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stdio_uart: add stdio_uart_onlcr option to convert LF -> CRLF on output #15619

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stdio_uart: add stdio_uart_onlcr option to convert LF -> CRLF on output #15619

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stdio_uart: add stdio_uart_onlcr option to convert LF -> CRLF on output 
Add USE_MODULE += "stdio_uart_onlcr" to enable it.
This is named after the "onlcr" stty flag, which does the same thing.
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@jimparis
jimparis committed Dec 16, 2020
commit 91640724b342a50678c3714fedd2df01de56c3c7
1 change: 1 addition & 0 deletions makefiles/pseudomodules.inc.mk
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Original file line number Diff line number Diff line change
Expand Up @@ -129,6 +129,7 @@ PSEUDOMODULES += soft_uart_modecfg
PSEUDOMODULES += stdin
PSEUDOMODULES += stdio_cdc_acm
PSEUDOMODULES += stdio_ethos
PSEUDOMODULES += stdio_uart_onlcr
PSEUDOMODULES += stdio_uart_rx
PSEUDOMODULES += stm32_eth
PSEUDOMODULES += stm32_eth_auto
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12 changes: 12 additions & 0 deletions sys/stdio_uart/stdio_uart.c
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Expand Up @@ -92,7 +92,19 @@ ssize_t stdio_write(const void* buffer, size_t len)
#ifdef MODULE_STDIO_ETHOS
ethos_send_frame(&ethos, (const uint8_t *)buffer, len, ETHOS_FRAME_TYPE_TEXT);
#else
#ifdef MODULE_STDIO_UART_ONLCR
const uint8_t *buf = (const uint8_t *)buffer;
const uint8_t cr = '\r';
size_t rem = len;
while (rem--) {
if (*buf == '\n')
uart_write(STDIO_UART_DEV, &cr, 1);
uart_write(STDIO_UART_DEV, buf, 1);
buf++;
}
#else
uart_write(STDIO_UART_DEV, (const uint8_t *)buffer, len);
#endif
#endif
return len;
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Suggested change
#ifdef MODULE_STDIO_UART_ONLCR
const uint8_t *buf = (const uint8_t *)buffer;
const uint8_t cr = '\r';
size_t rem = len;
while (rem--) {
if (*buf == '\n')
uart_write(STDIO_UART_DEV, &cr, 1);
uart_write(STDIO_UART_DEV, buf, 1);
buf++;
}
#else
uart_write(STDIO_UART_DEV, (const uint8_t *)buffer, len);
#endif
#endif
return len;
ssize_t result = len;
if (IS_USED(MODULE_STDIO_UART_CRLF)) {
static const uint8_t crlf[2] = { (uint8_t)'\r', (uint8_t)'\n' };
while (len) {
size_t chunk_len = 0;
const uint8_t *buf = buffer;
while ((chunk_len < len) && (buf[chunk_len] != (uint8_t)'\n')) {
chunk_len++;
}
uart_write(STDIO_UART_DEV, buf, chunk_len);
buf += chunk_len;
len -= chunk_len;
if (chunk_len) {
uart_write(STDIO_UART_DEV, crlf, sizeof(crlf));
buf++;
len--;
}
}
}
else {
uart_write(STDIO_UART_DEV, (const uint8_t *)buffer, len);
}
return result;

Three things:

  • The current implementation that writes one char at a time. However, some UART drivers do support DMA. In that case this would keep the CPU busy all the time, while without CRLF conversion the CPU would only need to setup the DMA transfer to the UART peripheral (and like attend to an IRQ once it is done). The suggested change will be more efficient, as it will result in two DMA transfers per line rather than in one DMA transfer per character
  • There is no need for preprocessor conditionals here, you can just use a C level if. That way, the compile tests will check syntactical correctness for both branches. And with any optimization level better than -O0 the compiler will eliminate the dead branch, so that there is no overhead to pay
  • I am personally not super happy with name stdio_uart_onlcr, as it is not immediately clear to me what onlcr stands for. How about something like stdio_uart_crlf instead?

}