From patchwork Mon Sep 24 06:19:03 2012
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Subject: Powerpc 8xx CPM_UART too many interrupts
From: Christophe Leroy <christophe.leroy@c-s.fr>
X-Patchwork-Id: 186298
Message-Id: <201209240619.q8O6J3Ft029443@localhost.localdomain>
To: Alan Cox <alan@linux.intel.com>, Vitaly Bordug <vitb@kernel.crashing.org>, 
 Marcelo Tosatti <marcelo@kvack.org>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
 linux-serial@vger.kernel.org
Date: Mon, 24 Sep 2012 08:19:03 +0200

Setting the fifo to only 1 byte generates one interrupt every 1ms at 9600 bauds.
This is too much. This patch reduces the threshold to speeds below 2400 bauds 
like in the 8250 UART driver.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>

--- linux-3.5-vanilla/drivers/tty/serial/cpm_uart/cpm_uart_core.c	2012-07-21 22:58:29.000000000 +0200
+++ linux-3.5/drivers/tty/serial/cpm_uart/cpm_uart_core.c	2012-08-09 17:38:37.000000000 +0200
@@ -71,7 +71,7 @@
 
 /**************************************************************/
 
-#define HW_BUF_SPD_THRESHOLD    9600
+#define HW_BUF_SPD_THRESHOLD    2400
 
 /*
  * Check, if transmit buffers are processed
@@ -505,7 +505,7 @@
 	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 ||
+	if (baud < HW_BUF_SPD_THRESHOLD ||
 	    (pinfo->port.state && pinfo->port.state->port.tty->low_latency))
 		pinfo->rx_fifosize = 1;
 	else