliable

The problem with the BootROM is that the SRC registers are not set
accordingly in case of a failed primary boot. E.g. if the device is
configured to boot from an eMMC and the eMMC is empty or image is
corrupt, the BootROM goes into 'recovery boot mode' (reference manual
Figure 8-1) and the last possible recovery option is the serial
downloader. In such case the SRC registers still indicate that the
device was booted from an eMMC instead of serial-download.

This commit ports the U-Boot commit [1] with slightly adaptions suggested
by Ahmad to Barebox. Also we need to reorder the imx6_init() else we
reset the otg-controller to early.

[1] https://source.denx.de/u-boot/u-boot/-/commit/e203dcf23e9eabc2e4f3d0b079457cd1516f2081

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.barebox.org/20210812083651.3576-1-m.felsch@pengutronix.de
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

2 files changed, 34 insertions, 2 deletions

diff --git a/arch/arm/mach-imx/boot.c b/arch/arm/mach-imx/boot.c
index 2b66bbf71e..fcb3c10064 100644
--- a/arch/arm/mach-imx/boot.c
+++ b/arch/arm/mach-imx/boot.c
@@ -8,6 +8,7 @@
 #include <magicvar.h>
 
 #include <io.h>
+#include <mach/clock-imx6.h>
 #include <mach/generic.h>
 #include <mach/imx25-regs.h>
 #include <mach/imx27-regs.h>
@@ -23,6 +24,7 @@
 #include <mach/imx8mq.h>
 #include <mach/imx6.h>
 
+#include <soc/fsl/fsl_udc.h>
 
 static void
 imx_boot_save_loc(void (*get_boot_source)(enum bootsource *, int *))
@@ -397,6 +399,11 @@ static u32 imx6_get_src_boot_mode(void __iomem *src_base)
 	return readl(src_base + IMX6_SRC_SBMR1);
 }
 
+static inline bool imx6_usboh3_clk_active(void)
+{
+	return (readl(MXC_CCM_CCGR6) & 0x3) == 0x3;
+}
+
 void imx6_get_boot_source(enum bootsource *src, int *instance)
 {
 	void __iomem *src_base = IOMEM(MX6_SRC_BASE_ADDR);
@@ -410,6 +417,26 @@ void imx6_get_boot_source(enum bootsource *src, int *instance)
 
 	bootsrc = imx53_bootsource_internal(bootmode);
 
+	/*
+	 * imx6_bootsource_serial() can't detect cases where the boot ROM
+	 * decided to use the serial downloader as a fall back (primary
+	 * boot source failed).
+	 *
+	 * Infer that the boot ROM used the USB serial downloader by
+	 * checking whether both the UDC and the clock enabling access
+	 * to its MMIO region are currently active...
+	 * This assumes:
+	 * - On fresh boots, PBL doesn't itself start a stopped UDC
+	 * - In barebox proper, boot source is saved before the UDC driver
+	 *   may enable the UDC
+	 */
+
+	if (imx6_usboh3_clk_active() &&
+	    is_chipidea_udc_running(IOMEM(MX6_OTG_BASE_ADDR))) {
+		*src = BOOTSOURCE_SERIAL;
+		return;
+	}
+
 	if (imx6_bootsource_serial(sbmr2) ||
 	    imx6_bootsource_serial_forced(bootsrc)) {
 		*src = BOOTSOURCE_SERIAL;
diff --git a/arch/arm/mach-imx/imx6.c b/arch/arm/mach-imx/imx6.c
index 9ccb391384..3ee42fd966 100644
--- a/arch/arm/mach-imx/imx6.c
+++ b/arch/arm/mach-imx/imx6.c
@@ -43,6 +43,11 @@ static void imx6_init_lowlevel(void)
 	uint32_t periph_sel_2;
 	uint32_t reg;
 
+	/*
+	 * Before reset the controller imx6_boot_save_loc() must be called to
+	 * detect serial-downloader fall back boots. For further information
+	 * check the comment in imx6_get_boot_source().
+	 */
 	if ((readl(MXC_CCM_CCGR6) & 0x3))
 		imx_reset_otg_controller(IOMEM(MX6_OTG_BASE_ADDR));
 
@@ -205,10 +210,10 @@ int imx6_init(void)
 	void __iomem *src = IOMEM(MX6_SRC_BASE_ADDR);
 	u64 mx6_uid;
 
-	imx6_init_lowlevel();
-
 	imx6_boot_save_loc();
 
+	imx6_init_lowlevel();
+
 	mx6_silicon_revision = imx6_cpu_revision();
 	mx6_uid = imx6_uid();