media: chips-media: wave5: Add hrtimer based polling support

media: chips-media: wave5: Add hrtimer based polling support

Add support for starting a polling timer in case an interrupt is not
available. This helps to keep the VPU functional in SoCs such as AM62A,
where the hardware interrupt hookup may not be present due to an SoC errata
[1].

The timer is shared across all instances of encoders and decoders and is
started when the first instance of an encoder or decoder is opened and
stopped when the last instance is closed, thus avoiding per instance
polling and saving CPU bandwidth. As VPU driver manages this instance
related tracking and synchronization, the aforementioned shared timer
related polling logic is implemented within the VPU driver itself. This
scheme may also be useful in general too (even if irq is present) for
non-realtime multi-instance VPU use-cases (for e.g 32 instances of VPU
being run together) where system is running already under high interrupt
load and switching to polling may help mitigate this as the polling thread
is shared across all the VPU instances.

Hrtimer is chosen for polling here as it provides precise timing and
scheduling and the API seems better suited for periodic polling task such
as this.  As a general rule of thumb,

Worst case latency with hrtimer = Actual latency (achievable with irq)
                                  + Polling interval

NOTE (the meaning of terms used above is as follows):
- Latency: Time taken to process one frame
- Actual Latency : Time taken by hardware to process one frame and signal
  it to OS (i.e. if latency that was possible to achieve if irq line was
present)

There is a trade-off between latency and CPU usage when deciding the value
for polling interval. With aggressive polling intervals (i.e. going with
even lesser values) the CPU usage increases although worst case latencies
get better. On the contrary, with greater polling intervals worst case
latencies will increase although the CPU usage will decrease.

The 5ms offered a good balance between the two as we were able to reach
close to actual latencies (as achievable with irq) without incurring too
much of CPU as seen in below experiments and thus 5ms is chosen as default
polling interval.

- 1x 640x480@25 Encoding using different hrtimer polling intervals [2]
- 4x 1080p30 Transcode (File->decode->encode->file) irq vs polling
  comparison [3]
- 1x 1080p Transcode (File->decode->encode->file) irq vs polling comparison
  [4]
- 1080p60 Streaming use-case irq vs polling comparison [5]
- 1x 1080p30 sanity decode and encode tests [6]

The polling interval can also be changed using vpu_poll_interval module
param in case user want to change it as per their use-case requirement
keeping in mind above trade-off.

Parse the irq number and if not present, initialize the hrtimer and the
polling worker thread before proceeding with v4l2 device registrations.

Based on interrupt status, we use a worker thread to iterate over the
interrupt status for each instance and send completion event as being done
in irq thread function.

Move the core functionality of the irq thread function to a separate
function wave5_vpu_handle_irq so that it can be used by both the worker
thread when using polling mode and irq thread when using interrupt mode.

Protect the hrtimer access and instance list with device specific mutex
locks to avoid race conditions while different instances of encoder and
decoder are started together.

[1] https://www.ti.com/lit/pdf/spruj16
(Ref: Section 4.2.3.3 Resets, Interrupts, and Clocks)
[2] https://gist.github.com/devarsht/ee9664d3403d1212ef477a027b71896c
[3] https://gist.github.com/devarsht/3a58b4f201430dfc61697c7e224e74c2
[4] https://gist.github.com/devarsht/a6480f1f2cbdf8dd694d698309d81fb0
[5] https://gist.github.com/devarsht/44aaa4322454e85e01a8d65ac47c5edb
[6] https://gist.github.com/devarsht/2f956bcc6152dba728ce08cebdcebe1d

Signed-off-by: Devarsh Thakkar <devarsht@ti.com>
Tested-by: Jackson Lee <jackson.lee@chipsnmedia.com>
Signed-off-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>