Is ddb(4) over a USB-to-Serial port possible?

[Julian Elischer]

On 22/11/17 4:33 pm, Hans Petter Selasky wrote:
> On 11/22/17 08:40, Julian Elischer wrote:
>> On 22/11/17 3:08 pm, Farhan Khan wrote:
>>> Hi all
>>>
>>> I am reading here in section "10.5, On-Line Kernel Debugging User 
>>> Remote GDB" that you can run ddb(4) over a serial cable. I am 
>>> interested in setting this up.
>>>
>>> My development box does not have a serial port (however, I do see 
>>> a uart0 device in the kernel messages), so I attached a USB serial 
>>> port attached to a machine with a serial port. The device uses the 
>>> uslcom(4) driver and is listed as "Silicon Labs CP2102 USB to UART 
>>> Bridge Controller". When I enter ddb and enter "gdb" I receive the 
>>> error "The remote GDB backend could not be selected."
>>>
>>> The hint.uart.0.port in /boot/device.hints is still set to 0x3F8, 
>>> which corresponds to the aforementioned kernel messages. I am not 
>>> certain if this is the same port as the USB-based serial port. 
>>> Does this feature work over USB?
>>>
>>> Thanks,
>>> Farhan Khan
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>>>
>> generally speaking the answer is no, unless the device is emulated 
>> using SMI or some other mechanism that is independent of FreeBSD.
>>
>> uart0 is not the device on the USB but a hardware device at 3F8, 
>> regardless of whether it actually exists. Since the USB stack is 
>> not really functional when in the debugger, it can not be used to 
>> communicate with the debugger.
>>
>> I find a better answer is to fire up the test machine under bhyve, 
>> in which case its console is accessible through any stream session. 
>> (e.g. ssh)..
>>
>
> Hi,
>
> There is another setting for this:
>
> Try setting cons_baud and cons_unit below. USB serial supports 
> polling from the debugger after panic.
>
> hw.usb.ucom.cons_baud: 9600
> hw.usb.ucom.cons_subunit: 0
> hw.usb.ucom.cons_unit: -1
> hw.usb.ucom.debug: 0
> hw.usb.ucom.pps_mode: 0
>
wow.. I will need to try this when I have the appropriate setup.
is this in the kenv? is it in loader.conf  or sysctl.conf?

>
>
> --HPS
>