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>

1 files changed, 19 insertions, 0 deletions

diff --git a/drivers/serial/cpm_uart/cpm_uart_core.c b/drivers/serial/cpm_uart/cpm_uart_core.c
index f2b8adcc6c92..8692ff98fc07 100644
--- a/drivers/serial/cpm_uart/cpm_uart_core.c
+++ b/drivers/serial/cpm_uart/cpm_uart_core.c
@@ -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);
         }