Debug socket introduction

Debug-socket

Debug-socket is proxy running on host to interact with target, the functionality of debug-socket in software development, as shown in the following picture.

Fig.1 Socket debug in SW development

According to the riscv-debug specification, if any kernel contains standard debug modules, simply follow the: “RISC-V external debugging support version xxx”. For standard debug module:

Fig.2 RISC-V debug overview

We choose to use a software-based debug socket instead of a standard debug module to implement the debug function, both of which have the same effect and can be used for debugging of the soc. For our debug-socket, see debug-socket connections overview.

Fig.3 Debug socket connection overview

Basically, the debug-socket implements basic functions required by gdb, with the help of hardware-provided breakpoint, watchpoint, trace buffer, and many other features.

Debug-socket supported command list

The full-stack debug tool development is under way, you can use the raw debug-socket interface to debug for now. Debug socket offers a big list of commands, however the following commands are the ones used most frequently:

Command	Usage	Function
b0	b0 addr	set a breakpoint at hw breakpoint 0 with addr
b1	b1 addr	set a breakpoint at hw breakpoint 1 with addr
b2	b2 addr	set a breakpoint at hw breakpoint 2 with addr
b3	b3 addr	set a breakpoint at hw breakpoint 3 with addr
d0	d0	disable breakpoint at hw breakpoint 0
d1	d1	disable breakpoint at hw breakpoint 1
d2	d2	disable breakpoint at hw breakpoint 2
d3	d3	disable breakpoint at hw breakpoint 3
wp (not supported for now)	wp	show watchpoint configure
bp	bp	show breakpoint configure
c	c	continue to run
stall	stall	make cpu stall
step N	step N	run next N instructions
gpr(not supported for now)	gpr	print all general purpose register
q	q	quit debug-socket
wb_pc	wb_pc	show current excute instruction pc
if_pc	if_pc	show current fetch instruction pc
minstret	minstret	show m-mode excuted instruction count
mstatus	mstatus	show mstatus value
mcause	mcause	show mcause value
mepc	mepc	show mepc value
mip	mip	show mip value
mie	mie	show mie value
hpmcounter_3~hpmcounter_10	hpmcounter_3 hpmcounter_4 hpmcounter_5 hpmcounter_6 hpmcounter_7 hpmcounter_8 hpmcounter_9 hpmcounter_10	show PMU counter values
dump	dump 0x00f00000 0x00f00080 rb/dma	dump content from start address to end address
read	read 0x00f00000 rb/dma	read content from specified address, rb for device register & dma for memory
write	write 0x00f00008 1 rb/dma	write value to specified address, rb for device register & dma for memory
uart1	uart1	show uart1 cfg
gpio	gpio	show gpio cfg
rtc	rtc	show rtc cfg
wdt	wdt	show wdt cfg
i2c0	i2c0	show i2c controller’s cfg

Classical debug process

When encounter some error in program, you can use debug-socket to debug the program:

1. type ‘minstret’ twice to analysis if the CPU is stall or not, if the two values of minstret is the same value, the CPU is stalled

: minstret
Do Read to Addr 0x1002b0 (minstret), Got Data 0x2409734f
Please enter command: (All Data in HEX no matter 0x is added or not)
: minstret
Do Read to Addr 0x1002b0 (minstret), Got Data 0x240aa177
Please enter command: (All Data in HEX no matter 0x is added or not)
:

2. if the CPU is not stalled, type ‘wb_pc’

: wb_pc
Do Read to Addr 0x100258 (wb_pc), Got Data 0x80009430
Please enter command: (All Data in HEX no matter 0x is added or not)
:

3. use ‘b0 addr’ to set a breakpoint, the program will stop when run into addr

: b0 80008e48
add breakpoint0, pc_addr = 0x80008e48
Please enter command: (All Data in HEX no matter 0x is added or not)
:

4. then, you can use ‘read addr dma’ to check some var value

: read 800102c4 dma
Do Read to Addr 0x800102c4, Got Data 0x6ffffffff
Please enter command: (All Data in HEX no matter 0x is added or not)
:

5. type ‘step N’ to run N instructions

: step 10
pc = 0x80000300
pc = 0x80000304
pc = 0x80000308
pc = 0x8000030c
pc = 0x80000310
pc = 0x80000314
pc = 0x80000318
pc = 0x8000031c
pc = 0x80000320
pc = 0x80000324
Please enter command: (All Data in HEX no matter 0x is added or not)
:

6. re-check some var

: read 800102c4 dma
Do Read to Addr 0x800102c4, Got Data 0x6ffffffff
Please enter command: (All Data in HEX no matter 0x is added or not)
:

7. continue to run until run into the breakpoint again

: c
Continue
Please enter command: (All Data in HEX no matter 0x is added or not)
:

8. disable breakpoint

: d0
del hw breakpoint1
Please enter command: (All Data in HEX no matter 0x is added or not)
:

9. continue

: c
Continue
Please enter command: (All Data in HEX no matter 0x is added or not)
: