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serial: cpm_uat: reducing CPM serial latency

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Setting Rx FIFO size to 1 reduces reader process wakeup latency up to
20x for baud rates <= 9600. This patch sets the Rx FIFO size to 1 for
baud rates <= 9600 or if low latency has been requested for the tty.

Signed-off-by: Michael Trimarchi <michael@gandalf.sssup.it>
Signed-off-by: Bruno Morelli <bruno@evidence.eu.com>
Signed-off-by: Baurzhan Ismagulov <ibr@radix50.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
Baurzhan Ismagulov 2010-11-11 10:53:03 +01:00 committed by Greg Kroah-Hartman
parent 63295cb2ff
commit 5b04ec4ad8
1 changed files with 19 additions and 0 deletions
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19
drivers/serial/cpm_uart/cpm_uart_core.c
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@ -72,6 +72,8 @@ static void cpm_uart_initbd(struct uart_cpm_port *pinfo);
/**************************************************************/
#define HW_BUF_SPD_THRESHOLD 9600
/*
* Check, if transmit buffers are processed
*/
@ -503,6 +505,11 @@ static void cpm_uart_set_termios(struct uart_port *port,
pr_debug("CPM uart[%d]:set_termios\n", port->line);
baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk / 16);
if (baud <= HW_BUF_SPD_THRESHOLD ||
(pinfo->port.state && pinfo->port.state->port.tty->low_latency))
pinfo->rx_fifosize = 1;
else
pinfo->rx_fifosize = RX_BUF_SIZE;
/* Character length programmed into the mode register is the
* sum of: 1 start bit, number of data bits, 0 or 1 parity bit,
@ -594,6 +601,17 @@ static void cpm_uart_set_termios(struct uart_port *port,
*/
bits++;
if (IS_SMC(pinfo)) {
/*
* MRBLR can be changed while an SMC/SCC is operating only
* if it is done in a single bus cycle with one 16-bit move
* (not two 8-bit bus cycles back-to-back). This occurs when
* the cp shifts control to the next RxBD, so the change does
* not take effect immediately. To guarantee the exact RxBD
* on which the change occurs, change MRBLR only while the
* SMC/SCC receiver is disabled.
*/
out_be16(&pinfo->smcup->smc_mrblr, pinfo->rx_fifosize);
/* Set the mode register. We want to keep a copy of the
* enables, because we want to put them back if they were
* present.
@ -604,6 +622,7 @@ static void cpm_uart_set_termios(struct uart_port *port,
out_be16(&smcp->smc_smcmr, smcr_mk_clen(bits) | cval |
SMCMR_SM_UART | prev_mode);
} else {
out_be16(&pinfo->sccup->scc_genscc.scc_mrblr, pinfo->rx_fifosize);
out_be16(&sccp->scc_psmr, (sbits << 12) | scval);
}