linux-kernel - Re: [PATCH] pm_qos: Add system bus performance parameter

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Date:	Fri, 27 Aug 2010 07:31:46 -0700
From:	Kevin Hilman <khilman@...prootsystems.com>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	James Bottomley <James.Bottomley@...e.de>,
	mark gross <markgross@...gnar.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jonathan Corbet <corbet@....net>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [PATCH] pm_qos: Add system bus performance parameter

Saravana Kannan <skannan@...eaurora.org> writes:

> Some drivers/devices might need some minimum system bus performance to
> provide acceptable service. Provide a PM QoS parameter to send these requests
> to.
>
> The new parameter is named "system bus performance" since it is generic enough
> for the unit of the request to be frequency, bandwidth or something else that
> might be appropriate. It's up to each implementation of the QoS provider to
> define what the unit of the request would be.
>
> Signed-off-by: Saravana Kannan <skannan@...eaurora.org>

With this current design, only one system-wide bus would be managed.
What if a platform has more than one independently scalable bus?

I think the only scalable way to handle this kind of thing is to have
per-device QoS constraints that can then be combined/aggregated by parent
devices/busses.

At LPC this year, I've proposed per-device QoS constraints[1] as a topic
for the PM mini-conf.  I hope some folks from the MSM camp can be there
for these discussions.

Kevin

[1] http://www.linuxplumbersconf.org/2010/ocw/proposals/819

> ---
>  kernel/pm_qos_params.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/pm_qos_params.c b/kernel/pm_qos_params.c
> index 996a4de..1a44a67 100644
> --- a/kernel/pm_qos_params.c
> +++ b/kernel/pm_qos_params.c
> @@ -93,12 +93,21 @@ static struct pm_qos_object network_throughput_pm_qos = {
>  	.type = PM_QOS_MAX,
>  };
>  
> +static BLOCKING_NOTIFIER_HEAD(system_bus_performance_notifier);
> +static struct pm_qos_object system_bus_performance_pm_qos = {
> +	.requests = PLIST_HEAD_INIT(system_bus_performance_pm_qos.requests, pm_qos_lock),
> +	.notifiers = &system_bus_performance_notifier,
> +	.name = "system_bus_performance",
> +	.default_value = 0,
> +	.type = PM_QOS_MAX,
> +};
>  
>  static struct pm_qos_object *pm_qos_array[] = {
>  	&null_pm_qos,
>  	&cpu_dma_pm_qos,
>  	&network_lat_pm_qos,
>  	&network_throughput_pm_qos
> +	&system_bus_performance_pm_qos
>  };
>  
>  static ssize_t pm_qos_power_write(struct file *filp, const char __user *buf,
--
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