[RFC] Add initial support for FCR register #96
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In order to add support for controlling FIFO using FIFO control register we need to persist the state of FCR. Thus, add FCR to state of serial console. Signed-off-by: Jinank Jain <[email protected]>
Currently I am handling partial writes to FIFO control register which deals with clearing RX/TX queues. There are other bits which deals with disabling/enabling FIFO along with setting different ITL mode. I think those can be added a later point when the need arises for it. This should fix the behavior from FreeBSD's point of view. Signed-off-by: Jinank Jain <[email protected]>
jinankjain
requested review from
andreeaflorescu,
lauralt,
gsserge,
roypat and
JonathanWoollett-Light
as code owners
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@cperciva is this something that would help with the FreeBsd use case? |
| @@ -542,6 +546,13 @@ impl<T: Trigger, EV: SerialEvents, W: Write> Serial<T, EV, W> { | |||
| } | |||
| // We want to enable only the interrupts that are available for 16550A (and below). | |||
| IER_OFFSET => self.interrupt_enable = value & IER_UART_VALID_BITS, | |||
| FCR_OFFSET => { | |||
| if value & FCR_FIFO_RESET_RX != 0 || value & FCR_FIFO_RESET_TX != 0 { | |||
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I'm a bit confused by this. Resetting the RX buffer isn't the same thing as resetting the TX buffer.
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@andreeaflorescu We have a workaround in FreeBSD right now, but yes it would be good to have the FCR fixed so we don't need the workaround. |
Yap, I remember that you had a workaround, I was wondering if this would help with removing the workaround 😄 |
This particular workaround is fairly lightweight -- we check if flushing the FIFO resulted in the queue being empty, and if not we drain it byte by byte -- so we'll probably keep it around for a bit in case it helps with other systems. I've seen the printf triggered on some EC2 instances too, so (unless you can tell me that Nitro uses rust-vmm in its firmware?) it's probably going to be useful to keep it there anyway. |
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But if you mean "will this make the workaround path stop being used when FreeBSD boots on Firecracker", I think the answer is yes. :-) |
Summary of the PR
The goal of this pull request is to add enough FCR support to satisfy the requirement of FreeBSD. Thus, only adding support for clearing RX and TX queue while omitting other features such as Fifo enable/disable.
I would like to get the community feedback on:
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