tcl/target/gd32vf103: work around broken ndmreset

On this chip, the ndmreset bit in the RISC-V debug module doesn't
trigger a system reset like it should. To work around this, add a custom
"reset-assert" handler in its config file that resets the system by
writing to memory-mapped registers.

I've tested this workaround on a Sipeed Longan Nano dev board with a
GD32VF103CBT6 chip. It works correctly for both "reset run" and "reset
halt" (halting at pc=0 for the latter).

I originally submitted[1] this workaround to the riscv-openocd fork of
OpenOCD. That fork's maintainers accepted it, but have not upstreamed it
like they have several other of my changes.

[1] https://github.com/riscv/riscv-openocd/pull/538

Change-Id: I7482990755b300fcbe4963c9a599d599bc02684d
Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6957
Tested-by: jenkins
Reviewed-by: zapb <dev@zapb.de>

tcl/target/gd32vf103.cfg

@@ -28,6 +28,12 @@ jtag newtap $_CHIPNAME cpu -irlen 5 -expected-id 0x1000563d
 set _TARGETNAME $_CHIPNAME.cpu
 target create $_TARGETNAME riscv -chain-position $_TARGETNAME
 
+proc default_mem_access {} {
+	riscv set_mem_access progbuf
+}
+
+default_mem_access
+
 $_TARGETNAME configure -work-area-phys 0x20000000 -work-area-size $_WORKAREASIZE -work-area-backup 0
 
 set _FLASHNAME $_CHIPNAME.flash
@@ -40,3 +46,74 @@ $_TARGETNAME configure -event reset-init {
 	# DBGMCU_CR |= DBG_WWDG_STOP | DBG_IWDG_STOP
 	mmw 0xE0042004 0x00000300 0
 }
+
+# On this chip, ndmreset (the debug module bit that triggers a software reset)
+# doesn't work. So for JTAG connections without an SRST, we need to trigger a
+# reset manually. This is an undocumented reset sequence that's used by the
+# JTAG flashing script in the vendor-supplied GD32VF103 PlatformIO plugin:
+#
+#   https://github.com/sipeed/platform-gd32v/commit/f9cbb44819bc05dd2010cc815c32be0486800cc2
+#
+$_TARGETNAME configure -event reset-assert {
+	set dmcontrol 		0x10
+	set dmcontrol_dmactive	[expr {1 << 0}]
+	set dmcontrol_ackhavereset [expr {1 << 28}]
+	set dmcontrol_haltreq	[expr {1 << 31}]
+
+	global _RESETMODE
+
+	# If hardware NRST signal is connected and configured (reset_config srst_only)
+	# the device has been recently reset in 'jtag arp_init-reset', therefore
+	# DM_DMSTATUS_ANYHAVERESET reads 1.
+	# The following 'halt' command checks this status bit
+	# and shows 'Hart 0 unexpectedly reset!' if set.
+	# Prevent this message by sending an acknowledge first.
+	set val [expr {$dmcontrol_dmactive | $dmcontrol_ackhavereset}]
+	riscv dmi_write $dmcontrol $val
+
+	# Halt the core so that we can write to memory. We do this first so
+	# that it doesn't clobber our dmcontrol configuration.
+	halt
+
+	# Set haltreq appropriately for the type of reset we're doing. This
+	# replicates what the generic RISC-V reset_assert() function would
+	# do if we weren't overriding it. The $_RESETMODE hack sucks, but
+	# it's the least invasive way to determine whether we need to halt.
+	#
+	# If we didn't override the generic handler, we'd actually still have
+	# to do this: the default handler sets ndmreset, which prevents memory
+	# access even though it doesn't actually trigger a reset on this chip.
+	# So we'd need to unset it here, which involves a write to dmcontrol,
+	# Since haltreq is write-only and there's no way to leave it unchanged,
+	# we'd have to figure out its proper value anyway.
+	set val $dmcontrol_dmactive
+	if {$_RESETMODE ne "run"} {
+		set val [expr {$val | $dmcontrol_haltreq}]
+	}
+	riscv dmi_write $dmcontrol $val
+
+	# Unlock 0xe0042008 so that the next write triggers a reset
+	mww 0xe004200c 0x4b5a6978
+
+	# We need to trigger the reset using abstract memory access, since
+	# progbuf access tries to read a status code out of a core register
+	# after the write happens, which fails when the core is in reset.
+	riscv set_mem_access abstract
+
+	# Go!
+	mww 0xe0042008 0x1
+
+	# Put the memory access mode back to what it was.
+	default_mem_access
+}
+
+# Capture the mode of a given reset so that we can use it later in the
+# reset-assert handler.
+proc init_reset { mode } {
+	global _RESETMODE
+	set _RESETMODE $mode
+
+	if {[using_jtag]} {
+		jtag arp_init-reset
+	}
+}