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Subject[RFC PATCH] PM / QoS: Introduce new classes: DMA-Throughput and DVFS-Latency
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1. CPU_DMA_THROUGHPUT

This might look simliar to CPU_DMA_LATENCY. However, there are H/W
blocks that creates QoS requirement based on DMA throughput, not
latency, while their (those QoS requester H/W blocks) services are
short-term bursts that cannot be effectively responsed by DVFS
mechanisms (CPUFreq and Devfreq).

In the Exynos4412 systems that are being tested, such H/W blocks include
MFC (multi-function codec)'s decoding and enconding features, TV-out
(including HDMI), and Cameras. When the display is operated at 60Hz,
each chunk of task should be done within 16ms and the workload on DMA is
not well spread and fluctuates between frames; some frame requires more
and some do not and within a frame, the workload also fluctuates
heavily and the tasks within a frame are usually not parallelized; they
are processed through specific H/W blocks, not CPU cores. They often
have PPMU capabilities; however, they need to be polled very frequently
in order to let DVFS mechanisms react properly. (less than 5ms).

For such specific tasks, allowing them to request QoS requirements seems
adequete because DVFS mechanisms (as long as the polling rate is 5ms or
longer) cannot follow up with them. Besides, the device drivers know
when to request and cancel QoS exactly.

2. DVFS_LATENCY

Both CPUFreq and Devfreq have response latency to a sudden workload
increase. With near-100% (e.g., 95%) up-threshold, the average response
latency is approximately 1.5 x polling-rate.

A specific polling rate (e.g., 100ms) may generally fit for its system;
however, there could be exceptions for that. For example,
- When a user input suddenly starts: typing, clicking, moving cursors, and
such, the user might need the full performance immediately. However,
we do not know whether the full performance is actually needed or not
until we calculate the utilization; thus, we need to calculate it
faster with user inputs or any similar events. Specifying QoS on CPU
processing power or Memory bandwidth at every user input is an
overkill because there are many cases where such speed-up isn't
necessary.
- When a device driver needs a faster performance response from DVFS
mechanism. This could be addressed by simply putting QoS requests.
However, such QoS requests may keep the system running fast
unnecessary in some cases, especially if a) the device's resource
usage bursts with some duration (e.g., 100ms-long bursts) and
b) the driver doesn't know when such burst come. MMC/WiFi often had
such behaviors although there are possibilities that part (b) might
be addressed with further efforts.

The cases shown above can be tackled with putting QoS requests on the
response time or latency of DVFS mechanism, which is directly related to
its polling interval (if the DVFS mechanism is polling based).

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
---
include/linux/pm_qos.h | 6 +++++-
kernel/power/qos.c | 31 ++++++++++++++++++++++++++++++-
2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/include/linux/pm_qos.h b/include/linux/pm_qos.h
index e5bbcba..f8ccb7b 100644
--- a/include/linux/pm_qos.h
+++ b/include/linux/pm_qos.h
@@ -13,13 +13,17 @@
#define PM_QOS_CPU_DMA_LATENCY 1
#define PM_QOS_NETWORK_LATENCY 2
#define PM_QOS_NETWORK_THROUGHPUT 3
+#define PM_QOS_CPU_DMA_THROUGHPUT 4
+#define PM_QOS_DVFS_RESPONSE_LATENCY 5

-#define PM_QOS_NUM_CLASSES 4
+#define PM_QOS_NUM_CLASSES 6
#define PM_QOS_DEFAULT_VALUE -1

#define PM_QOS_CPU_DMA_LAT_DEFAULT_VALUE (2000 * USEC_PER_SEC)
#define PM_QOS_NETWORK_LAT_DEFAULT_VALUE (2000 * USEC_PER_SEC)
#define PM_QOS_NETWORK_THROUGHPUT_DEFAULT_VALUE 0
+#define PM_QOS_CPU_DMA_THROUGHPUT_DEFAULT_VALUE 0
+#define PM_QOS_DVFS_LAT_DEFAULT_VALUE (2000 * USEC_PER_SEC)
#define PM_QOS_DEV_LAT_DEFAULT_VALUE 0

struct pm_qos_request {
diff --git a/kernel/power/qos.c b/kernel/power/qos.c
index 995e3bd..b15e0b7 100644
--- a/kernel/power/qos.c
+++ b/kernel/power/qos.c
@@ -101,11 +101,40 @@ static struct pm_qos_object network_throughput_pm_qos = {
};


+static BLOCKING_NOTIFIER_HEAD(cpu_dma_throughput_notifier);
+static struct pm_qos_constraints cpu_dma_tput_constraints = {
+ .list = PLIST_HEAD_INIT(cpu_dma_tput_constraints.list),
+ .target_value = PM_QOS_CPU_DMA_THROUGHPUT_DEFAULT_VALUE,
+ .default_value = PM_QOS_CPU_DMA_THROUGHPUT_DEFAULT_VALUE,
+ .type = PM_QOS_MAX,
+ .notifiers = &cpu_dma_throughput_notifier,
+};
+static struct pm_qos_object cpu_dma_throughput_pm_qos = {
+ .constraints = &cpu_dma_tput_constraints,
+ .name = "cpu_dma_throughput",
+};
+
+
+static BLOCKING_NOTIFIER_HEAD(dvfs_lat_notifier);
+static struct pm_qos_constraints dvfs_lat_constraints = {
+ .list = PLIST_HEAD_INIT(dvfs_lat_constraints.list),
+ .target_value = PM_QOS_DVFS_LAT_DEFAULT_VALUE,
+ .default_value = PM_QOS_DVFS_LAT_DEFAULT_VALUE,
+ .type = PM_QOS_MIN,
+ .notifiers = &dvfs_lat_notifier,
+};
+static struct pm_qos_object dvfs_lat_pm_qos = {
+ .constraints = &dvfs_lat_constraints,
+ .name = "dvfs_latency",
+};
+
static struct pm_qos_object *pm_qos_array[] = {
&null_pm_qos,
&cpu_dma_pm_qos,
&network_lat_pm_qos,
- &network_throughput_pm_qos
+ &network_throughput_pm_qos,
+ &cpu_dma_throughput_pm_qos,
+ &dvfs_lat_pm_qos,
};

static ssize_t pm_qos_power_write(struct file *filp, const char __user *buf,
--
1.7.4.1


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