linux-kernel - Re: [PATCH 2/3] scsi: add hooks for host runtime power management

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Date:	Wed, 2 Nov 2011 10:53:18 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Lin Ming <ming.m.lin@...el.com>
cc:	linux-kernel@...r.kernel.org, <linux-ide@...r.kernel.org>,
	<linux-scsi@...r.kernel.org>, <linux-pm@...r.kernel.org>,
	Jeff Garzik <jgarzik@...ox.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	James Bottomley <JBottomley@...allels.com>,
	Tejun Heo <tj@...nel.org>, Huang Ying <ying.huang@...el.com>,
	Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH 2/3] scsi: add hooks for host runtime power management

On Wed, 2 Nov 2011, Lin Ming wrote:

> Adds ->suspend and ->resume callbacks in struct scsi_host_template
> to do host runtime power management.
> 
> Signed-off-by: Lin Ming <ming.m.lin@...el.com>
> ---
>  drivers/scsi/scsi_pm.c   |   34 +++++++++++++++++++++++++++++++---
>  include/scsi/scsi_host.h |    8 ++++++++
>  2 files changed, 39 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/scsi/scsi_pm.c b/drivers/scsi/scsi_pm.c
> index 1be6c5a..5742bed2 100644
> --- a/drivers/scsi/scsi_pm.c
> +++ b/drivers/scsi/scsi_pm.c
> @@ -42,6 +42,28 @@ static int scsi_dev_type_resume(struct device *dev)
>  	return err;
>  }
>  
> +static int scsi_host_suspend(struct device *dev)
> +{
> +	struct Scsi_Host *shost = dev_to_shost(dev);
> +	int err = 0;
> +
> +	if (shost->hostt->suspend)
> +		err = shost->hostt->suspend(shost);
> +
> +	return err;
> +}
> +
> +static int scsi_host_resume(struct device *dev)
> +{
> +	struct Scsi_Host *shost = dev_to_shost(dev);
> +	int err = 0;
> +
> +	if (shost->hostt->resume)
> +		err = shost->hostt->resume(shost);
> +
> +	return err;
> +}

These don't need to be independent routines.  You can put them inline 
in scsi_runtime_suspend() and scsi_runtime_resume() below; that's how 
the device code is written.

> @@ -132,7 +160,7 @@ static int scsi_runtime_idle(struct device *dev)
>  
>  	/* Insert hooks here for targets, hosts, and transport classes */
>  
> -	if (scsi_is_sdev_device(dev))
> +	if (scsi_is_sdev_device(dev) || scsi_is_host_device(dev))
>  		err = pm_schedule_suspend(dev, 100);

This is wrong.  The 100-ms delay is meant for devices only.  There's 
no reason to delay suspending a host once everything beneath it is 
suspended.  Just leave the code the way it is now.

>  	else
>  		err = pm_runtime_suspend(dev);
> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index f1f2644..512e2a0 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -356,6 +356,14 @@ struct scsi_host_template {
>  	enum blk_eh_timer_return (*eh_timed_out)(struct scsi_cmnd *);
>  
>  	/*
> +	 * Optional routine for scsi host runtime pm.
> +	 *
> +	 * Status: OPTIONAL
> +	 */
> +	int (*suspend)(struct Scsi_Host *);
> +	int (*resume)(struct Scsi_Host *);

The comment should explain what drivers are supposed to do when these 
hooks are called.  In particular, the suspend callback should _not_ 
power-down the entire host adapter; it should power-down only the 
circuitry that controls the interface to the SCSI bus.  The upstream 
interface to the CPU should remain at full power.

If there's no way to power-down just that part of the host adapter then 
there's no need to have these callbacks at all.  The low-level driver 
can simply use the runtime PM API on its own struct device.

Alan Stern

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