Date:   Fri, 25 May 2018 18:08:56 -0700
From:   David Collins <collinsd@...eaurora.org>
To:     Rajendra Nayak <rnayak@...eaurora.org>, viresh.kumar@...aro.org,
        sboyd@...nel.org, andy.gross@...aro.org, ulf.hansson@...aro.org
Cc:     devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Lina Iyer <ilina@...eaurora.org>
Subject: Re: [PATCH v2 5/6] soc: qcom: rpmh powerdomain driver

Hello Rajendra,

On 05/25/2018 03:01 AM, Rajendra Nayak wrote:
> The RPMh powerdomain driver aggregates the corner votes from various

s/powerdomain/power domain/

This applies to all instances in this patch.  "Power domain" seems to be
the preferred spelling in the kernel.


> consumers for the ARC resources and communicates it to RPMh.
> 
> We also add data for all powerdomains on sdm845 as part of the patch.
> The driver can be extended to support other SoCs which support RPMh
> 
> Signed-off-by: Rajendra Nayak <rnayak@...eaurora.org>
> ---
>  .../devicetree/bindings/power/qcom,rpmhpd.txt |  65 ++++
>  drivers/soc/qcom/Kconfig                      |   9 +
>  drivers/soc/qcom/Makefile                     |   1 +
>  drivers/soc/qcom/rpmhpd.c                     | 360 ++++++++++++++++++
>  4 files changed, 435 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/power/qcom,rpmhpd.txt
>  create mode 100644 drivers/soc/qcom/rpmhpd.c
...
> +++ b/Documentation/devicetree/bindings/power/qcom,rpmhpd.txt

I think that this binding documentation should be in a patch separate from
the driver.


> @@ -0,0 +1,65 @@
> +Qualcomm RPMh Powerdomains

s/Qualcomm/Qualcomm Technologies, Inc./


> +
> +* For RPMh powerdomains, we communicate a performance state to RPMh

Does this line need to start with '*'?


> +which then translates it into a corresponding voltage on a rail
> +
> +Required Properties:
> + - compatible: Should be one of the following
> +	* qcom,sdm845-rpmhpd: RPMh powerdomain for the sdm845 family of SoC
> + - power-domain-cells: number of cells in power domain specifier
> +	must be 1
> + - operating-points-v2: Phandle to the OPP table for the power-domain.
> +	Refer to Documentation/devicetree/bindings/power/power_domain.txt
> +	and Documentation/devicetree/bindings/opp/qcom-opp.txt for more details
> +
> +Example:
> +
> +	rpmhpd: power-controller {
> +		compatible = "qcom,sdm845-rpmhpd";
> +		#power-domain-cells = <1>;
> +		operating-points-v2 = <&rpmhpd_opp_table>,
> +				      <&rpmhpd_opp_table>,
> +				      <&rpmhpd_opp_table>,
> +				      <&rpmhpd_opp_table>,
> +				      <&rpmhpd_opp_table>,
> +				      <&rpmhpd_opp_table>,
> +				      <&rpmhpd_opp_table>,
> +				      <&rpmhpd_opp_table>,
> +				      <&rpmhpd_opp_table>;

Can this be changed to simply:
  operating-points-v2 = <&rpmhpd_opp_table>;

The opp binding documentation [1] states that this should be ok:

    If only one phandle is available, then the same OPP table will be used
    for all power domains provided by the power domain provider.


> +	};
> +
> +	rpmhpd_opp_table: opp-table {
> +		compatible = "operating-points-v2-qcom-level", "operating-points-v2";
> +
> +		rpmhpd_opp1: opp@1 {

Is there any significance to the 1 through 8 values in these OPP table
nodes?  If not, then could this be changed to something like:

rpmhpd_retention: opp@16 {
...
rpmhpd_turbo_l1: opp@416 {


> +			qcom-corner = <16>;

s/qcom-corner/qcom,level/g


> +		};
> +
> +		rpmhpd_opp2: opp@2 {
> +			qcom-corner = <48>;
> +		};
> +
> +		rpmhpd_opp3: opp@3 {
> +			qcom-corner = <64>;
> +		};
> +
> +		rpmhpd_opp4: opp@4 {
> +			qcom-corner = <128>;
> +		};
> +
> +		rpmhpd_opp5: opp@5 {
> +			qcom-corner = <192>;
> +		};
> +
> +		rpmhpd_opp6: opp@6 {
> +			qcom-corner = <256>;
> +		};
> +
> +		rpmhpd_opp7: opp@7 {
> +			qcom-corner = <320>;
> +		};

Can you please add 336 and 384 to your example?  384 at least should be
present as it corresponds to the Turbo level which all supplies support.


