ASoC: qcom: q6apm-dai: add SCM buffer assignment for mDSP platforms

On platforms such as Qualcomm Shikra, audio is served by the modem DSP
(mDSP) which runs in a stage-2 protected context. Unlike ADSP targets
where SMMU-mapped system RAM is directly accessible, the mDSP cannot
reach the PCM buffers unless they are explicitly SCM-assigned to the
appropriate Virtual Machine IDs (VMIDs). Without this assignment, audio
does not function on these platforms.

At probe time, the driver reads the GPR domain_id from the parent APM
device to determine which DSP serves audio. When domain_id matches
GPR_DOMAIN_ID_ADSP the existing SMMU/iommus path is taken and no new
code is exercised. When domain_id matches GPR_DOMAIN_ID_MODEM, the
driver verifies that qcom_scm is available (deferring otherwise) and
that iommus is absent (the two mechanisms are mutually exclusive), then
enables the SCM assignment path.

In the SCM path the driver parses the optional memory-region entries in
DT. The first entry (memory-region[0] / audio_heap_mem) is the
control-path carveout used by the DSP firmware for command traffic; since
the mDSP operates on stage-2 protected memory, this carveout must be
SCM-assigned at probe time itself for the firmware to function. It is
SCM-assigned to HLOS (retained as source owner, RW) plus MSS_MSA and
LPASS (consumer VMIDs, both RW) and restored to HLOS-only ownership via
devm_add_action_or_reset() at device removal. The second entry
(memory-region[1]) is the data-path buffer pool from which per-stream
DMA buffers are carved out. This pool is attached via
of_reserved_mem_device_init_by_idx() so that PCM buffers allocate
directly from the carveout instead of system RAM.

The data-path pool (memory-region[1]) is SCM-assigned to the consumer
VMIDs as a single whole-pool operation at probe time, rather than
per-stream in pcm_new()/compr_open(). qcom_scm_assign_mem() consumes an
entry in a small fixed-size TZ memory-protection table that is shared
platform-wide; assigning individual per-stream slices (up to
Q6APM_POOL_MAX_STREAMS times) exhausts that table and hangs the SMC
call, which was observed as a crash while bringing up the sound card on
Shikra. Since the whole pool is already accessible to the mDSP once
assigned, pcm_new()/pcm_free()/compr_open()/compr_free() skip the
per-buffer SCM assign/unassign entirely when the data-path pool is in
use (has_reserved_mem), and only fall back to per-buffer assignment when
use_scm_assign is set without a data-path pool present.

Compressed audio streams follow the same pattern: the DMA buffer
allocated in compr_open() is SCM-assigned immediately after allocation
and unassigned in compr_free() before the buffer is freed, unless it was
carved from the pre-assigned data-path pool.

The VMIDs are static per SoC and hardcoded in the driver (HLOS, MSS_MSA,
LPASS) rather than read from DT, following the upstream pattern used by
rmtfs_mem and qcom_q6v5_pas. Buffer constraints are capped at
reserved_buf_size when the data-path pool is present, and
snd_pcm_set_fixed_buffer_all() is used for both paths so the carveout is
not subject to the preallocate_dma module parameter.

All new code paths are gated on use_scm_assign (false when domain_id is
not GPR_DOMAIN_ID_MODEM), ensuring existing ADSP/iommus targets are
completely unaffected.

Co-developed-by: Mohit Sharma <mohit.sharma@oss.qualcomm.com>
Signed-off-by: Mohit Sharma <mohit.sharma@oss.qualcomm.com>
Signed-off-by: Ajay Kumar Nandam <ajay.nandam@oss.qualcomm.com>
Link: https://patch.msgid.link/20260923-vmid-v4-v6-4-3d61a0319b1c@oss.qualcomm.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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