serial: Add semihosting driver
This adds a serial driver which uses semihosting calls to read and write
to the host's console. For convenience, if CONFIG_DM_SERIAL is enabled,
we will instantiate a serial driver. This allows users to enable this
driver (which has no physical device) without modifying their device
trees or board files. We also implement a non-DM driver for SPL, or for
much faster output in U-Boot proper.
There are three ways to print to the console:
Method Baud
================== =====
smh_putc in a loop 170
smh_puts 1600
smh_write with :tt 20000
================== =====
These speeds were measured using a 175 character message with a J-Link
adapter. For reference, U-Boot typically prints around 2700 characters
during boot on this board. There are two major factors affecting the
speed of these functions. First, each breakpoint incurs a delay. Second,
each debugger memory transaction incurs a delay. smh_putc has a
breakpoint and memory transaction for every character. smh_puts has one
breakpoint, but still has to use a transaction for every character. This
is because we don't know the length up front, so OpenOCD has to check if
each character is nul. smh_write has only one breakpoint and one memory
transfer.
DM serial drivers can only implement a putc interface, so we are stuck
with the slowest API. Non-DM drivers can implement puts, which is vastly
more efficient. When the driver starts up, we try to open :tt. Since
this is an extension, this may fail. If it does, we fall back to
smh_puts. We don't check :semihosting-features, since there are
nonconforming implementations (OpenOCD) which don't implement it (but
*do* implement :tt).
Some semihosting implementations (QEMU) don't handle READC properly. To
work around this, we try to use open/read (much like for stdin) if
possible.
There is no non-blocking I/O available, so we don't implement pending.
This will cause __serial_tstc to always return true. If
CONFIG_SERIAL_RX_BUFFER is enabled, _serial_tstc will try and read
characters forever. To avoid this, we depend on this config being
disabled.
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 610f806..a07fab2 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -399,6 +399,15 @@
start up driver model. The driver will be available until the real
driver model serial is running.
+config DEBUG_UART_SEMIHOSTING
+ bool "semihosting"
+ depends on SEMIHOSTING_SERIAL
+ help
+ Select this to enable the debug UART using the semihosting driver.
+ This provides basic serial output from the console without needing to
+ start up driver model. The driver will be available until the real
+ driver model serial is running.
+
config DEBUG_UART_SIFIVE
bool "SiFive UART"
depends on SIFIVE_SERIAL
@@ -782,6 +791,19 @@
on systems with RCar or SH SoCs, say Y to this option. If unsure,
say N.
+config SEMIHOSTING_SERIAL
+ bool "Semihosting UART support"
+ depends on SEMIHOSTING && !SERIAL_RX_BUFFER
+ help
+ Select this to enable a serial UART using semihosting. Special halt
+ instructions will be issued which an external debugger (such as a
+ JTAG emulator) may interpret. The debugger will display U-Boot's
+ console output on the host system.
+
+ Enable this option only if you are using a debugger which supports
+ semihosting. If you are not using a debugger, this driver will halt
+ the boot.
+
config UNIPHIER_SERIAL
bool "Support for UniPhier on-chip UART"
depends on ARCH_UNIPHIER