> +		rpmhpd_opp8: opp@8 {
> +			qcom-corner = <416>;
> +		};
> +	};

How are consumers of these power domains supposed to identify which domain
within <&rpmhpd> to use (e.g. VDD_CX vs VDD_MX)?  If the answer is a plain
integer index, then could you please add per-platform #define constants in
a DT header file which explicitly define the meaning for each index?

How do consumers of these power domains identify which level they want to
set for a specific power domain (e.g. Nominal vs Turbo)?

Would it be helpful to provide a DT header file with #define constants for
the cross-platform sparse level mapping?  This is done in [2] for the
downstream rpmh-regulator driver that handles ARC managed regulators.


Would it be ok to add some consumer DT nodes in this binding file example
so that it is clear how consumers interact with the rpmhpd?


> diff --git a/drivers/soc/qcom/Kconfig b/drivers/soc/qcom/Kconfig
> index a7a405178967..1faed239701d 100644
> --- a/drivers/soc/qcom/Kconfig
> +++ b/drivers/soc/qcom/Kconfig
> @@ -74,6 +74,15 @@ config QCOM_RMTFS_MEM
>  
>  	  Say y here if you intend to boot the modem remoteproc.
>  
> +config QCOM_RPMHPD
> +	tristate "Qualcomm RPMh Powerdomain driver"

s/Qualcomm/Qualcomm Technologies, Inc./


> +	depends on QCOM_RPMH && QCOM_COMMAND_DB
> +	help
> +	  QCOM RPMh powerdomain driver to support powerdomain with
> +	  performance states. The driver communicates a performance state
> +	  value to RPMh which then translates it into corresponding voltage
> +	  for the voltage rail.
> +
>  config QCOM_RPMPD
>  	tristate "Qualcomm RPM Powerdomain driver"
>  	depends on MFD_QCOM_RPM && QCOM_SMD_RPM
> diff --git a/drivers/soc/qcom/Makefile b/drivers/soc/qcom/Makefile
> index 9550c170de93..499513f63bef 100644
> --- a/drivers/soc/qcom/Makefile
> +++ b/drivers/soc/qcom/Makefile
> @@ -16,3 +16,4 @@ obj-$(CONFIG_QCOM_SMSM)	+= smsm.o
>  obj-$(CONFIG_QCOM_WCNSS_CTRL) += wcnss_ctrl.o
>  obj-$(CONFIG_QCOM_APR) += apr.o
>  obj-$(CONFIG_QCOM_RPMPD) += rpmpd.o
> +obj-$(CONFIG_QCOM_RPMHPD) += rpmhpd.o
> diff --git a/drivers/soc/qcom/rpmhpd.c b/drivers/soc/qcom/rpmhpd.c
> new file mode 100644
> index 000000000000..a79f094ad326
> --- /dev/null
> +++ b/drivers/soc/qcom/rpmhpd.c
> @@ -0,0 +1,360 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2018, The Linux Foundation. All rights reserved.
> + */

Minor: The copyright comment could be made into a single line comment.


