From ea27064d212bb340c1b9760a40ee89cfff0a20c9 Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benjamin.valentin@ml-pa.com>
Date: Mon, 24 Aug 2020 18:28:30 +0200
Subject: [PATCH 1/2] picolibc: make stdout buffering optional

---
 makefiles/pseudomodules.inc.mk           |  1 +
 sys/picolibc_syscalls_default/syscalls.c | 28 +++++++++++++++++++++---
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/makefiles/pseudomodules.inc.mk b/makefiles/pseudomodules.inc.mk
index 510e6fecd8..b444af4ef7 100644
--- a/makefiles/pseudomodules.inc.mk
+++ b/makefiles/pseudomodules.inc.mk
@@ -86,6 +86,7 @@ PSEUDOMODULES += newlib_gnu_source
 PSEUDOMODULES += newlib_nano
 PSEUDOMODULES += openthread
 PSEUDOMODULES += picolibc
+PSEUDOMODULES += picolibc_stdout_buffered
 PSEUDOMODULES += pktqueue
 PSEUDOMODULES += posix_headers
 PSEUDOMODULES += printf_float
diff --git a/sys/picolibc_syscalls_default/syscalls.c b/sys/picolibc_syscalls_default/syscalls.c
index 1782610d08..48b0b903e6 100644
--- a/sys/picolibc_syscalls_default/syscalls.c
+++ b/sys/picolibc_syscalls_default/syscalls.c
@@ -63,10 +63,12 @@ int kill(pid_t pid, int sig)
 
 #include "mutex.h"
 
-static mutex_t picolibc_put_mutex = MUTEX_INIT;
-
+#ifndef PICOLIBC_STDOUT_BUFSIZE
 #define PICOLIBC_STDOUT_BUFSIZE 64
+#endif
 
+#ifdef MODULE_PICOLIBC_STDOUT_BUFFERED
+static mutex_t picolibc_put_mutex = MUTEX_INIT;
 static char picolibc_stdout[PICOLIBC_STDOUT_BUFSIZE];
 static int picolibc_stdout_queued;
 
@@ -81,10 +83,14 @@ static void _picolibc_flush(void)
 static int picolibc_put(char c, FILE *file)
 {
     (void)file;
+
     mutex_lock(&picolibc_put_mutex);
     picolibc_stdout[picolibc_stdout_queued++] = c;
-    if (picolibc_stdout_queued == PICOLIBC_STDOUT_BUFSIZE || c == '\n')
+
+    if (picolibc_stdout_queued == PICOLIBC_STDOUT_BUFSIZE || c == '\n') {
         _picolibc_flush();
+    }
+
     mutex_unlock(&picolibc_put_mutex);
     return 1;
 }
@@ -98,6 +104,22 @@ static int picolibc_flush(FILE *file)
     return 0;
 }
 
+#else
+int picolibc_put(char c, FILE *file)
+{
+    (void)file;
+    stdio_write(&c, 1);
+    return 1;
+}
+
+static int picolibc_flush(FILE *file)
+{
+    (void)file;
+    return 0;
+}
+
+#endif
+
 static int picolibc_get(FILE *file)
 {
     (void)file;

From c94860d8fc6137e2d333c0c2eb098f41e2976e09 Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benpicco@googlemail.com>
Date: Tue, 1 Sep 2020 12:04:26 +0200
Subject: [PATCH 2/2] makefiles/stdio: enable stdout buffering for CDC ACM,
 ethos, semihosting

CDC ACM, ethos, Semihosting and SLIP all benefit from sending data out
in larger chunks and will benefit from stdout buffering.

`stdio_rtt` does have an internal TX buffer, so we don't need to do any
buffering on top.

With plain UART there was a slight advantage *without* buffering when
testing with `tests/periph_uart_nonblocking` (with the non-blocking feature
disabled).

Since the removal of the buffering saves us some RAM and ROM, disable it by
default there.

This will be different with DMA enabled UARTs.
---
 makefiles/stdio.inc.mk | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/makefiles/stdio.inc.mk b/makefiles/stdio.inc.mk
index 4c7749d2f9..5c1d3bff22 100644
--- a/makefiles/stdio.inc.mk
+++ b/makefiles/stdio.inc.mk
@@ -55,3 +55,10 @@ ifneq (,$(filter stdio_semihosting,$(USEMODULE)))
   USEMODULE += xtimer
   FEATURES_REQUIRED += cpu_core_cortexm
 endif
+
+# enable stdout buffering for modules that benefit from sending out buffers in larger chunks
+ifneq (,$(filter picolibc,$(USEMODULE)))
+  ifneq (,$(filter stdio_cdc_acm stdio_ethos slipdev_stdio stdio_semihosting,$(USEMODULE)))
+    USEMODULE += picolibc_stdout_buffered
+  endif
+endif