On Wed, Jul 14, 2021 at 11:39:34PM +0300, Ramon Fried wrote: > On Wed, Jul 14, 2021 at 11:56 AM Stephan Gerhold <step...@gerhold.net> wrote: > > > > For some reason, the DragonBoard 410c aborts autoboot immediately if > > U-Boot is started without LK. It looks like it picks up a single broken > > character via serial and therefore believes a key was pressed to abort > > autoboot. > > > > After some debugging, it seems like adding some delay after pinctrl > > setup but before UART initialization fixes the issue. It's also worth > > mentioning that unlike when booting from LK, the pinctrl setup is > > actually necessary when booting U-Boot without LK since UART is broken > > if the pinctrl line is removed. > > > > I suspect that reconfiguring the pins might take some time to stabilize > > and if the UART controller is enabled too quickly it will pick up some > > random noise. Adding a few milliseconds of delay fixes the issue and > > shouldn't have any other negative side effects. > > > > 3ms seems to be the minimum delay required in my tests, use 5ms instead > > just to be sure. > > > > Signed-off-by: Stephan Gerhold <step...@gerhold.net> > > --- > > > > drivers/serial/serial_msm.c | 4 ++++ > > 1 file changed, 4 insertions(+) > > > > diff --git a/drivers/serial/serial_msm.c b/drivers/serial/serial_msm.c > > index d8dd5c1104..ac4d0824b9 100644 > > --- a/drivers/serial/serial_msm.c > > +++ b/drivers/serial/serial_msm.c > > @@ -18,6 +18,7 @@ > > #include <asm/global_data.h> > > #include <asm/io.h> > > #include <linux/compiler.h> > > +#include <linux/delay.h> > > #include <dm/pinctrl.h> > > > > /* Serial registers - this driver works in uartdm mode*/ > > @@ -193,6 +194,9 @@ static int msm_uart_clk_init(struct udevice *dev) > > > > static void uart_dm_init(struct msm_serial_data *priv) > > { > > + /* Delay initialization for a bit to let pins stabilize if > > necessary */ > > + mdelay(5); > > + > > writel(priv->clk_bit_rate, priv->base + UARTDM_CSR); > > writel(0x0, priv->base + UARTDM_MR1); > > writel(MSM_BOOT_UART_DM_8_N_1_MODE, priv->base + UARTDM_MR2); > > -- > > 2.32.0 > > > Eh...this is not a nice fix, perhaps just clearing the RX buffer after > initialization will work better ?
I don't like it much either but this is the best I came up with even after spending several hours trying to track down the problem. I also tried porting more register initialization from LK, probably even tried clearing the RX buffer again after initialization. The problem is: In my tests there was no way to get rid of the delay, because the character doesn't show up until a few milliseconds after initialization. What you suggest would only work like this: <initialize UART> mdelay(5); // Wait for the weird character to show up <clear RX buffer> Which is not much different (and in fact will take longer) than: mdelay(5); <initialize UART> So unfortunately I can't think of a way to get rid of the delay. :( I'm not sure if this is some strange problem of my DB410c, I'd actually be curious if the problem can also be reproduced on others. Stephan