Raw printk support

Unless you are lucky enough to have an ICE for debugging hard issues involving out-of-band contexts, you might have to resort to basic printk-style debugging over a serial line. If so, do NOT rely on the regular printk() routine for this, this won’t work in most cases, potentially adding even more sources of lockups to the whole gloomy picture. Although the printk() machinery can be used from out-of-band context, the output is deferred until the root stage gets back in control, which means that:

  • you can’t reliably trace out-of-band code on the spot, deferred output issued from an out-of-band context, or from a section of code running with interrupts disabled in the CPU may appear after subsequent in-band messages under some circumstances, due to a buffering effect.

  • if the debug traces are sent at high pace (e.g. from an out-of-band IRQ handler every few hundreds of microseconds), the machine is likely to come to a stall due to the massive output the heavy printk() machinery would have to handle, leading to an apparent lockup.

The only sane option for printk-like debugging in demanding out-of-band context is using the raw_printk() routine for issuing raw debug messages to a serial console, so that you may get some sensible feedback for understanding what is going on with the execution flow. This feature should be enabled by turning on CONFIG_RAW_PRINTK, otherwise all output sent to raw_printk() is discarded.

Because a regular (in-band) serial console driver won’t be usable from out-of-band context, enabling raw printk support requires adapting the serial console driver your platform is using, by adding a raw write handler to the console description. Just like the regular write() handler, the raw output handler receives a console pointer, the character string to output and its length as parameters. This handler should send the characters to the UART as quickly as possible, with little to no preparation.

All output formatted by the generic raw_printk() routine is passed to the raw write handler of the current serial console driver if present. Calls to the raw output handler are serialized in raw_printk() by holding a hard spinlock, which means that interrupts are disabled in the CPU when running the handler.

A raw write handler is normally derived from the regular write handler for the same serial console device, skipping any in-band locking construct, only waiting for the bare minimum time for the output to drain in the UART since we want to keep interrupt latency low.

Adding RAW_PRINTK support to the AMBA PL011 serial driver

--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -2206,6 +2206,40 @@ static void pl011_console_putchar(struct uart_port *port, int ch)
 	pl011_write(ch, uap, REG_DR);
 }
 
+#ifdef CONFIG_RAW_PRINTK
+
+/*
+ * The uart clk stays on all along in the current implementation,
+ * despite what pl011_console_write() suggests, so for the time being,
+ * just emit the characters assuming the chip is clocked. If the clock
+ * ends up being turned off after writing, we may need to clk_enable()
+ * it at console setup, relying on the non-zero enable_count for
+ * keeping pl011_console_write() from disabling it.
+ */
+static void
+pl011_console_write_raw(struct console *co, const char *s, unsigned int count)
+{
+	struct uart_amba_port *uap = amba_ports[co->index];
+	unsigned int old_cr, new_cr, status;
+
+	old_cr = readw(uap->port.membase + UART011_CR);
+	new_cr = old_cr & ~UART011_CR_CTSEN;
+	new_cr |= UART01x_CR_UARTEN | UART011_CR_TXE;
+	writew(new_cr, uap->port.membase + UART011_CR);
+
+	while (count-- > 0) {
+		if (*s == '\n')
+			pl011_console_putchar(&uap->port, '\r');
+		pl011_console_putchar(&uap->port, *s++);
+	}
+	do
+		status = readw(uap->port.membase + UART01x_FR);
+	while (status & UART01x_FR_BUSY);
+	writew(old_cr, uap->port.membase + UART011_CR);
+}
+
+#endif  /* !CONFIG_RAW_PRINTK */
+
 static void
 pl011_console_write(struct console *co, const char *s, unsigned int count)
 {
@@ -2406,6 +2440,9 @@ static struct console amba_console = {
 	.device		= uart_console_device,
 	.setup		= pl011_console_setup,
 	.match		= pl011_console_match,
+#ifdef CONFIG_RAW_PRINTK
+	.write_raw	= pl011_console_write_raw,
+#endif
 	.flags		= CON_PRINTBUFFER | CON_ANYTIME,
 	.index		= -1,
 	.data		= &amba_reg,

ARM-specific raw console driver

The vanilla ARM kernel port already provides an UART-based raw output routine called printascii() when CONFIG_DEBUG_LL is enabled, provided the right debug UART channel is defined too (CONFIG_DEBUG_UART_xx).

When CONFIG_RAW_PRINTK and CONFIG_DEBUG_LL are both defined in the kernel configuration, the ARM implementation of Dovetail automatically registers a special console device for emitting debug output (see arch/arm/kernel/raw_printk.c), which redirects calls to its raw write handler by raw_printk() to printascii(). In other words, if CONFIG_DEBUG_LL already provides you with a functional debug output channel, you don’t need the active serial console driver to implement a raw write handler for enabling raw_printk(), the raw console device should handle raw_printk() requests just fine.

Enabling CONFIG_DEBUG_LL with a wrong UART debug channel is a common cause of lockup at boot. You do want to make sure that the proper CONFIG_DEBUG_UART_xx symbol matching your hardware is selected along with CONFIG_DEBUG_LL.