Re: [PATCH v2 2/3] ASoC: qcom: q6apm-dai: add VMID-based SCM assignment for mDSP buffers

[Ajay Kumar Nandam <ajay.nandam@oss.qualcomm.com>]

On 8/27/2026 4:22 AM, Mark Brown wrote:
> On Wed, Aug 26, 2026 at 12:10:13AM +0530, Ajay Kumar Nandam wrote:
>> 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.
> 
>> -		ret = snd_pcm_set_fixed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV, component->dev, size);
>> +		if (pdata->has_reserved_mem)
>> +			ret = snd_pcm_set_managed_buffer_all(pcm,
>> +							     SNDRV_DMA_TYPE_DEV, component->dev,
>> +							     pdata->reserved_buf_size,
>> +							     pdata->reserved_buf_size);
> 
> Does this do the right thing if the ALSA core preallocate_dma module
> param is set to 0 for some reason?  I'm not clear why you would...

Good catch.  With preallocate_dma=0 the managed-buffer path defers
allocation to stream open, but the reserved-memory carveout is a
fixed-size pool that must always be mapped upfront — lazy allocation
from the system page allocator would bypass the SCM-assigned region
entirely.

Will switch back to snd_pcm_set_fixed_buffer_all() for the
reserved-memory path in v3; the carveout is fixed by nature and
should not be subject to the preallocate knob.

Thanks & Regards
Ajay Kumar Nandam