> +
> +#include <linux/err.h>
> +#include <linux/export.h>
> +#include <linux/init.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/mutex.h>
> +#include <linux/pm_domain.h>
> +#include <linux/slab.h>
> +#include <linux/of.h>
> +#include <linux/of_device.h>
> +#include <linux/platform_device.h>
> +#include <linux/pm_opp.h>
> +#include <soc/qcom/cmd-db.h>
> +#include <soc/qcom/rpmh.h>
> +
> +#define domain_to_rpmhpd(domain) container_of(domain, struct rpmhpd, pd)
> +
> +#define DEFINE_RPMHPD_AO(_platform, _name, _active)			\
> +	static struct rpmhpd _platform##_##_active;			\
> +	static struct rpmhpd _platform##_##_name = {			\
> +		.pd = {	.name = #_name,	},				\
> +		.peer = &_platform##_##_active,				\
> +		.res_name = #_name".lvl",				\
> +	};								\
> +	static struct rpmhpd _platform##_##_active = {			\
> +		.pd = { .name = #_active, },				\
> +		.peer = &_platform##_##_name,				\
> +		.active_only = true,					\
> +		.res_name = #_name".lvl",				\
> +	}
> +
> +#define DEFINE_RPMHPD(_platform, _name)					\
> +	static struct rpmhpd _platform##_##_name = {			\
> +		.pd = { .name = #_name, },				\
> +		.res_name = #_name".lvl",				\
> +	}
> +
> +/*
> + * This is the number of bytes used for each command DB aux data entry of an
> + * ARC resource.
> + */
> +#define RPMH_ARC_LEVEL_SIZE	2
> +#define RPMH_ARC_MAX_LEVELS	16
> +
> +struct rpmhpd {
> +	struct device *dev;
> +	struct generic_pm_domain pd;
> +	struct rpmhpd *peer;
> +	const bool active_only;
> +	unsigned long corner;

Does this actually need to be unsigned long?  It looks like unsigned int
is being passed in from rpmhpd_set_performance().  Also, hlvl values sent
to RPMh will only every be in the range 0 - 15.

If you change the type here, then can you also please change long to int
in to_active_sleep() and rpmhpd_aggregate_corner() below?


> +	u32	level[RPMH_ARC_MAX_LEVELS];
> +	int	level_count;
> +	bool enabled;
> +	const char *res_name;
> +	u32	addr;
> +};

Can you please indent the fields of this struct to the same column with tabs?


> +
> +struct rpmhpd_desc {
> +	struct rpmhpd **rpmhpds;
> +	size_t num_pds;
> +};

This struct could be removed and the per-platform arrays could instead be
NULL terminated.

> +
> +static DEFINE_MUTEX(rpmhpd_lock);
> +
> +/* sdm845 RPMh powerdomains */
> +DEFINE_RPMHPD(sdm845, ebi);
> +DEFINE_RPMHPD_AO(sdm845, mx, mx_ao);
> +DEFINE_RPMHPD_AO(sdm845, cx, cx_ao);
> +DEFINE_RPMHPD(sdm845, lmx);
> +DEFINE_RPMHPD(sdm845, lcx);
> +DEFINE_RPMHPD(sdm845, gfx);
> +DEFINE_RPMHPD(sdm845, mss);
> +
> +static struct rpmhpd *sdm845_rpmhpds[] = {
> +	[0] = &sdm845_ebi,

If you are going to explicitly index these elements, then can you please
use #define constants from a DT header file that specifies meaningful
names?  The existing 0-8 indexing is no better than implicit indexing.


> +	[1] = &sdm845_mx,
> +	[2] = &sdm845_mx_ao,
> +	[3] = &sdm845_cx,
> +	[4] = &sdm845_cx_ao,
> +	[5] = &sdm845_lmx,
> +	[6] = &sdm845_lcx,
> +	[7] = &sdm845_gfx,
> +	[8] = &sdm845_mss,
> +};
> +
> +static const struct rpmhpd_desc sdm845_desc = {
> +	.rpmhpds = sdm845_rpmhpds,
> +	.num_pds = ARRAY_SIZE(sdm845_rpmhpds),
> +};
> +
> +static const struct of_device_id rpmhpd_match_table[] = {
> +	{ .compatible = "qcom,sdm845-rpmhpd", .data = &sdm845_desc },
> +	{ }
> +};
> +MODULE_DEVICE_TABLE(of, rpmhpd_match_table);
> +
> +static int rpmhpd_send_corner(struct rpmhpd *pd, int state, unsigned int corner)
> +{
> +	struct tcs_cmd cmd = {
> +		.addr = pd->addr,
> +		.data = corner,
> +	};
> +
> +	return rpmh_write(pd->dev, state, &cmd, 1);

This can be optimized by calling rpmh_write_async() whenever the corner
being sent is smaller than the last value sent.  That way, no time is
wasted waiting for an ACK when decreasing voltage.  Would you mind adding
the necessary check and previous request caching for this?


