Re: ns8250: UART FCR is broken

On 2022-10-26 at 17:15 EDT, Colin Percival <cperciva@freebsd.org> wrote:

>On 10/26/22 13:48, Ed Maste wrote:
>>On Mon, 24 Oct 2022 at 22:11, void <void@f-m.fm> wrote:
>>>this started appearing in dmesg
>>>
>>>ns8250: UART FCR is broken
>>>ns8250: UART FCR is broken
>>
>>This message was added as part of Colin's work to support FreeBSD in 
>>the Firecracker VMM 
>>https://cgit.freebsd.org/src/commit/?id=c4b68e7e53bb352be3fa16995b99764c03097e66
>>
>>In this case it indicates that bhyve has the same bug/missing 
>>functionality as Firecracker -- it doesn't implement the FCR_XMT_RST 
>>or FCR_RCV_RST bits. You can safely ignore the message, and it will 
>>disappear once someone adds the required support to bhyve. We should 
>>probably also have the kernel emit the message only once. I've CC'd 
>>Colin for comment.
>
>Indeed, looking at usr.sbin/bhyve/uart_emul.c it looks like FCR_XMT_RST is 
>not emulated.  This is different from Firecracker, which doesn't emulate 
>either anything from the FCR and where I was seeing the receive side not 
>being flushed, but I'm glad my warning was able to flag a bug. :-)
>
>If "void" is comfortable with kernel hacking, it would be great to 
>confirm >that the warning is indeed coming from the transmit side not 
>being flushed; a printf("drain = %d\n", drain); would be sufficient.
>
>And yes, only emitting this warning once per device (or once per boot?) 
>would probably be good.

I got the same message on real hardware, no virtualization involved as 
either host or guest. Is that expected at all?

Relevant lines from /var/run/dmesg.boot:

ns8250: UART FCR is broken
ns8250: UART FCR is broken
uart0: <16550 or compatible> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0
uart0: console (115200,n,8,1)
ns8250: UART FCR is broken
ns8250: UART FCR is broken
uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 on acpi0
ns8250: UART FCR is broken

Thanks,
Matteo