Re: [RFC v2 PATCH 2/4] block: add queue runtime pm callbacks

Alan Stern
2012-05-17T18:56:59Z

On Thu, 17 May 2012, Alan Stern wrote:

> On Thu, 17 May 2012, Lin Ming wrote:
>
> > Add runtime pm suspend/resume callbacks to request queue.
> > As an example, implement these callbacks in sd driver.
>
> This is not the way to do it. The block subsystem should not use
> suspend/resume callbacks.
>
> Instead, there should be block functions that can be called by client
> drivers: block_pre_runtime_suspend, block_post_runtime_suspend,
> bock_pre_runtime_resume, and block_post_runtime_resume.
>
> They should do something like this:
>
> block_pre_runtime_suspend:
> If any requests are in the queue, return -EBUSY.
> Otherwise set q->rpm_status to RPM_SUSPENDING and
> return 0.
>
> block_post_runtime_suspend:
> If the suspend succeeded then set q->rpm_status to
> RPM_SUSPENDED. Otherwise set it to RPM_ACTIVE and
> call pm_runtime_mark_last_busy().
>
> block_pre_runtime_resume:
> Set q->rpm_status to RPM_RESUMING.
>
> block_post_runtime_resume:
> If the resume succeeded then set q->rpm_status to
> RPM_ACTIVE and call pm_runtime_mark_last_busy() and
> pm_runtime_request_autosuspend().
> Otherwise set q->rpm_status to RPM_SUSPENDED.
>
> There should also be an initialization function for client drivers to
> call. block_runtime_pm_init() should call pm_runtime_mark_last_busy(),
> pm_runtime_use_autosuspend(), and pm_runtime_autosuspend().
>
> Next, you have to modify the parts of the block layer that run when a
> new request is added to the queue or a request is removed.
>
> When a request is added:
> If q->rpm_status is RPM_SUSPENDED, or if q->rpm_status
> is RPM_SUSPENDING and the REQ_PM flag isn't set, call
> pm_runtime_request_resume().
>
> When a request finishes:
> Call pm_runtime_mark_last_busy().
>
> Next, you have to change the parts of the block layer responsible for
> taking a request from the queue and handing it to the lower-level
> driver (both peek and get). If q->rpm_status is RPM_SUSPENDED, they
> shouldn't do anything -- act as though the queue is empty. If
> q->rpm_status is RPM_SUSPENDING or RPM_RESUMING, they should hand over
> the request only if it has the REQ_PM flag set.
>
> For this to work, the block layer has to know what struct device
> pointer to pass to the pm_runtime_* routines. You'll have to add that
> information to the request_queue structure; I guess q->dev can get set
> by block_pm_runtime_init(). In fact, when that's done you won't need
> q->rpm_status any more. You'll be able to use q->dev->power.rpm_status
> directly, and you won't have to update it because the PM core does that
> for you.
>
> (Or maybe it would be easier to make q->rpm_status be a pointer to
> q->dev->power.rpm_status. That way, if CONFIG_PM_RUNTIME isn't enabled
> or block_runtime_pm_init() hasn't been called, you can have
> q->rpm_status simply point to a static value that is permanently set to
> RPM_ACTIVE.)
>
> I may have left some parts out from this brief description. Hopefully
> you'll be able to figure out the general idea and get it to work.

Oh, yes -- I knew I had forgotten something.

The client driver has to be changed. That means scsi_sysfs_add_sdev()
should call block_pm_runtime_init().

scsi_runtime_suspend() should call block_pre_runtime_suspend() and fail
if that routine returns an error. Then after calling
scsi_dev_type_suspend(), it should call block_post_runtime_suspend()
to let it know whether the suspend operation succeeded.

Similarly, scsi_runtime_resume() should call block_pre_runtime_resume()
before scsi_dev_type_resume() and block_post_runtime_resume()
afterward.

Once that's done, you can remove the scsi_autpm stuff from sd_open()
and sd_release().

Alan Stern