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

[Feng Tang]

Asynchronous driver probing can help much on kerenl 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 family of HWs, some of which
  could use async probling 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 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, which prevents these spi/i2c controller drivers to be async
  probed. For plaforms not using these spi/i2c slave devices, they can
  use this cmdline option to benefit from the async probing.

Suggested-by: Alexander Duyck <alexander.duyck@xxxxxxxxx>
Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
---
 drivers/base/dd.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 8ac10af..1f257561 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 +678,26 @@ 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);
+}
+
+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 - 1);
+	return 0;
+}
+__setup("driver_async_probe=", save_async_options);
+
 bool driver_allows_async_probing(struct device_driver *drv)
 {
+	if (unlikely(cmdline_requested_async_probing(drv->name)))
+		return true;
+
 	switch (drv->probe_type) {
 	case PROBE_PREFER_ASYNCHRONOUS:
 		return true;
-- 
2.7.4