serial: cpm_uat: reducing CPM serial latency
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>
diff --git a/drivers/serial/cpm_uart/cpm_uart_core.c b/drivers/serial/cpm_uart/cpm_uart_core.c
index f2b8adc..8692ff9 100644
--- a/drivers/serial/cpm_uart/cpm_uart_core.c
+++ b/drivers/serial/cpm_uart/cpm_uart_core.c
@@ -72,6 +72,8 @@
/**************************************************************/
+#define HW_BUF_SPD_THRESHOLD 9600
+
/*
* Check, if transmit buffers are processed
*/
@@ -503,6 +505,11 @@
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 @@
*/
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 @@
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);
}