Re: [PATCH 2/2] tty: serial: OMAP: transmit FIFO threshold interrupts don't wake the chip

[Govindraj <govindraj.ti@gmail.com>]

On Sat, Jan 21, 2012 at 12:57 PM, Paul Walmsley <paul@pwsan.com> wrote:
> It seems that when the transmit FIFO threshold is reached on OMAP
> UARTs, it does not result in a PRCM wakeup.  This appears to be a
> silicon bug.  This means that if the MPU powerdomain is in a low-power
> state, the MPU will not be awakened to refill the FIFO until the next
> interrupt from another device.
>
> The best solution, at least for the short term, would be for the OMAP
> serial driver to call a OMAP subarchitecture function to prevent the
> MPU powerdomain from entering a low power state while the FIFO has
> data to transmit.  However, we no longer have a clean way to do this,
> since patches that add platform_data function pointers have been
> deprecated by the OMAP maintainer.  So we attempt to work around this
> as well.  The workarounds depend on the setting of CONFIG_CPU_IDLE.
>
> When CONFIG_CPU_IDLE=n, the driver will now only transmit one byte at
> a time.  This causes the transmit FIFO threshold interrupt to stay
> active until there is no more data to be sent.  Thus, the MPU
> powerdomain stays on during transmits.  Aside from that energy
> consumption penalty, each transmitted byte results in a huge number of
> UART interrupts -- about five per byte.  This wastes CPU time and is
> quite inefficient, but is probably the most expedient workaround in
> this case.
>
> When CONFIG_CPU_IDLE=y, there is a slightly more direct workaround:
> the PM QoS constraint can be abused to keep the MPU powerdomain on.
> This results in a normal number of interrupts, but, similar to the
> above workaround, wastes power by preventing the MPU from entering
> WFI.
>
> Future patches are planned for the 3.4 merge window to implement more
> efficient, but also more disruptive, workarounds to these problems.
>

With these two patches number of interrupts seems to increase by 24x
after boot up.

[...]

/ # cat /proc/interrupts | grep "UART2"
 74:       3902      INTC  OMAP UART2
/ #
[...]

without these two patches + cpu_idle enabled

[...]

/ #
/ # cat /proc/interrupts | grep "UART2"
 74:        158      INTC  OMAP UART2
/ #

[...]


I am using beagle xm and 3.3rc1

Looks like there are far two many uart irqs which
keeps the mpu busy and thus preventing uart sluggishness.

--
Thanks,
Govindraj.R


> DMA operation is unaffected by this patch.
>
> Signed-off-by: Paul Walmsley <paul@pwsan.com>
> Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
> Cc: Govindraj Raja <govindraj.r@ti.com>
> Cc: Kevin Hilman <khilman@ti.com>
> ---
>  arch/arm/plat-omap/include/plat/omap-serial.h |    1
>  drivers/tty/serial/omap-serial.c              |   51 +++++++++++++++++++++++++
>  2 files changed, 51 insertions(+), 1 deletions(-)
>
> diff --git a/arch/arm/plat-omap/include/plat/omap-serial.h b/arch/arm/plat-omap/include/plat/omap-serial.h
> index 9ff4444..12a64eb 100644
> --- a/arch/arm/plat-omap/include/plat/omap-serial.h
> +++ b/arch/arm/plat-omap/include/plat/omap-serial.h
> @@ -131,6 +131,7 @@ struct uart_omap_port {
>        u32                     context_loss_cnt;
>        u32                     errata;
>        u8                      wakeups_enabled;
> +       u8                      max_tx_count;
>
>        struct pm_qos_request   pm_qos_request;
>        u32                     latency;
> diff --git a/drivers/tty/serial/omap-serial.c b/drivers/tty/serial/omap-serial.c
> index 9de7d71..621fde5 100644
> --- a/drivers/tty/serial/omap-serial.c
> +++ b/drivers/tty/serial/omap-serial.c
> @@ -88,6 +88,49 @@ static inline void serial_omap_clear_fifos(struct uart_omap_port *up)
>        serial_out(up, UART_FCR, 0);
>  }
>
> +/**
> + * serial_omap_block_cpu_low_power_state - prevent MPU pwrdm from leaving ON
> + * @up: struct uart_omap_port *
> + *
> + * Prevent the MPU powerdomain from entering a power state lower than
> + * ON.  (It should be sufficient to prevent it from entering INACTIVE,
> + * but there is presently no easy way to do this.)  This works around
> + * a suspected silicon bug in the OMAP UART IP blocks.  The UARTs should
> + * wake the PRCM when the transmit FIFO threshold interrupt is raised, but
> + * they do not.   See also serial_omap_allow_cpu_low_power_state().  No
> + * return value.
> + */
> +static void serial_omap_block_cpu_low_power_state(struct uart_omap_port *up)
> +{
> +#ifdef CONFIG_CPU_IDLE
> +       up->latency = 1;
> +       schedule_work(&up->qos_work);
> +#else
> +       up->max_tx_count = 1;
> +#endif
> +}
> +
> +/**
> + * serial_omap_allow_cpu_low_power_state - remove power state restriction on MPU
> + * @up: struct uart_omap_port *
> + *
> + * Cancel the effects of serial_omap_block_cpu_low_power_state().
> + * This should allow the MPU powerdomain to enter a power state lower
> + * than ON, assuming the rest of the kernel is not restricting it.
> + * This works around a suspected silicon bug in the OMAP UART IP
> + * blocks.  The UARTs should wake the PRCM when the transmit FIFO
> + * threshold interrupt is raised, but they do not.  No return value.
> + */
> +static void serial_omap_allow_cpu_low_power_state(struct uart_omap_port *up)
> +{
> +#ifdef CONFIG_CPU_IDLE
> +       up->latency = up->calc_latency;
> +       schedule_work(&up->qos_work);
> +#else
> +       up->max_tx_count = up->port.fifosize / 4;
> +#endif
> +}
> +
>  /*
>  * serial_omap_get_divisor - calculate divisor value
>  * @port: uart port info
> @@ -163,6 +206,9 @@ static void serial_omap_stop_tx(struct uart_port *port)
>                serial_out(up, UART_IER, up->ier);
>        }
>
> +       if (!up->use_dma)
> +               serial_omap_allow_cpu_low_power_state(up);
> +
>        pm_runtime_mark_last_busy(&up->pdev->dev);
>        pm_runtime_put_autosuspend(&up->pdev->dev);
>  }
> @@ -264,7 +310,7 @@ static void transmit_chars(struct uart_omap_port *up)
>                serial_omap_stop_tx(&up->port);
>                return;
>        }
> -       count = up->port.fifosize / 4;
> +       count = up->max_tx_count;
>        do {
>                serial_out(up, UART_TX, xmit->buf[xmit->tail]);
>                xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
> @@ -297,6 +343,7 @@ static void serial_omap_start_tx(struct uart_port *port)
>
>        if (!up->use_dma) {
>                pm_runtime_get_sync(&up->pdev->dev);
> +               serial_omap_block_cpu_low_power_state(up);
>                serial_omap_enable_ier_thri(up);
>                pm_runtime_mark_last_busy(&up->pdev->dev);
>                pm_runtime_put_autosuspend(&up->pdev->dev);
> @@ -1421,6 +1468,8 @@ static int serial_omap_probe(struct platform_device *pdev)
>        up->port.fifosize = 64;
>        up->port.ops = &serial_omap_pops;
>
> +       up->max_tx_count = up->port.fifosize / 4;
> +
>        if (pdev->dev.of_node)
>                up->port.line = of_alias_get_id(pdev->dev.of_node, "serial");
>        else
>
>
> --
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