fix(msm8916): add timeout for crash console TX flush

Resetting the UART DM controller while there are still remaining
characters in the FIFO often results in corruption on the UART receiver
side. To avoid this the msm8916 crash console implementation tries to
wait until the TX FIFO is empty.

Unfortunately this might spin forever if the transmitter was disabled
before it has fully finished transmitting. In this case the TXEMT bit
console_uartdm_core_flush is waiting for will never get set.

There seems to be no good way to detect if the transmitter is actually
enabled via the status registers. However, the TX FIFO is fairly small
and should not take too long to get flushed, so fix this by simply
limiting the amount of iterations with a short timeout.

Move the code to console_uartdm_core_init to ensure that this always
happens before resetting the transmitter (also during initialization).

Change-Id: I5bb43cb0b6c029bcd15e253d60d36c0b310e108b
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>

diff --git a/plat/qti/msm8916/aarch64/msm8916_helpers.S b/plat/qti/msm8916/aarch64/msm8916_helpers.S
index dad9968ad365c0c7e5fee398bfa574aa46106550..528c5a42ef747443e95a18d20b155b47aef7c443 100644 (file)
--- a/plat/qti/msm8916/aarch64/msm8916_helpers.S
+++ b/plat/qti/msm8916/aarch64/msm8916_helpers.S
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, Stephan Gerhold <stephan@gerhold.net>
+ * Copyright (c) 2021-2023, Stephan Gerhold <stephan@gerhold.net>
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -31,17 +31,6 @@
         */
 func plat_crash_console_init
        mov     x1, #BLSP_UART2_BASE
-
-       /*
-        * If the non-secure world has been actively using the UART there might
-        * be still some characters left to be sent in the FIFO. In that case,
-        * resetting the transmitter too early might cause all output to become
-        * corrupted. To avoid that, try to flush (wait until FIFO empty) first.
-        */
-       mov     x4, lr
-       bl      console_uartdm_core_flush
-       mov     lr, x4
-
        mov     x0, #1
        b       console_uartdm_core_init
 endfunc plat_crash_console_init

diff --git a/plat/qti/msm8916/aarch64/uartdm_console.S b/plat/qti/msm8916/aarch64/uartdm_console.S
index e142179399553f310754944f60af6271be8115b9..6c65daf04820e44b2ebc4bf3d164b1d9e35f0595 100644 (file)
--- a/plat/qti/msm8916/aarch64/uartdm_console.S
+++ b/plat/qti/msm8916/aarch64/uartdm_console.S
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2022, Stephan Gerhold <stephan@gerhold.net>
+ * Copyright (c) 2021-2023, Stephan Gerhold <stephan@gerhold.net>
  *
  * Based on aarch64/skeleton_console.S:
  * Copyright (c) 2015-2020, ARM Limited and Contributors. All rights reserved.
@@ -65,7 +65,21 @@ endfunc console_uartdm_register
         * -----------------------------------------------------------
         */
 func console_uartdm_core_init
-       /* Reset receiver */
+       /*
+        * Try to flush remaining characters from the TX FIFO before resetting
+        * the transmitter. Unfortunately there is no good way to check if
+        * the transmitter is actually enabled (and will finish eventually),
+        * so use a timeout to avoid looping forever.
+        */
+       mov     w2, #65536
+1:
+       ldr     w3, [x1, #UART_DM_SR]
+       tbnz    w3, #UART_DM_SR_TXEMT_BIT, 2f
+       subs    w2, w2, #1
+       b.ne    1b
+       /* Timeout */
+
+2:     /* Reset receiver */
        mov     w3, #UART_DM_CR_RESET_RX
        str     w3, [x1, #UART_DM_CR]