Chihchung just pointed out that we don't need extra built-in module to do that.
We will fix the output_source and output_sink, add two extra section just controlling the switching.
Then we disable/enable one of the extra section with variable (not using purpose).
I will take a close look on how this works.

I think we want to provide a non-stopping listening experience so the steps involving enable/disable devices will not work.  We want something that could be toggled or applied on-the-fly, so approach in #1 sounds promising.

Hi Chinyue, after taking a look again in the code, I agree it should be fine to do this dsp reload without enable/disable devices.

Following what cras_iodev_update_dsp does, we enable the left-right-swap variable to change the dsp pipeline. Since the pipeline is protected by a mutex. We don't need to worry it might mess up audio thread. In audio thread, when it runs apply_dsp, it will acquire the mutex first, so after the quick switch in cmd_load_pipeline

        /* This locking is short to avoild blocking audio thread. */
        pthread_mutex_lock(&ctx->mutex);
        old_pipeline = ctx->pipeline;
        ctx->pipeline = pipeline;
        pthread_mutex_unlock(&ctx->mutex);

audio thread can acquire the mutex and get the new dsp pipeline.
It should happen without user noticing it.

Thanks for the suggestion!

https://chromium-review.googlesource.com/#/c/386701/1
https://chromium-review.googlesource.com/#/c/386702/1
https://chromium-review.googlesource.com/#/c/386703/1
should add the support.
The change in dsp.ini is not uploaded yet.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/adhd/+/9e6d58368b2f2d1a3a9d9281433470a6319e9847

commit 9e6d58368b2f2d1a3a9d9281433470a6319e9847
Author: Cheng-Yi Chiang <cychiang@chromium.org>
Date: Sat Sep 17 19:01:53 2016

CRAS: dps_mod_builtin - Add a swap_lr built-in module

This module swaps data on left and right channels.

BUG= chromium:645051 
TEST=Insert one swap_lr section in dsp.ini and check left and right
channels are swapped.
    [swap_lr]
    library=builtin
    label=swap_lr
    input_0={swap_lr:0}
    input_1={swap_lr:1}
    output_2={dst:1}
    output_3={dst:0}

Change-Id: I687e69bcc13375436f9535db97514b3a60580376
Reviewed-on: https://chromium-review.googlesource.com/386701
Commit-Ready: Cheng-Yi Chiang <cychiang@chromium.org>
Tested-by: Cheng-Yi Chiang <cychiang@chromium.org>
Reviewed-by: Chinyue Chen <chinyue@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>

[modify] https://crrev.com/9e6d58368b2f2d1a3a9d9281433470a6319e9847/cras/src/server/cras_dsp_mod_builtin.c

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/adhd/+/4f86aad321250a527075476dac94241c24817521

commit 4f86aad321250a527075476dac94241c24817521
Author: Cheng-Yi Chiang <cychiang@chromium.org>
Date: Mon Sep 19 16:37:26 2016

CRAS: dsp - Add function to set a boolean variable

Rename cras_dsp_set_variable to cras_dsp_set_variable_string.
Add cras_dsp_set_variable_boolean to set a boolean variable.
Remove unused cras_dsp_sync declaration.

BUG= chromium:645051 
TEST=make check

Signed-off-by: Cheng-Yi Chiang <cychiang@chromium.org>
Change-Id: I6ef667629b5c170bb7cd854fc89c31aa2064aa03
Reviewed-on: https://chromium-review.googlesource.com/386702
Reviewed-by: Dylan Reid <dgreid@chromium.org>

[modify] https://crrev.com/4f86aad321250a527075476dac94241c24817521/cras/src/server/cras_dsp.c
[modify] https://crrev.com/4f86aad321250a527075476dac94241c24817521/cras/src/server/cras_dsp.h
[modify] https://crrev.com/4f86aad321250a527075476dac94241c24817521/cras/src/server/cras_iodev.c
[modify] https://crrev.com/4f86aad321250a527075476dac94241c24817521/cras/src/tests/dsp_unittest.cc
[modify] https://crrev.com/4f86aad321250a527075476dac94241c24817521/cras/src/tests/iodev_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/adhd/+/c9f56d936f63f56186b6daaa3f0c9abed828531a

commit c9f56d936f63f56186b6daaa3f0c9abed828531a
Author: Cheng-Yi Chiang <cychiang@chromium.org>
Date: Mon Sep 19 16:46:46 2016

CRAS: iodev - Add function to swap left and right channel using dsp

Add cras_iodev_dsp_set_swap_mode_for_node to swap left and right channel
using dsp. It is used as the default implementation of
set_swap_mode_for_node ops on alsa_io device.

