serial/uart/8250: Add tunable RX interrupt trigger I/F of FIFO buffers

Add tunable RX interrupt trigger I/F of FIFO buffers.

Serial devices are used as not only message communication devices but control or sending communication devices. For the latter uses, normally small data will be exchanged, so user applications want to receive data unit as soon as possible for real-time tendency. If we have a sensor which sends a 1 byte data each time and must control a device based on the sensor feedback, the RX interrupt should be triggered for each data.

According to HW specification of serial UART devices, RX interrupt trigger can be changed, but the trigger is hard-coded. For example, RX interrupt trigger in 16550A can be set to 1, 4, 8, or 14 bytes for HW, but current driver sets the trigger to only 8bytes.

This patch makes some devices change RX interrupt trigger from userland.

<How to use>
- Read current setting

 # cat /sys/class/tty/ttyS0/rx_trig_bytes
 8

- Write user setting

 # echo 1 > /sys/class/tty/ttyS0/rx_trig_bytes
 # cat /sys/class/tty/ttyS0/rx_trig_bytes
 1

<Support uart devices>
- 16550A and Tegra (1, 4, 8, or 14 bytes)
- 16650V2 (8, 16, 24, or 28 bytes)
- 16654 (8, 16, 56, or 60 bytes)
- 16750 (1, 16, 32, or 56 bytes)

Changed in V2:
- Use _IOW for TIOCSFIFORTRIG definition
- Pass the interrupt trigger value itself

Changes in V3:
- Change I/F from ioctl(2) to sysfs(rx_int_trig)

Changes in V4:
- Introduce fifo_bug flag in uart_8250_port structure
This is enabled only when parity is enabled and UART_BUG_PARITY is enabled for up->bugs. If this flag is enabled, user cannot set RX trigger.
- Return -EOPNOTSUPP when it does not support device at convert_fcr2val() and at convert_val2rxtrig()
- Set the nearest lower RX trigger when users input a meaningless value at convert_val2rxtrig()
- Check whether p->fcr is existing at serial8250_clear_and_reinit_fifos()
- Set fcr = up->fcr in the begging of serial8250_do_set_termios()

Changes in V5:
- Support Tegra, 16650V2, 16654, and 16750
- Store default FCR value to up->fcr when the port is first created
- Add rx_trig_byte[] in uart_config[] for each device and use rx_trig_byte[] in convert_fcr2val() and convert_val2rxtrig()

Changes in V5.1:
- Fix FCR_RX_TRIG_MAX_STATE definition

Changes in V6:
- Move FCR_RX_TRIG_* definition in 8250.h to include/uapi/linux/serial_reg.h, rename those to UART_FCR_R_TRIG_*, and use UART_FCR_TRIGGER_MASK to UART_FCR_R_TRIG_BITS()
- Change following function names:
convert_fcr2val() => fcr_get_rxtrig_bytes() convert_val2rxtrig() => bytes_to_fcr_rxtrig()
- Fix typo in serial8250_do_set_termios()
- Delete the verbose error message pr_info() in bytes_to_fcr_rxtrig()
- Rename *rx_int_trig/rx_trig* to *rxtrig* for several functions or variables (but UI remains rx_int_trig)
- Change the meaningless variable name 'val' to 'bytes' following functions:

    fcr_get_rxtrig_bytes(), bytes_to_fcr_rxtrig(), do_set_rxtrig(),
    do_serial8250_set_rxtrig(), and serial8250_set_attr_rxtrig()

- Use up->fcr in order to get rxtrig_bytes instead of rx_trig_raw in fcr_get_rxtrig_bytes()
- Use conf_type->rxtrig_bytes[0] instead of switch statement for support check in register_dev_spec_attr_grp()
- Delete the checking whether a user changed FCR or not when minimum buffer is needed in serial8250_do_set_termios()

Changes in V7:
- Add Documentation
- Change I/F name from rx_int_trig to rx_trig_bytes because the name rx_int_trig is hard to understand how users specify the value

---
 Documentation/ABI/testing/sysfs-tty |   16 +++
 drivers/tty/serial/8250/8250.h      |    2 
 drivers/tty/serial/8250/8250_core.c |  173 ++++++++++++++++++++++++++++++++---
 drivers/tty/serial/serial_core.c    |   18 ++--
 include/linux/serial_8250.h         |    2 
 include/linux/serial_core.h         |    4 +
 include/uapi/linux/serial_reg.h     |    5 +
 7 files changed, 198 insertions(+), 22 deletions(-)

index ad22fb0..9eb3c2b 100644
@@ -138,3 +138,19 @@ Description:
 
 		 These sysfs values expose the TIOCGSERIAL interface via
 		 sysfs rather than via ioctls.
+
+What:		/sys/class/tty/ttyS0/rx_trig_bytes
+Date:		May 2014
+Contact:	Yoshihiro YUNOMAE <[email protected]>
+Description:
+		 Shows current RX interrupt trigger bytes or sets the
+		 user specified value to change it for the FIFO buffer.
+		 Users can show or set this value regardless of opening the
+		 serial device file or not.
+
+		 The RX trigger can be set one of four kinds of values for UART
+		 serials. When users input a meaning less value to this I/F,
+		 the RX trigger is changed to the nearest lower value for the
+		 device specification. For example, when user sets 7bytes on
+		 16550A, which has 1/4/8/14 bytes trigger, the RX trigger is
+		 automatically changed to 4 bytes.