> +};
> +
> +static void to_active_sleep(struct rpmhpd *pd, unsigned long corner,
> +			    unsigned long *active, unsigned long *sleep)
> +{
> +	*active = corner;
> +
> +	if (pd->active_only)
> +		*sleep = 0;
> +	else
> +		*sleep = *active;
> +}
> +
> +static int rpmhpd_aggregate_corner(struct rpmhpd *pd)
> +{
> +	int ret;
> +	struct rpmhpd *peer = pd->peer;
> +	unsigned long active_corner, sleep_corner;
> +	unsigned long this_corner = 0, this_sleep_corner = 0;
> +	unsigned long peer_corner = 0, peer_sleep_corner = 0;

s/this_corner/this_active_corner/
s/peer_corner/peer_active_corner/

This is more consistent and I think that it makes the code a little more
readable.


> +
> +	to_active_sleep(pd, pd->corner, &this_corner, &this_sleep_corner);
> +
> +	if (peer && peer->enabled)
> +		to_active_sleep(peer, peer->corner, &peer_corner,
> +				&peer_sleep_corner);
> +
> +	active_corner = max(this_corner, peer_corner);
> +
> +	ret = rpmhpd_send_corner(pd, RPMH_ACTIVE_ONLY_STATE, active_corner);
> +	if (ret)
> +		return ret;
> +
> +	sleep_corner = max(this_sleep_corner, peer_sleep_corner);
> +
> +	return rpmhpd_send_corner(pd, RPMH_SLEEP_STATE, sleep_corner);
> +}

This aggregation function as well as the rpmhpd_send_corner() calls below
are not sufficient for RPMh.  There are 3 states that must all be used:
RPMH_ACTIVE_ONLY_STATE, RPMH_WAKE_ONLY_STATE, and RPMH_SLEEP_STATE.  The
naming is somewhat confusing as rpmhpd is defining a different concept of
active-only.

For power domains without active-only or peers, only
RPMH_ACTIVE_ONLY_STATE should be used.  This instructs RPMh to issue the
request immediately.

For power domains with active-only, requests will need to be made for all
three.  active_corner would be sent for both RPMH_ACTIVE_ONLY_STATE (so
that the request takes effect immediately) and RPMH_WAKE_ONLY_STATE (so
that the request is inserted into the wake TCS).  sleep_corner would be
sent for RPMH_SLEEP_STATE (so that the request is inserted into the sleep
TCS).

You can see how this is handled in the RPMh clock driver in patch [3].

You may be able to get away with using only RPMH_SLEEP_STATE and
RPMH_ACTIVE_ONLY_STATE assuming that you issue the RPMH_SLEEP_STATE
request first due to the rpmh driver caching behavior added in the
cache_rpm_request() function in [4].  However, could you please confirm
with Lina that this usage will continue to work in the future?  I'm not
sure what guarantees are made at the rpmh API level.


> +
> +static int rpmhpd_power_on(struct generic_pm_domain *domain)
> +{
> +	int ret = 0;
> +	struct rpmhpd *pd = domain_to_rpmhpd(domain);

Minor: It might look a little nicer to list 'pd' definition first amongst
the local variables in this function as well as those below.


> +
> +	mutex_lock(&rpmhpd_lock);
> +
> +	pd->enabled = true;
> +
> +	if (pd->corner)
> +		ret = rpmhpd_aggregate_corner(pd);
> +
> +	mutex_unlock(&rpmhpd_lock);
> +
> +	return ret;
> +}
> +
> +static int rpmhpd_power_off(struct generic_pm_domain *domain)
> +{
> +	int ret;
> +	struct rpmhpd *pd = domain_to_rpmhpd(domain);
> +
> +	mutex_lock(&rpmhpd_lock);
> +
> +	if (pd->level[0] == 0) {
> +		ret = rpmhpd_send_corner(pd, RPMH_ACTIVE_ONLY_STATE, 0);
> +		if (ret)
> +			return ret;
> +
> +		ret = rpmhpd_send_corner(pd, RPMH_SLEEP_STATE, 0);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	pd->enabled = false;
> +
> +	mutex_unlock(&rpmhpd_lock);
> +
> +	return 0;
> +}
> +
> +static int rpmhpd_set_performance(struct generic_pm_domain *domain,
> +			     unsigned int state)
> +{
> +	int ret = 0;
> +	struct rpmhpd *pd = domain_to_rpmhpd(domain);
> +
> +	mutex_lock(&rpmhpd_lock);
> +
> +	pd->corner = state;

What is the numbering space for 'state'?  I assume that it is a vlvl value
corresponding to qcom,level in a DT OPP table node.  If so, additional
logic is required.

