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    SubjectRe: [RFC PATCH v2 4/4] tty/serial8250: Make use of IRQ_HANDLED_MANY interface
    On Fri, 16 Feb 2024, Leonardo Bras wrote:

    > For every TX byte an IRQ is requested.
    > On threaded IRQs, the handler calls serial8250_tx_chars can send multiple
    > bytes, limited to it's queue size (tx_loadsz).

    Perhaps I'm missing something here but I don't understand what this tries
    to say.

    - 8250 driver gets TX empty IRQ
    - We write x bytes to FIFO
    - UART blasts those bits to wire, eventually emptying FIFO
    - We get the next TX empty IRQ

    What in this makes "for every TX byte an IRQ is requested" true? There's
    one IRQ only for every x bytes TX'ed as far as I can tell!?!

    --
    i.

    > When this happens, the handler return IRQ_HANDLED with reduces the
    > unhandled IRQ counter only by 1, even though many requests have been
    > handled at once.
    >
    > This causes the unhandled IRQ counter to go up until it reaches the maximum
    > and causes the registered IRQ to be disabled, thus breaking the serial
    > console.
    >
    > Make use of the newly introduced IRQ_HANDLED_MANY interface to return the
    > number of requests handled, so the unhandled IRQ counter can get decreased
    > accordingly.
    >
    > Signed-off-by: Leonardo Bras <leobras@redhat.com>
    > ---
    > include/linux/serial_8250.h | 2 +-
    > drivers/tty/serial/8250/8250_core.c | 13 ++++++++-----
    > drivers/tty/serial/8250/8250_port.c | 16 ++++++++++------
    > 3 files changed, 19 insertions(+), 12 deletions(-)
    >
    > diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
    > index ec46e3b49ee99..c9d4271b71d70 100644
    > --- a/include/linux/serial_8250.h
    > +++ b/include/linux/serial_8250.h
    > @@ -200,7 +200,7 @@ int fsl8250_handle_irq(struct uart_port *port);
    > int serial8250_handle_irq(struct uart_port *port, unsigned int iir);
    > u16 serial8250_rx_chars(struct uart_8250_port *up, u16 lsr);
    > void serial8250_read_char(struct uart_8250_port *up, u16 lsr);
    > -void serial8250_tx_chars(struct uart_8250_port *up);
    > +int serial8250_tx_chars(struct uart_8250_port *up);
    > unsigned int serial8250_modem_status(struct uart_8250_port *up);
    > void serial8250_init_port(struct uart_8250_port *up);
    > void serial8250_set_defaults(struct uart_8250_port *up);
    > diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
    > index ae637155fe7cd..2fab9102eec45 100644
    > --- a/drivers/tty/serial/8250/8250_core.c
    > +++ b/drivers/tty/serial/8250/8250_core.c
    > @@ -110,7 +110,7 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
    > {
    > struct irq_info *i = dev_id;
    > struct list_head *l, *end = NULL;
    > - int pass_counter = 0, handled = 0;
    > + int pass_counter = 0, handled_total = 0;
    >
    > pr_debug("%s(%d): start\n", __func__, irq);
    >
    > @@ -120,15 +120,18 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
    > do {
    > struct uart_8250_port *up;
    > struct uart_port *port;
    > + int handled;
    >
    > up = list_entry(l, struct uart_8250_port, list);
    > port = &up->port;
    >
    > - if (port->handle_irq(port)) {
    > - handled = 1;
    > + handled = port->handle_irq(port);
    > + if (handled) {
    > + handled_total += handled;
    > end = NULL;
    > - } else if (end == NULL)
    > + } else if (end == NULL) {
    > end = l;
    > + }
    >
    > l = l->next;
    >
    > @@ -140,7 +143,7 @@ static irqreturn_t serial8250_interrupt(int irq, void *dev_id)
    >
    > pr_debug("%s(%d): end\n", __func__, irq);
    >
    > - return IRQ_RETVAL(handled);
    > + return IRQ_RETVAL_MANY(handled_total);
    > }
    >
    > /*
    > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
    > index f799c34f1603c..74d53507a73d4 100644
    > --- a/drivers/tty/serial/8250/8250_port.c
    > +++ b/drivers/tty/serial/8250/8250_port.c
    > @@ -1802,7 +1802,7 @@ u16 serial8250_rx_chars(struct uart_8250_port *up, u16 lsr)
    > }
    > EXPORT_SYMBOL_GPL(serial8250_rx_chars);
    >
    > -void serial8250_tx_chars(struct uart_8250_port *up)
    > +int serial8250_tx_chars(struct uart_8250_port *up)
    > {
    > struct uart_port *port = &up->port;
    > struct circ_buf *xmit = &port->state->xmit;
    > @@ -1810,15 +1810,15 @@ void serial8250_tx_chars(struct uart_8250_port *up)
    >
    > if (port->x_char) {
    > uart_xchar_out(port, UART_TX);
    > - return;
    > + return 0;
    > }
    > if (uart_tx_stopped(port)) {
    > serial8250_stop_tx(port);
    > - return;
    > + return 0;
    > }
    > if (uart_circ_empty(xmit)) {
    > __stop_tx(up);
    > - return;
    > + return 0;
    > }
    >
    > count = up->tx_loadsz;
    > @@ -1858,6 +1858,9 @@ void serial8250_tx_chars(struct uart_8250_port *up)
    > */
    > if (uart_circ_empty(xmit) && !(up->capabilities & UART_CAP_RPM))
    > __stop_tx(up);
    > +
    > + /* Return number of chars sent */
    > + return up->tx_loadsz - count;
    > }
    > EXPORT_SYMBOL_GPL(serial8250_tx_chars);
    >
    > @@ -1923,6 +1926,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
    > bool skip_rx = false;
    > unsigned long flags;
    > u16 status;
    > + int handled = 0;
    >
    > if (iir & UART_IIR_NO_INT)
    > return 0;
    > @@ -1956,14 +1960,14 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
    > serial8250_modem_status(up);
    > if ((status & UART_LSR_THRE) && (up->ier & UART_IER_THRI)) {
    > if (!up->dma || up->dma->tx_err)
    > - serial8250_tx_chars(up);
    > + handled = serial8250_tx_chars(up);
    > else if (!up->dma->tx_running)
    > __stop_tx(up);
    > }
    >
    > uart_unlock_and_check_sysrq_irqrestore(port, flags);
    >
    > - return 1;
    > + return handled ? : 1;
    > }
    > EXPORT_SYMBOL_GPL(serial8250_handle_irq);
    >
    >


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