Re: [PATCH v3 1/2] serial: core: Start managing serial controllers to enable runtime PM

[Ilpo Järvinen]

On Wed, 23 Nov 2022, Tony Lindgren wrote:

> We want to enable runtime PM for serial port device drivers in a generic
> way. To do this, we want to have the serial core layer manage the
> registered physical serial controller devices.
> 
> The serial layer has a few challenges to deal with:
> 
> 1. The serial port mapping to a physical serial port controller device
>    is currently not easily available after the physical serial controller
>    struct device gets registered uart_add_one_port() time
> 
> 2. The serial port device drivers have their own driver data. So we cannot
>    currently start making use of serial core generic data easily without
>    changing all the serial port device drivers
> 
> To find the serial ports for a controller based on struct device, let's
> add a new data structure for a serial_controller. On registering a port,
> we can use the drv->state array to find the associated serial port
> controller and initialize the serial core controller.
> 
> As some serial port device drivers enable runtime PM in their probe before
> registering with the serial core layer, and some do not enable runtime PM
> at all currently, we need check the state in the serial core layer on
> uart_port_startup(). We need to also consider that a serial port device
> may have multiple ports.
> 
> Initially we enable runtime PM for all the serial port controller devices.
> This allows us to add runtime PM calls and properly handle any errors
> without a need for serial layer specific runtime PM wrapper functions.
> 
> After this patch no functional changes for the serial port device drivers
> are intended. We just enable runtime PM and keep the runtime PM usage
> count until all the serial controller ports are unregistered. For drivers
> implementing PM runtime, we just keep track of the runtime PM
> configuration.
> 
> For the serial port drivers, the serial core layer has the following use
> cases to deal with:
> 
> 1. If a serial port device driver does not implement runtime PM, the
>    device state is set to active state, and the runtime PM usage count
>    is kept until the last port for a device is unregistered
> 
> 2. If a serial port device driver implements runtime PM, the runtime PM
>    usage count is kept until the last port for the device is unregistered
> 
> 3. If a serial port device driver implements runtime PM autosuspend,
>    autosuspend is not prevented. This currently gets set only for the
>    8250_omap driver to keep runtime PM working for it
> 
> For system suspend, things should be mostly detached from the runtime PM.
> The serial port device drivers may call pm_runtime_force_suspend() and
> pm_runtime_force_resume() as needed.
> 
> Suggested-by: Johan Hovold <johan@xxxxxxxxxx>
> Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>
> ---
> Changes since v2:
> 
> - Make each serial port a proper device as suggested by Greg. This is
>   a separate patch that flushes the TX on runtime PM resume
> 
> Changes since v1:
> 
> - Use kref as suggested by Andy
> 
> - Fix memory leak on error as noted by Andy
> 
> - Use use unsigned char for supports_autosuspend as suggested by Andy
> 
> - Coding style improvments as suggested by Andy
> 
> ---
>  drivers/tty/serial/8250/8250_core.c |   1 +
>  drivers/tty/serial/8250/8250_omap.c |   1 +
>  drivers/tty/serial/serial_core.c    | 179 ++++++++++++++++++++++++++++
>  include/linux/serial_core.h         |   4 +
>  4 files changed, 185 insertions(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -1004,6 +1004,7 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
>  		uart->port.regshift     = up->port.regshift;
>  		uart->port.iotype       = up->port.iotype;
>  		uart->port.flags        = up->port.flags | UPF_BOOT_AUTOCONF;
> +		uart->port.supports_autosuspend = up->port.supports_autosuspend;
>  		uart->bugs		= up->bugs;
>  		uart->port.mapbase      = up->port.mapbase;
>  		uart->port.mapsize      = up->port.mapsize;
> diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
> --- a/drivers/tty/serial/8250/8250_omap.c
> +++ b/drivers/tty/serial/8250/8250_omap.c
> @@ -1330,6 +1330,7 @@ static int omap8250_probe(struct platform_device *pdev)
>  	up.rs485_start_tx = serial8250_em485_start_tx;
>  	up.rs485_stop_tx = serial8250_em485_stop_tx;
>  	up.port.has_sysrq = IS_ENABLED(CONFIG_SERIAL_8250_CONSOLE);
> +	up.port.supports_autosuspend = 1;
>  
>  	ret = of_alias_get_id(np, "serial");
>  	if (ret < 0) {
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -16,6 +16,7 @@
>  #include <linux/console.h>
>  #include <linux/gpio/consumer.h>
>  #include <linux/of.h>
> +#include <linux/pm_runtime.h>
>  #include <linux/proc_fs.h>
>  #include <linux/seq_file.h>
>  #include <linux/device.h>
> @@ -30,6 +31,20 @@
>  #include <linux/irq.h>
>  #include <linux/uaccess.h>
>  
> +/*
> + * Serial port device specific data for serial core.
> + *
> + * Each port device can have multiple ports with struct uart_state allocated
> + * for each port. The array of ports is kept in struct uart_driver.
> + */
> +struct serial_controller {
> +	struct device *dev;			/* Physical controller device */
> +	struct uart_driver *drv;		/* For port specific uart_state */
> +	struct kref ref;			/* Registered port count */
> +	unsigned long implements_pm_runtime:1;
> +	unsigned long supports_autosuspend:1;
> +};
> +
>  /*
>   * This is used to lock changes in serial line configuration.
>   */
> @@ -177,6 +192,162 @@ static void uart_port_dtr_rts(struct uart_port *uport, int raise)
>  		uart_clear_mctrl(uport, TIOCM_DTR | TIOCM_RTS);
>  }
>  
> +static struct serial_controller *to_controller(struct uart_port *port)
> +{
> +	if (!port->dev)
> +		return NULL;
> +
> +	return port->state->controller;
> +}
> +
> +/*
> + * Starts runtime PM for the serial controller device if not already started
> + * by the serial port driver. Called from uart_add_one_port() with port_mutex
> + * held.
> + */
> +static int serial_core_pm_runtime_start(struct uart_port *port)
> +{
> +	struct serial_controller *controller = to_controller(port);
> +	struct device *dev = port->dev;
> +	int ret = 0;
> +
> +	if (kref_get_unless_zero(&controller->ref))
> +		return 0;
> +
> +	/* Init controller device on first reference */
> +	kref_init(&controller->ref);
> +
> +	/* Always enable autosuspend and consider child devices for serdev */
> +	pm_runtime_use_autosuspend(dev);
> +	pm_suspend_ignore_children(dev, false);
> +
> +	/*
> +	 * If the port driver did not enable runtime PM in probe, do it now.
> +	 * Devices that did not enable runtime PM get set active so we can
> +	 * properly handle the returned errors for runtime PM calls.
> +	 */
> +	if (!pm_runtime_enabled(dev)) {
> +		pm_runtime_set_active(dev);
> +		pm_runtime_enable(dev);
> +	} else {
> +		controller->implements_pm_runtime = 1;
> +	}
> +
> +	/*
> +	 * Keep the port device enabled unless autosuspend is supported.
> +	 * Released on port shutdown.
> +	 */
> +	if (!controller->supports_autosuspend) {
> +		ret = pm_runtime_resume_and_get(dev);

Should this be done regardless of autosuspend if the port is console?

There's a problem in the current place where this being called from 
though, uart_console_enabled() doesn't return the correct value with at 
least some devices this early:

https://lore.kernel.org/linux-serial/AS8PR04MB84047F39CD10C00CEE29213F92219@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

-- 
 i.