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char/cadence_uart: Implement Tx flow control

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If the UART back-end blocks, buffer in the Tx FIFO to try again later.
This stops the IO-thread busy waiting on char back-ends (which causes
all sorts of performance problems).

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 4bea048b3ab38425701d82ccc1ab92545c26b79c.1388626249.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
This commit is contained in:
Peter Crosthwaite 2014-01-06 10:16:40 +00:00 committed by Peter Maydell
parent 1e5d8cacb7
commit 38acd64b1c
1 changed files with 29 additions and 2 deletions
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31
hw/char/cadence_uart.c
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@ -286,6 +286,34 @@ static void uart_write_rx_fifo(void *opaque, const uint8_t *buf, int size)
uart_update_status(s);
}
static gboolean cadence_uart_xmit(GIOChannel *chan, GIOCondition cond,
void *opaque)
{
UartState *s = opaque;
int ret;
/* instant drain the fifo when there's no back-end */
if (!s->chr) {
s->tx_count = 0;
}
if (!s->tx_count) {
return FALSE;
}
ret = qemu_chr_fe_write(s->chr, s->tx_fifo, s->tx_count);
s->tx_count -= ret;
memmove(s->tx_fifo, s->tx_fifo + ret, s->tx_count);
if (s->tx_count) {
int r = qemu_chr_fe_add_watch(s->chr, G_IO_OUT, cadence_uart_xmit, s);
assert(r);
}
uart_update_status(s);
return FALSE;
}
static void uart_write_tx_fifo(UartState *s, const uint8_t *buf, int size)
{
if ((s->r[R_CR] & UART_CR_TX_DIS) || !(s->r[R_CR] & UART_CR_TX_EN)) {
@ -306,8 +334,7 @@ static void uart_write_tx_fifo(UartState *s, const uint8_t *buf, int size)
memcpy(s->tx_fifo + s->tx_count, buf, size);
s->tx_count += size;
qemu_chr_fe_write_all(s->chr, s->tx_fifo, s->tx_count);
s->tx_count = 0;
cadence_uart_xmit(NULL, G_IO_OUT, s);
}
static void uart_receive(void *opaque, const uint8_t *buf, int size)