index 1ebf853..1b08c91 100644
@@ -12,6 +12,7 @@
  */
 
 #include <linux/serial_8250.h>
+#include <linux/serial_reg.h>
 #include <linux/dmaengine.h>
 
 struct uart_8250_dma {

@@ -60,6 +61,7 @@ struct serial8250_config {
 	unsigned short	fifo_size;
 	unsigned short	tx_loadsz;
 	unsigned char	fcr;
+	unsigned char	rxtrig_bytes[UART_FCR_R_TRIG_MAX_STATE];
 	unsigned int	flags;
 };
 

index b68550d..524cb49 100644
@@ -2552,15 +2552,16 @@ static struct attribute *tty_dev_attrs[] = {
 	NULL,
 	};
 
-static const struct attribute_group tty_dev_attr_group = {
+static struct attribute_group tty_dev_attr_group = {
 	.attrs = tty_dev_attrs,
 	};
 
-static const struct attribute_group *tty_dev_attr_groups[] = {
-	&tty_dev_attr_group,
-	NULL
-	};
-
+static void make_uport_attr_grps(struct uart_port *uport)
+{
+	uport->attr_grps[0] = &tty_dev_attr_group;
+	if (uport->dev_spec_attr_group)
+		uport->attr_grps[1] = uport->dev_spec_attr_group;
+}
 
 /**
  *	uart_add_one_port - attach a driver-defined port structure

@@ -2611,12 +2612,15 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
 
 	uart_configure_port(drv, state, uport);
 
+	make_uport_attr_grps(uport);
+
 	/*
 	 * Register the port whether it's detected or not.  This allows
 	 * setserial to be used to alter this port's parameters.
 	 */
 	tty_dev = tty_port_register_device_attr(port, drv->tty_driver,
-			uport->line, uport->dev, port, tty_dev_attr_groups);
+			uport->line, uport->dev, port,
+			(const struct attribute_group **)uport->attr_grps);
 	if (likely(!IS_ERR(tty_dev))) {
 		device_set_wakeup_capable(tty_dev, 1);
 	} else {

index af47a8a..8d0c215 100644
@@ -74,8 +74,10 @@ struct uart_8250_port {
 	struct list_head	list;		/* ports on this IRQ */
 	unsigned short		capabilities;	/* port capabilities */
 	unsigned short		bugs;		/* port bugs */
+	bool			fifo_bug;	/* min RX trigger if enabled */
 	unsigned int		tx_loadsz;	/* transmit fifo load size */
 	unsigned char		acr;
+	unsigned char		fcr;
 	unsigned char		ier;
 	unsigned char		lcr;
 	unsigned char		mcr;

index f729be9..7e3122f 100644
@@ -113,6 +113,8 @@ struct uart_icount {
 
 typedef unsigned int __bitwise__ upf_t;
 
+#define MAX_ATTR_GRPS		3
+
 struct uart_port {
 	spinlock_t		lock;			/* port lock */
 	unsigned long		iobase;			/* in/out[bwl] */

@@ -195,6 +197,8 @@ struct uart_port {
 	unsigned int		line;			/* port index */
 	resource_size_t		mapbase;		/* for ioremap */
 	struct device		*dev;			/* parent device */
+	struct attribute_group	*dev_spec_attr_group;	/* specific attribute */
+	struct attribute_group	*attr_grps[MAX_ATTR_GRPS];/* all attr. groups */
 	unsigned char		hub6;			/* this should be in the 8250 driver */
 	unsigned char		suspended;
 	unsigned char		irq_wake;

index e632260..936ecc5 100644
@@ -88,6 +88,11 @@
 #define UART_FCR6_T_TRIGGER_30	0x30 /* Mask for transmit trigger set at 30 */
 #define UART_FCR7_64BYTE	0x20 /* Go into 64 byte mode (TI16C750) */
 
+#define UART_FCR_R_TRIG_SHIFT		6
+#define UART_FCR_R_TRIG_BITS(x)		\
+	(((x) & UART_FCR_TRIGGER_MASK) >> UART_FCR_R_TRIG_SHIFT)
+#define UART_FCR_R_TRIG_MAX_STATE	4
+
 #define UART_LCR	3	/* Out: Line Control Register */
 /*
  * Note: if the word length is 5 bits (UART_LCR_WLEN5), then setting 

I'd prefer it if you could split the attr_grps logic out into a separate patch, the first one in a 2 patch series. I don't think it is quite correct what you are doing here, but given the other changes going on at the same time, it's hard to pick it apart.

So, can you split this up into 2 patches and resend please?

thanks,

greg k-h

Hi Greg,

Thank you for your review.

OK, I understood. If this patch will be separated as you say, other people will be able to understand easily when attr_grps will have been introduced.

Sure. I'll do it soon.

Thank you,
Yoshihiro YUNOMAE

Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: [email protected]