The feature of runtime pm in mmc_get_card& mmc_put_card

今天在porting sandisk 開發的flow，發現有兩個function是目前使用的kernel沒有定義的。
mmc_get_card 以及 mmc_put_card。
google了一下其實就是把原本mmc_claim_host 以及mmc_release_host加上runtime pm相關的function包起來而成。

[PATCH V3 3/4] mmc: block: Enable runtime pm for mmc blkdevice

從這版patch中可以看到mmc_get_card 以及 mmc_put_card的定義。也取代了原本的mmc_claim_host 以及mmc_release_host。

void mmc_get_card(struct mmc_card *card)
{
 pm_runtime_get_sync(&card->dev);
 mmc_claim_host(card->host);
}
EXPORT_SYMBOL(mmc_get_card);

/*
 * This is a helper function, which releases the host and drops the runtime
 * pm reference for the card device.
 */
void mmc_put_card(struct mmc_card *card)
{
 mmc_release_host(card->host);
 pm_runtime_mark_last_busy(&card->dev);
 pm_runtime_put_autosuspend(&card->dev);
}
EXPORT_SYMBOL(mmc_put_card);

從Documentation/power/runtime_pm.txt可以找到這幾個相關function的簡單敘述。

int pm_runtime_get_sync(struct device *dev);

    - increment the device's usage counter, run       

    pm_runtime_resume(dev) and return its result

int pm_runtime_resume(struct device *dev);

    - execute the subsystem-level resume callback for the device; 

   returns 0 on success, 1 if the device's runtime PM status was already 'active' 

   or error code on failure, where -EAGAIN means it may be safe to attempt to

   resume the device again in future, but 'power.runtime_error' should be checked 

   additionally, and -EACCES means that 'power.disable_depth' is different from 0

void pm_runtime_mark_last_busy(struct device *dev);
    - set the power.last_busy field to the current time

int pm_runtime_put_autosuspend(struct device *dev);
    - decrement the device's usage counter; if the result is 0 then run
    pm_request_autosuspend(dev) and return its result

int pm_request_autosuspend(struct device *dev);
    - schedule the execution of the subsystem-level suspend callback for the
    device when the autosuspend delay has expired; if the delay has already
    expired then the work item is queued up immediately

在文件中有下面這麼一段，這邊想提到的是既然mmc_get_card會引用到pm_runtime_resume，根據文件應該在其他地方會先enable runtime pm.

If the default initial runtime PM status of the device (i.e. 'suspended')
reflects the actual state of the device, its bus type's or its driver's
->probe() callback will likely need to wake it up using one of the PM core's
helper functions described in Section 4.  In that case, pm_runtime_resume()
should be used.  Of course, for this purpose the device's runtime PM has to be
enabled earlier by calling pm_runtime_enable().

再來文件中提到下面這段，也就理解了為何在mmc_put_card呼叫這兩個function。

Inactivity is determined based on the power.last_busy field.  Drivers should
call pm_runtime_mark_last_busy() to update this field after carrying out I/O,
typically just before calling pm_runtime_put_autosuspend().