From: Iris Johnson <iris@modwiz.com>

When the frontend device has no space for a read the fd is removed
from polling to allow time for the guest to read and clear the buffer.
Without the call to qemu_chr_fe_accept_input(), the poll will not be
broken out of when the guest has cleared the buffer causing significant
IO delays that get worse with smaller buffers.

Buglink: https://bugs.launchpad.net/qemu/+bug/1913341
Signed-off-by: Iris Johnson <iris@modwiz.com>
Message-id: 20210130184016.1787097-1-iris@modwiz.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 hw/char/exynos4210_uart.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/char/exynos4210_uart.c b/hw/char/exynos4210_uart.c
index 9b21d201b34..80d401a3795 100644
--- a/hw/char/exynos4210_uart.c
+++ b/hw/char/exynos4210_uart.c
@@ -519,6 +519,7 @@ static uint64_t exynos4210_uart_read(void *opaque, hwaddr offset,
             s->reg[I_(UTRSTAT)] &= ~UTRSTAT_Rx_BUFFER_DATA_READY;
             res = s->reg[I_(URXH)];
         }
+        qemu_chr_fe_accept_input(&s->chr);
         exynos4210_uart_update_dmabusy(s);
         trace_exynos_uart_read(s->channel, offset,
                                exynos4210_uart_regname(offset), res);
-- 
2.20.1