2016-06-05 18:33 GMT+08:00 Alan Young <consult.awy@xxxxxxxxx>: > On 05/06/16 02:14, Raymond Yau wrote: > > the point only increment in DMA brust size , so it is not sample accurate > > > https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/tree/include/linux/dmaengine.h > > * @DMA_RESIDUE_GRANULARITY_BURST: Residue is updated after each transferred > > * burst. This is typically only supported if the hardware has a progress * register of some sort (E.g. a register with the current read/write address * or a register with the amount of bursts/beats/bytes that have been * transferred or still need to be transferred). > > if the driver point callback does not read from hardware register , it should use > > DMA_RESIDUE_GRANULARITY_DESCRIPTOR: Residue reporting is not support. The * DMA channel is only able to tell whether a descriptor has been completed or * not, which means residue reporting is not supported by this channel. The * residue field of the dma_tx_state field will always be 0. > > > > Yes, I understand that. And that is exactly my point. Because of this a > pcm_status() result is only accurate to a granularity of period in most > cases. > > In fact, some drivers, for example imx sdma, declare > DMA_RESIDUE_GRANULARITY_BURST accuracy because sometimes they may have such > an accuracy but in practice, at least for audio, they only actually achieve > period accuracy. > > Regardless of what value of DMA_RESIDUE_GRANULARITY_xxx that a driver > claims to support, it is not really defined how fine a burst might be. So > the end result is, from the point of view of audio, that the resulting > position obtained by the pointer() call is pretty inaccurate. Hence my > proposal to attempt to improve the accuracy of the pcm_status() result > given the above constraints. > http://git.alsa-project.org/?p=alsa-lib.git;a=commitdiff;h=cf40ea169aad366b222283f431addafea6327149;hp=4bdb09126a32feb4394eaeb1d400d87e7c968770 HDA has hardware specific feature (audio wallclock) for the timestamp and period wakeup can be disabled only a few pci drivers which read the residue value from hardware register (e.g. hda-intel, oxygen , .) you have to measure the granularity since the unit may be different for usb, pcie and firewire sound card as the application is based on the pointer for read/write, you still need to use small period size and CPU power if you want to determine the value returned by snd_pcm_rewindable is safe or not > > _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
