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Subject[RFC 1/2] serial/8250.c: Change to singly linked handler chain.
I posted this to linux-serial yesterday, but it has attracted no comment
yet, so I'm submitting it to a wider audience.

This is a prerequisite to the interesting patch 2/2. This boots and
runs, but only on a single-port interrupt, so that's not very good
testing. It's out here for style comments.

This just converts the list of struct uart_8250_port attached to a single
IRQ from doubly linked to singly. It should have no behavior differences.


---
drivers/serial/8250.c | 93 +++++++++++++++++++++++++++----------------------
1 files changed, 51 insertions(+), 42 deletions(-)
diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 18caa95..7f66335 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -116,7 +116,7 @@ static unsigned int probe_rsa_count;
struct uart_8250_port {
struct uart_port port;
struct timer_list timer; /* "no irq" timer */
- struct list_head list; /* ports on this IRQ */
+ struct uart_8250_port *next; /* ports on this IRQ */
unsigned short capabilities; /* port capabilities */
unsigned short bugs; /* port bugs */
unsigned int tx_loadsz; /* transmit fifo load size */
@@ -146,7 +146,7 @@ struct uart_8250_port {

struct irq_info {
spinlock_t lock;
- struct list_head *head;
+ struct uart_8250_port *head;
};

static struct irq_info irq_lists[NR_IRQS];
@@ -1458,23 +1458,39 @@ serial8250_handle_port(struct uart_8250_port *up)
*
* This means we need to loop through all ports. checking that they
* don't have an interrupt pending.
+ *
+ * Fencepost consideration: serial8250_handle_port only polls each
+ * interrupt source once; it does not itself loop until the port
+ * has deasserted the interrupt line. Therefore, it is necessary to
+ * loop here until each port has had its IIR read in the idle state.
+ * "end" points to the beginning of the most recent run of idle ports.
+ * It is NULL if the current port is not idle. The loop ends when we
+ * are about to check end again.
*/
static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
{
struct irq_info *i = dev_id;
- struct list_head *l, *end = NULL;
+ struct uart_8250_port *up, *end = NULL;
int pass_counter = 0, handled = 0;

DEBUG_INTR("serial8250_interrupt(%d)...", irq);

spin_lock(&i->lock);

- l = i->head;
+ up = i->head;
do {
- struct uart_8250_port *up;
unsigned int iir;

- up = list_entry(l, struct uart_8250_port, list);
+ if (up == NULL) {
+ if (pass_counter++ > PASS_LIMIT) {
+ /* If we hit this, we're dead. */
+ printk(KERN_ERR "serial8250: too much work for "
+ "irq%d\n", irq);
+ break;
+ }
+ up = i->head;
+ continue;
+ }

iir = serial_in(up, UART_IIR);
if (!(iir & UART_IIR_NO_INT)) {
@@ -1486,9 +1505,10 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
} else if (up->port.iotype == UPIO_DWAPB &&
(iir & UART_IIR_BUSY) == UART_IIR_BUSY) {
/* The DesignWare APB UART has an Busy Detect (0x07)
- * interrupt meaning an LCR write attempt occured while the
- * UART was busy. The interrupt must be cleared by reading
- * the UART status register (USR) and the LCR re-written. */
+ * interrupt meaning an LCR write attempt occured
+ * while the UART was busy. The interrupt must
+ * be cleared by reading the UART status register
+ * (USR) and re-writing the LCR. */
unsigned int status;
status = *(volatile u32 *)up->port.private_data;
serial_out(up, UART_LCR, up->lcr);
@@ -1497,17 +1517,10 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)

end = NULL;
} else if (end == NULL)
- end = l;
+ end = up;

- l = l->next;
-
- if (l == i->head && pass_counter++ > PASS_LIMIT) {
- /* If we hit this, we're dead. */
- printk(KERN_ERR "serial8250: too much work for "
- "irq%d\n", irq);
- break;
- }
- } while (l != end);
+ up = up->next;
+ } while (up != end);

spin_unlock(&i->lock);

@@ -1525,39 +1538,35 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
*/
static void serial_do_unlink(struct irq_info *i, struct uart_8250_port *up)
{
+ struct uart_8250_port *p, **pptr = &i->head;
+
spin_lock_irq(&i->lock);

- if (!list_empty(i->head)) {
- if (i->head == &up->list)
- i->head = i->head->next;
- list_del(&up->list);
- } else {
- BUG_ON(i->head != &up->list);
- i->head = NULL;
+ while ((p = *pptr) != up) {
+ BUG_ON(p == NULL);
+ pptr = &p->next;
}

+ *pptr = up->next;
+
spin_unlock_irq(&i->lock);
}

static int serial_link_irq_chain(struct uart_8250_port *up)
{
struct irq_info *i = irq_lists + up->port.irq;
- int ret, irq_flags = up->port.flags & UPF_SHARE_IRQ ? IRQF_SHARED : 0;
+ int ret = 0;
+ struct uart_8250_port *head;

spin_lock_irq(&i->lock);
+ up->next = head = i->head;
+ i->head = up;
+ spin_unlock_irq(&i->lock);

- if (i->head) {
- list_add(&up->list, i->head);
- spin_unlock_irq(&i->lock);
-
- ret = 0;
- } else {
- INIT_LIST_HEAD(&up->list);
- i->head = &up->list;
- spin_unlock_irq(&i->lock);
-
+ if (!head) {
+ int irq_flag = up->port.flags & UPF_SHARE_IRQ ? IRQF_SHARED : 0;
ret = request_irq(up->port.irq, serial8250_interrupt,
- irq_flags, "serial", i);
+ irq_flag, "serial", i);
if (ret < 0)
serial_do_unlink(i, up);
}
@@ -1570,8 +1579,8 @@ static void serial_unlink_irq_chain(struct uart_8250_port *up)
struct irq_info *i = irq_lists + up->port.irq;

BUG_ON(i->head == NULL);
-
- if (list_empty(i->head))
+ serial_do_unlink(i, up);
+ if (i->head == NULL);
free_irq(up->port.irq, i);

serial_do_unlink(i, up);
--
1.5.6.5

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