When using RPMh, the platform and supply independent vlvl sparse numbering
space is used by consumers so that they can always have consistent values.
 However, the actual requests sent to RPMh ARC must be in the hlvl
numbering space (i.e. 0 - 15(max)).  In the case of this driver, the
acceptable hlvl values for a given power domain are 0 to pd->level_count - 1.

I suspect that you need to add something like this here:

int i;

for (i = 0; i < pd->level_count; i++)
	if (state <= pd->level[i])
		break;

if (i == pd->level_count) {
	ret = -EINVAL;
	dev_err(pd->dev, "invalid state=%u for domain %s",
		state, pd->pd.name);
	goto out;
}

pd->corner = i;


Note that a given power domain will likely not support all of the vlvl
values listed in the DT OPP table nodes.


> +
> +	if (!pd->enabled)
> +		goto out;
> +
> +	ret = rpmhpd_aggregate_corner(pd);
> +out:
> +	mutex_unlock(&rpmhpd_lock);
> +
> +	return ret;
> +}
> +
> +static unsigned int rpmhpd_get_performance(struct generic_pm_domain *genpd,
> +					   struct dev_pm_opp *opp)> +{
> +	struct device_node *np;
> +	unsigned int corner = 0;
> +
> +	np = dev_pm_opp_get_of_node(opp);
> +	if (of_property_read_u32(np, "qcom,level", &corner)) {
> +		pr_err("%s: missing 'qcom,level' property\n", __func__);
> +		return 0;

Why return 0 instead of an error?


> +	}
> +
> +	of_node_put(np);
> +
> +	return corner;
> +}

Is there an API to determine the currently operating performance state of
a given power domain?  Is this information accessible from userspace?  We
will definitely need this for general debugging.


> +
> +static int rpmhpd_update_level_mapping(struct rpmhpd *rpmhpd)
> +{
> +	u8 *buf;

This could be changed to the following in order to remove the need for
kzalloc() and kfree() calls below:

u8 buf[RPMH_ARC_MAX_LEVELS * RPMH_ARC_LEVEL_SIZE];


> +	int i, j, len, ret;
> +
> +	len = cmd_db_read_aux_data_len(rpmhpd->res_name);
> +	if (len <= 0)
> +		return len;
> +

A check like this is needed here:

if (len > RPMH_ARC_MAX_LEVELS * RPMH_ARC_LEVEL_SIZE)
	return -EINVAL;


> +	buf = kzalloc(len, GFP_KERNEL);
> +	if (!buf)
> +		return -ENOMEM;
> +
> +	ret = cmd_db_read_aux_data(rpmhpd->res_name, buf, len);
> +	if (ret < 0)
> +		return ret;
> +
> +	rpmhpd->level_count = len / RPMH_ARC_LEVEL_SIZE;
> +
> +	for (i = 0; i < rpmhpd->level_count; i++) {

If you make buf a fixed size array, then rpmhpd->level[i] = 0; is needed
here or a memset() outside of the for loop.


