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

[Hans Petter Selasky]

On 11/23/17 16:59, Julian Elischer wrote:
> 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?
> 

Hi,

/boot/loader.conf

or

kenv

or

sysctl

Before attaching the device.

--HPS