linux-kernel - Re: [PATCH v5 2/5] ACPI / processor_idle: Add support for Low Power Idle(LPI) states

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Message-ID: <573CA866.2050804@arm.com>
Date:	Wed, 18 May 2016 18:37:42 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	"Prakash, Prashanth" <pprakash@...eaurora.org>,
	linux-acpi@...r.kernel.org, "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Sudeep Holla <sudeep.holla@....com>, linux-kernel@...r.kernel.org,
	Vikas Sajjan <vikas.cha.sajjan@....com>,
	Sunil <sunil.vl@....com>,
	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	Al Stone <al.stone@...aro.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH v5 2/5] ACPI / processor_idle: Add support for Low Power
 Idle(LPI) states



On 17/05/16 18:46, Prakash, Prashanth wrote:
> Hi Sudeep,
>
> On 5/11/2016 9:37 AM, Sudeep Holla wrote:
>> +
>> +static int acpi_processor_get_lpi_info(struct acpi_processor *pr)
>> +{
>> +	int ret, i;
>> +	struct acpi_lpi_states_array *info;
>> +	struct acpi_device *d = NULL;
>> +	acpi_handle handle = pr->handle, pr_ahandle;
>> +	acpi_status status;
>> +
>> +	if (!osc_pc_lpi_support_confirmed)
>> +		return -EOPNOTSUPP;
>> +
>> +	max_leaf_depth = 0;
>> +	if (!acpi_has_method(handle, "_LPI"))
>> +		return -EINVAL;
>> +	flat_state_cnt = 0;
>> +
>> +	while (ACPI_SUCCESS(status = acpi_get_parent(handle, &pr_ahandle))) {
>> +		if (!acpi_has_method(handle, "_LPI"))
>> +			continue;
>> +
>> +		acpi_bus_get_device(handle, &d);
>> +		if (!strcmp(acpi_device_hid(d), ACPI_PROCESSOR_CONTAINER_HID))
>> +			break;
>> +
>> +		max_leaf_depth++;
>> +		handle = pr_ahandle;
>> +	}
>> +
> In the above loop, we break when we find a device with HID ==
> ACPI_PROCESSOR_CONTAINER_HID. Shouldn't we continue to parse as long as the
> parent HID == ACPI_PROCESSOR_CONTAINER_HID? This is required to make sure we
> parse states in levels higher than cluster level in processor hierarchy.
>

Yes, thanks for pointing that out. With just clusters in _LPI on my dev
board, I missed it.

> Also, I think it might be safe to break out of the loop if we didn't find
> _LPI package, instead of continuing. Given  the presence of LPI entry:
> "Enabled Parent State", I can't think of a non-ambiguous scenario where we
> might find LPI packages in state N and N+2, but not in N+1, as we will not
> be able to figure out which state in N enables which states in N+2.
> Thoughts?

Though I admit I haven't thought in detail on how to deal with the
asymmetric topology, but that was the reason why I continue instead of
breaking.

Excerpts from the spec: "... This example is symmetric but that is not a
requirement. For example, a system may contain a different number of
processors in different containers or an asymmetric hierarchy where one
side of the topology tree is deeper than another...."

-- 
Regards,
Sudeep

-- 
Regards,
Sudeep