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Date:   Thu, 13 Aug 2020 16:04:31 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-serial@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
        Jiri Slaby <jslaby@...e.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Jason Wessel <jason.wessel@...driver.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 4/5] serial: amba-pl011: Enable NMI aware uart port

On Thu, 13 Aug 2020 at 05:29, Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Tue, Jul 21, 2020 at 5:11 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> >
> > Allow serial device interrupt to be requested as an NMI during
> > initialization in polling mode. If the irqchip doesn't support serial
> > device interrupt as an NMI then fallback to it being as a normal IRQ.
> >
> > Currently this NMI aware uart port only supports NMI driven programmed
> > IO operation whereas DMA operation isn't supported.
> >
> > And while operating in NMI mode, RX always remains active irrespective
> > of whether corresponding TTY port is active or not. So we directly bail
> > out of startup, shutdown and rx_stop APIs if NMI mode is active.
> >
> > Also, get rid of modification to interrupts enable mask in pl011_hwinit()
> > as now we have a proper way to enable interrupts for NMI entry using
> > pl011_enable_interrupts().
> >
> > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > ---
> >  drivers/tty/serial/amba-pl011.c | 124 ++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 113 insertions(+), 11 deletions(-)
>
> Overall: I ran out of time to do a super full review, but presumably
> you're going to spin this series anyway and I'll look at it again
> then.  For now a few things I noticed below...
>

Sure and thanks for your review.

>
> > diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> > index 0983c5e..5df1c07 100644
> > --- a/drivers/tty/serial/amba-pl011.c
> > +++ b/drivers/tty/serial/amba-pl011.c
> > @@ -41,6 +41,8 @@
> >  #include <linux/sizes.h>
> >  #include <linux/io.h>
> >  #include <linux/acpi.h>
> > +#include <linux/irq.h>
> > +#include <linux/irqdesc.h>
> >
> >  #include "amba-pl011.h"
> >
> > @@ -347,6 +349,10 @@ static int pl011_fifo_to_tty(struct uart_amba_port *uap)
> >                 if (uart_handle_sysrq_char(&uap->port, ch & 255))
> >                         continue;
> >
> > +               if (uart_nmi_handle_char(&uap->port, ch, UART011_DR_OE, ch,
> > +                                        flag))
> > +                       continue;
> > +
> >                 uart_insert_char(&uap->port, ch, UART011_DR_OE, ch, flag);
> >         }
> >
> > @@ -1316,6 +1322,9 @@ static void pl011_stop_rx(struct uart_port *port)
> >         struct uart_amba_port *uap =
> >             container_of(port, struct uart_amba_port, port);
> >
> > +       if (uart_nmi_active(port))
> > +               return;
> > +
> >         uap->im &= ~(UART011_RXIM|UART011_RTIM|UART011_FEIM|
> >                      UART011_PEIM|UART011_BEIM|UART011_OEIM);
> >         pl011_write(uap->im, uap, REG_IMSC);
> > @@ -1604,13 +1613,6 @@ static int pl011_hwinit(struct uart_port *port)
> >                     UART011_FEIS | UART011_RTIS | UART011_RXIS,
> >                     uap, REG_ICR);
> >
> > -       /*
> > -        * Save interrupts enable mask, and enable RX interrupts in case if
> > -        * the interrupt is used for NMI entry.
> > -        */
> > -       uap->im = pl011_read(uap, REG_IMSC);
> > -       pl011_write(UART011_RTIM | UART011_RXIM, uap, REG_IMSC);
> > -
> >         if (dev_get_platdata(uap->port.dev)) {
> >                 struct amba_pl011_data *plat;
> >
> > @@ -1711,6 +1713,96 @@ static void pl011_put_poll_char(struct uart_port *port,
> >         pl011_write(ch, uap, REG_DR);
> >  }
> >
> > +static irqreturn_t pl011_nmi_int(int irq, void *dev_id)
> > +{
>
> I wish there was a better way to share code between this and
> pl011_int(), but I guess it'd be too ugly?  If nothing else it feels
> like you should do something to make it more obvious to anyone looking
> at them that they are sister functions and any change to one of them
> should be reflected in the other.  Maybe they should be logically next
> to each other?
>

Yes, I can make them sit logically next to each other.

>
> > +       struct uart_amba_port *uap = dev_id;
> > +       unsigned int status, pass_counter = AMBA_ISR_PASS_LIMIT;
> > +       int handled = 0;
> > +
> > +       status = pl011_read(uap, REG_MIS);
> > +       if (status) {
> > +               do {
> > +                       check_apply_cts_event_workaround(uap);
> > +
> > +                       pl011_write(status, uap, REG_ICR);
> > +
> > +                       if (status & (UART011_RTIS|UART011_RXIS)) {
> > +                               pl011_fifo_to_tty(uap);
> > +                               irq_work_queue(&uap->port.nmi_state.rx_work);
>
> It feels like it might be beneficial to not call irq_work_queue() in a
> loop.  It doesn't hurt but it feels like, at least, it's going to keep
> doing a bunch of atomic operations.  It's not like it'll cause the
> work to run any sooner because it has to run on the same CPU, right?
>

AFAIK, the loop here is about checking interrupt status if another
interrupt is raised while we are servicing the prior one. But I think
it would be an unlikely case here as we defer actual work and given
the slow serial transfer rate.

>
> > +                       }
> > +
> > +                       if (status & UART011_TXIS)
> > +                               irq_work_queue(&uap->port.nmi_state.tx_work);
>
> Here too...
>

Ditto.

