Re: ns8250: UART FCR is broken

From: Colin Percival <cperciva_at_freebsd.org>
Date: Wed, 26 Oct 2022 21:15:29 UTC 
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.

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid