Messages in this thread | | | Date | Thu, 13 Nov 2014 09:27:30 -0600 | From | Pierre-Louis Bossart <> | Subject | Re: [alsa-devel] Question on Compressed offload session |
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On 11/12/14, 9:02 PM, gsantosh@codeaurora.org wrote: > Hi All, > > The Question is for the compressed offload session. > > For a generic codec driver during the startup function it will set some of > the hw_constraints rule similarly like this. > > snd_pcm_hw_constraint_list(substream->runtime, 0, > SNDRV_PCM_HW_PARAM_RATE, > &constraints_12_24); > > pcm_lib.c will try to add the rule to the runtime structure by accessing > the pointers which will be initialized during opening of the session, > as The Constraints added by the codec driver will be updated in the > > struct snd_pcm_hw_constraints of runtime structure which will be part of > substream handle. > > But for the compressed offload I do not see the initialization done for HW > constraints, as done in pcm session > > 2092int snd_pcm_open_substream(struct snd_pcm *pcm, int stream, > 2093 struct file *file, > 2094 struct snd_pcm_substream **rsubstream) > > most of the existing drivers which has the hw_constraint_list code will > not be applicable for compress offload session, how to solve this?
You can't directly link physical output/input with the decoder/encoder in general. For decoders, the sample-rate may not always be known ahead of time, e.g. with AAC-SBR implicit signaling. There is no way to add constraints on open, there is an assumption that a sample-rate converter is part of the chain to take care of the difference between the output of the offloaded decoder and the back-end actual sampling frequency (same with number of channels and bit-width btw). Likewise if you encode the frequency may not be the same as what the backend provides and some SRC might be needed. -Pierre
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