>
> > +
> > +                       if (pass_counter-- == 0)
> > +                               break;
> > +
> > +                       status = pl011_read(uap, REG_MIS);
> > +               } while (status != 0);
> > +               handled = 1;
> > +       }
> > +
> > +       return IRQ_RETVAL(handled);
> > +}
> > +
> > +static int pl011_allocate_nmi(struct uart_amba_port *uap)
> > +{
> > +       int ret;
> > +
> > +       irq_set_status_flags(uap->port.irq, IRQ_NOAUTOEN);
> > +       ret = request_nmi(uap->port.irq, pl011_nmi_int, IRQF_PERCPU,
> > +                         "uart-pl011", uap);
> > +       if (ret) {
> > +               irq_clear_status_flags(uap->port.irq, IRQ_NOAUTOEN);
> > +               return ret;
> > +       }
> > +
> > +       enable_irq(uap->port.irq);
> > +
> > +       return ret;
> > +}
> > +
> > +static void pl011_tx_irq_callback(struct uart_port *port)
> > +{
> > +       struct uart_amba_port *uap =
> > +           container_of(port, struct uart_amba_port, port);
> > +
> > +       spin_lock(&port->lock);
> > +       pl011_tx_chars(uap, true);
> > +       spin_unlock(&port->lock);
> > +}
> > +
> > +static int pl011_poll_init(struct uart_port *port)
> > +{
> > +       struct uart_amba_port *uap =
> > +           container_of(port, struct uart_amba_port, port);
> > +       int retval;
> > +
> > +       retval = pl011_hwinit(port);
> > +       if (retval)
> > +               goto clk_dis;
>
> I don't think you want "goto clk_dis" here.
>

Yeah, will fix it to return here instead.

>
> > +
> > +       /* In case NMI isn't supported, fallback to normal interrupt mode */
> > +       retval = pl011_allocate_nmi(uap);
> > +       if (retval)
> > +               return 0;
> > +
> > +       retval = uart_nmi_state_init(port);
> > +       if (retval)
> > +               goto clk_dis;
>
> Wouldn't you also need to to somehow call free_nmi() in the error case?
>

Yes, will fix it.

>
> > +       port->nmi_state.tx_irq_callback = pl011_tx_irq_callback;
> > +       uart_set_nmi_active(port, true);
> > +
> > +       pl011_enable_interrupts(uap);
> > +
> > +       return 0;
> > +
> > + clk_dis:
> > +       clk_disable_unprepare(uap->clk);
> > +       return retval;
> > +}
> > +
> >  #endif /* CONFIG_CONSOLE_POLL */
> >
> >  static bool pl011_split_lcrh(const struct uart_amba_port *uap)
> > @@ -1736,8 +1828,6 @@ static void pl011_write_lcr_h(struct uart_amba_port *uap, unsigned int lcr_h)
> >
> >  static int pl011_allocate_irq(struct uart_amba_port *uap)
> >  {
> > -       pl011_write(uap->im, uap, REG_IMSC);
> > -
> >         return request_irq(uap->port.irq, pl011_int, IRQF_SHARED, "uart-pl011", uap);
> >  }
> >
> > @@ -1748,6 +1838,9 @@ static int pl011_startup(struct uart_port *port)
> >         unsigned int cr;
> >         int retval;
> >
> > +       if (uart_nmi_active(port))
> > +               return 0;
> > +
> >         retval = pl011_hwinit(port);
> >         if (retval)
> >                 goto clk_dis;
> > @@ -1790,6 +1883,9 @@ static int sbsa_uart_startup(struct uart_port *port)
> >                 container_of(port, struct uart_amba_port, port);
> >         int retval;
> >
> > +       if (uart_nmi_active(port))
> > +               return 0;
> > +
> >         retval = pl011_hwinit(port);
> >         if (retval)
> >                 return retval;
> > @@ -1859,6 +1955,9 @@ static void pl011_shutdown(struct uart_port *port)
> >         struct uart_amba_port *uap =
> >                 container_of(port, struct uart_amba_port, port);
> >
> > +       if (uart_nmi_active(port))
> > +               return;
> > +
> >         pl011_disable_interrupts(uap);
> >
> >         pl011_dma_shutdown(uap);
> > @@ -1891,6 +1990,9 @@ static void sbsa_uart_shutdown(struct uart_port *port)
> >         struct uart_amba_port *uap =
> >                 container_of(port, struct uart_amba_port, port);
> >
> > +       if (uart_nmi_active(port))
> > +               return;
> > +
> >         pl011_disable_interrupts(uap);
> >
> >         free_irq(uap->port.irq, uap);
> > @@ -2142,7 +2244,7 @@ static const struct uart_ops amba_pl011_pops = {
> >         .config_port    = pl011_config_port,
> >         .verify_port    = pl011_verify_port,
> >  #ifdef CONFIG_CONSOLE_POLL
> > -       .poll_init     = pl011_hwinit,
> > +       .poll_init     = pl011_poll_init,
>
> Do we need to add a "free" at this point?
>

Where do you envision its usage? As currently if we enable NMI once,
we would like it to be active throughout a boot cycle.

-Sumit

>
>
> >         .poll_get_char = pl011_get_poll_char,
> >         .poll_put_char = pl011_put_poll_char,
> >  #endif
> > @@ -2173,7 +2275,7 @@ static const struct uart_ops sbsa_uart_pops = {
> >         .config_port    = pl011_config_port,
> >         .verify_port    = pl011_verify_port,
> >  #ifdef CONFIG_CONSOLE_POLL
> > -       .poll_init     = pl011_hwinit,
> > +       .poll_init     = pl011_poll_init,
> >         .poll_get_char = pl011_get_poll_char,
> >         .poll_put_char = pl011_put_poll_char,
> >  #endif
> > --
> > 2.7.4
> >

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