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ns8250: Check if flush via FCR succeeded

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The emulated UART in the Firecracker VMM (aka the implementation in the
rust-vmm/vm-superio project) includes FIFOs but does not implement the
FCR register, which is used by ns8250_flush to flush the FIFOs.

Check the LSR to see if there is still data in the FIFOs and call
ns8250_drain if necessary.

Discussed with:	emaste, imp, jrtc27
Sponsored by:	https://patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D36979
This commit is contained in:
Colin Percival 2022-10-13 11:01:03 -07:00
parent 782105f7c8
commit c4b68e7e53
1 changed files with 19 additions and 0 deletions
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19
sys/dev/uart/uart_dev_ns8250.c
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@ -210,6 +210,8 @@ static void
ns8250_flush(struct uart_bas *bas, int what)
{
uint8_t fcr;
uint8_t lsr;
int drain = 0;
fcr = FCR_ENABLE;
#ifdef CPU_XBURST
@ -221,6 +223,23 @@ ns8250_flush(struct uart_bas *bas, int what)
fcr |= FCR_RCV_RST;
uart_setreg(bas, REG_FCR, fcr);
uart_barrier(bas);
/*
* Detect and work around emulated UARTs which don't implement the
* FCR register; on these systems we need to drain the FIFO since
* the flush we request doesn't happen. One such system is the
* Firecracker VMM, aka. the rust-vmm/vm-superio emulation code:
* https://github.com/rust-vmm/vm-superio/issues/83
*/
lsr = uart_getreg(bas, REG_LSR);
if ((lsr & LSR_TEMT) && (what & UART_FLUSH_TRANSMITTER))
drain |= UART_DRAIN_TRANSMITTER;
if ((lsr & LSR_RXRDY) && (what & UART_FLUSH_RECEIVER))
drain |= UART_DRAIN_RECEIVER;
if (drain != 0) {
printf("ns8250: UART FCR is broken\n");
ns8250_drain(bas, drain);
}
}
static int