Re: [RFC PATCH v2] async: Add cmdline option to specify drivers to be async probed

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

On Mon, 2019-01-28 at 09:20 +0800, Feng Tang wrote:
> Asynchronous driver probing can help much on kernel fastboot, and
> this option can provide a flexible way to optimize and quickly verify
> async driver probe.
> 
> Also it will help in below cases:
> * Some driver actually covers several families of HWs, some of which
>   could use async probing while others don't. So we can't simply
>   turn on the PROBE_PREFER_ASYNCHRONOUS flag in driver, but use this
>   cmdline option, like igb driver async patch discussed at
>   https://www.spinics.net/lists/netdev/msg545986.html
> 
> * For SOC (System on Chip) with multiple spi or i2c controllers, most
>   of the slave spi/i2c devices will be assigned with fixed controller
>   number, while async probing may make those controllers get different
>   index for each boot, which prevents those controller drivers to be
>   async probed. For platforms not using these spi/i2c slave devices,
>   they can use this cmdline option to benefit from the async probing.
> 
> Suggested-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> Signed-off-by: Feng Tang <feng.tang@intel.com>
> ---
>  drivers/base/dd.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 8ac10af..cfa704a 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -57,6 +57,10 @@ static atomic_t deferred_trigger_count = ATOMIC_INIT(0);
>  static struct dentry *deferred_devices;
>  static bool initcalls_done;
>  
> +/* Save the async probe drivers' name from kernel cmdline */
> +#define ASYNC_DRV_NAMES_MAX_LEN	256
> +static char async_probe_drv_names[ASYNC_DRV_NAMES_MAX_LEN];
> +
>  /*
>   * In some cases, like suspend to RAM or hibernation, It might be reasonable
>   * to prohibit probing of devices as it could be unsafe.
> @@ -674,8 +678,27 @@ int driver_probe_device(struct device_driver *drv, struct device *dev)
>  	return ret;
>  }
>  
> +static inline bool cmdline_requested_async_probing(const char *drv_name)
> +{
> +	return parse_option_str(async_probe_drv_names, drv_name);
> +}
> +
> +/* The format is like driver_async_probe=drv_name1,drv_name2,drv_name3 */
> +static int __init save_async_options(char *buf)
> +{
> +	if (strlen(buf) >= ASYNC_DRV_NAMES_MAX_LEN)
> +		printk(KERN_WARNING "Too long list for async_probe_drv_names!");
> +
> +	strlcpy(async_probe_drv_names, buf, ASYNC_DRV_NAMES_MAX_LEN);
> +	return 0;
> +}
> +__setup("driver_async_probe=", save_async_options);
> +
>  bool driver_allows_async_probing(struct device_driver *drv)
>  {
> +	if (cmdline_requested_async_probing(drv->name))
> +		return true;
> +

This piece still should be moved. Ideally it should be down in the
"default" case area we use to determine if the module parameter
async_probe was used or not. Where you currently have this will
overrride the driver behavior if it absolutely cannot use an
asynchronous probe.

>  	switch (drv->probe_type) {
>  	case PROBE_PREFER_ASYNCHRONOUS:
>  		return true;