> +		for (j = 0; j < RPMH_ARC_LEVEL_SIZE; j++)
> +			rpmhpd->level[i] |=
> +				buf[i * RPMH_ARC_LEVEL_SIZE + j] << (8 * j);
> +
> +		/*
> +		 * The AUX data may be zero padded.  These 0 valued entries at
> +		 * the end of the map must be ignored.
> +		 */
> +		if (i > 0 && rpmhpd->level[i] == 0) {
> +			rpmhpd->level_count = i;
> +			break;
> +		}
> +		pr_dbg("%s: ARC hlvl=%2d --> vlvl=%4u\n", rpmhpd->res_name, i,
> +		       rpmhpd->level[i]);
> +	}
> +
> +	kfree(buf);
> +	return 0;
> +}
> +
> +static int rpmhpd_probe(struct platform_device *pdev)
> +{
> +	int i, ret;
> +	size_t num;
> +	struct genpd_onecell_data *data;
> +	struct rpmhpd **rpmhpds;
> +	const struct rpmhpd_desc *desc;
> +
> +	desc = of_device_get_match_data(&pdev->dev);
> +	if (!desc)
> +		return -EINVAL;
> +
> +	rpmhpds = desc->rpmhpds;
> +	num = desc->num_pds;
> +
> +	data = devm_kzalloc(&pdev->dev, sizeof(*data), GFP_KERNEL);
> +	if (!data)
> +		return -ENOMEM;
> +
> +	data->domains = devm_kcalloc(&pdev->dev, num, sizeof(*data->domains),
> +				     GFP_KERNEL);
> +	data->num_domains = num;
> +
> +	ret = cmd_db_ready();
> +	if (ret) {
> +		if (ret != -EPROBE_DEFER)
> +			dev_err(&pdev->dev, "Command DB unavailable, ret=%d\n",
> +				ret);
> +		return ret;
> +	}
> +
> +	for (i = 0; i < num; i++) {
> +		if (!rpmhpds[i])
> +			continue;

Why is this check needed?


> +
> +		rpmhpds[i]->dev = &pdev->dev;
> +		rpmhpds[i]->addr = cmd_db_read_addr(rpmhpds[i]->res_name);
> +		if (!rpmhpds[i]->addr) {
> +			dev_err(&pdev->dev, "Could not find RPMh address for resource %s\n",
> +				rpmhpds[i]->res_name);
> +			return -ENODEV;
> +		}
> +
> +		ret = cmd_db_read_slave_id(rpmhpds[i]->res_name);
> +		if (ret != CMD_DB_HW_ARC) {
> +			dev_err(&pdev->dev, "RPMh slave ID mismatch\n");
> +			return -EINVAL;
> +		}
> +
> +		ret = rpmhpd_update_level_mapping(rpmhpds[i]);
> +		if (ret)
> +			return ret;
> +
> +		rpmhpds[i]->pd.power_off = rpmhpd_power_off;
> +		rpmhpds[i]->pd.power_on = rpmhpd_power_on;
> +		rpmhpds[i]->pd.set_performance_state = rpmhpd_set_performance;
> +		rpmhpds[i]->pd.opp_to_performance_state = rpmhpd_get_performance;
> +		pm_genpd_init(&rpmhpds[i]->pd, NULL, true);
> +
> +		data->domains[i] = &rpmhpds[i]->pd;
> +	}
> +
> +	return of_genpd_add_provider_onecell(pdev->dev.of_node, data);
> +}
> +
> +static int rpmhpd_remove(struct platform_device *pdev)
> +{
> +	of_genpd_del_provider(pdev->dev.of_node);
> +	return 0;
> +}
> +
> +static struct platform_driver rpmhpd_driver = {
> +	.driver = {
> +		.name = "qcom-rpmhpd",
> +		.of_match_table = rpmhpd_match_table,
> +	},
> +	.probe = rpmhpd_probe,
> +	.remove = rpmhpd_remove,
> +};
> +
> +static int __init rpmhpd_init(void)
> +{
> +	return platform_driver_register(&rpmhpd_driver);
> +}
> +core_initcall(rpmhpd_init);
> +
> +static void __exit rpmhpd_exit(void)
> +{
> +	platform_driver_unregister(&rpmhpd_driver);
> +}
> +module_exit(rpmhpd_exit);
> +
> +MODULE_DESCRIPTION("Qualcomm RPMh Power Domain Driver");

s/Qualcomm/Qualcomm Technologies, Inc./


> +MODULE_LICENSE("GPL v2");
> +MODULE_ALIAS("platform:qcom-rpmhpd");

Thanks,
David

[1]:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/opp/opp.txt?h=v4.17-rc6#n49
[2]:
https://source.codeaurora.org/quic/la/kernel/msm-4.9/tree/include/dt-bindings/regulator/qcom,rpmh-regulator.h?h=msm-4.9
[3]: https://lkml.org/lkml/2018/5/9/54
[4]: https://lkml.org/lkml/2018/5/24/536

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a Linux Foundation Collaborative Project

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