To use this function, there must be a swap_lr section in dsp.ini:

  [swap_lr]
  library=builtin
  label=swap_lr
  disable=swap_lr_disabled
  input_0={swap_lr:0}
  input_1={swap_lr:1}
  output_2={dst:1}
  output_3={dst:0}

where {swap_lr:0}, {swap_lr:1} are the output ports of the block which
was before output sink (usually eq2).

cras_iodev_dsp_set_swap_mode_for_node must support two use cases:
- User sets swap mode when device is not playing.
- User sets swap mode when device is playing.

In the first use case, cras_iodev_update_dsp sets swap_lr_disabled
variable based on left_right_swapped on active node.
For the second use case, we need to udpate dsp in
cras_iodev_dsp_set_swap_mode_for_node so the change takes effect right
away. It is fine to do so because pipeline in the context is protected
by a mutex.

BUG= chromium:645051 
TEST=with modified dsp, check left right swap works using
cras_test_client --swap_left_right n:m:1.

Signed-off-by: Cheng-Yi Chiang <cychiang@chromium.org>
Change-Id: I78fa29f3bd9edbe59eda73242cc855aa8781bce0
Reviewed-on: https://chromium-review.googlesource.com/386703
Reviewed-by: Dylan Reid <dgreid@chromium.org>

[modify] https://crrev.com/c9f56d936f63f56186b6daaa3f0c9abed828531a/cras/src/server/cras_dsp.c
[modify] https://crrev.com/c9f56d936f63f56186b6daaa3f0c9abed828531a/cras/src/tests/alsa_io_unittest.cc
[modify] https://crrev.com/c9f56d936f63f56186b6daaa3f0c9abed828531a/cras/src/server/cras_iodev.h
[modify] https://crrev.com/c9f56d936f63f56186b6daaa3f0c9abed828531a/cras/src/tests/iodev_unittest.cc
[modify] https://crrev.com/c9f56d936f63f56186b6daaa3f0c9abed828531a/cras/src/server/cras_iodev.c
[modify] https://crrev.com/c9f56d936f63f56186b6daaa3f0c9abed828531a/cras/src/server/cras_alsa_io.c

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/adhd/+/91e434c4cb91fdcc90fe893258d3c109ce6069c8

commit 91e434c4cb91fdcc90fe893258d3c109ce6069c8
Author: Cheng-Yi Chiang <cychiang@chromium.org>
Date: Wed Sep 21 10:40:53 2016

CRAS: dsp_ini: Add a swap_lr plugin before sink plugin

Add a swap_lr plugin before sink plugin and handle the port changes.
With this change, user does not need to change dsp.ini for left right
swap function.

BUG= chromium:645051 
TEST=cras_test_client --swap_left_right 8:1:1 on samus and check left
and right channel are swapped. There is no change in dsp ini file.

Signed-off-by: Cheng-Yi Chiang <cychiang@chromium.org>
Change-Id: Ifbb6823218d48bd87487d7d8f8d1b02a16bf9c8f
Reviewed-on: https://chromium-review.googlesource.com/387588

[modify] https://crrev.com/91e434c4cb91fdcc90fe893258d3c109ce6069c8/cras/src/tests/cras_dsp_pipeline_unittest.cc
[modify] https://crrev.com/91e434c4cb91fdcc90fe893258d3c109ce6069c8/cras/src/tests/dsp_ini_unittest.cc
[modify] https://crrev.com/91e434c4cb91fdcc90fe893258d3c109ce6069c8/cras/src/server/cras_dsp_ini.c

Verified in ChromeOS ToT Build 8848.0.0 / 55.0.2874.0 on Minnie.