On Wed, 27 May 2020 at 13:17, Benjamin GAIGNARD
[off-list ref] wrote:

On 5/27/20 12:09 PM, Valentin Schneider wrote:

quoted

Hi Benjamin,

On 26/05/20 16:16, Benjamin Gaignard wrote:

quoted

A first round [1] of discussions and suggestions have already be done on
this series but without found a solution to the problem. I resend it to
progress on this topic.

Apologies for sleeping on that previous thread.

So what had been suggested over there was to use uclamp to boost the
frequency of the handling thread; however if you use threaded IRQs you
get RT threads, which already get the max frequency by default (at least
with schedutil).

Does that not work for you, and if so, why?

That doesn't work because almost everything is done by the hardware blocks
without charge the CPU so the thread isn't running. I have done the
tests with schedutil
and ondemand scheduler (which is the one I'm targeting). I have no
issues when using
performance scheduler because it always keep the highest frequencies.

IMHO, the only way to ensure a min frequency for anything else than a
thread is to use freq_qos_add_request() just like cpufreq cooling
device but for the opposite QoS. This can be applied only on the
frequency domain of the CPU which handles the interrupt.
Have you also checked the wakeup latency of your idle state ?

quoted

quoted

When start streaming from the sensor the CPU load could remain very low
because almost all the capture pipeline is done in hardware (i.e. without
using the CPU) and let believe to cpufreq governor that it could use lower
frequencies. If the governor decides to use a too low frequency that
becomes a problem when we need to acknowledge the interrupt during the
blanking time.
The delay to ack the interrupt and perform all the other actions before
the next frame is very short and doesn't allow to the cpufreq governor to
provide the required burst of power. That led to drop the half of the frames.

To avoid this problem, DCMI driver informs the cpufreq governors by adding
a cpufreq minimum load QoS resquest.

Benjamin

[1] https://lkml.org/lkml/2020/4/24/360

Benjamin Gaignard (3):
   PM: QoS: Introduce cpufreq minimum load QoS
   cpufreq: governor: Use minimum load QoS
   media: stm32-dcmi: Inform cpufreq governors about cpu load needs

  drivers/cpufreq/cpufreq_governor.c        |   5 +
  drivers/media/platform/stm32/stm32-dcmi.c |   8 ++
  include/linux/pm_qos.h                    |  12 ++
  kernel/power/qos.c                        | 213 ++++++++++++++++++++++++++++++
  4 files changed, 238 insertions(+)

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