Carlo Caione wrote:
let's say that our application sends uning snd_pcm_writei() a buffer that is exactly 64 bytes and that our hwoff (appl_ofs)
The hardware and the software (appl) pointers are independent.
is pointing to a memory location 0 in the DMA area. When this buffer is taken from the SoC sound driver, the .copy callback writes this buffer as follow: 32 bytes (left channel) to memory locations from *hwoff to *hwoff+31, and 32 bytes (right channel) for *hwoff+32 to *hwoff+63. Problem is when my buffer is not multiple of 64 bytes, for example if the buffer size is < 64. If I receive a buffer of lenght 32 byte than my DMA memory is filled as follows: 16 bytes from *hwoff to *hwoff+15 and 16 bytes from *hwoff+32 to *hwoff+47, leaving 32 bytes in the block not initialized.
These bytes are written later. And if they are not written fast enough, you get an underrun. This is no difference from any other sound card.
In some way I should always guarantee that the size of the buffer passed to my .copy callback is multiple of 64 bytes.
The .copy callback just copies samples to memory; this is possible with any alignment.
The DMA runs asynchronously, and uses its own pointer.
Regards, Clemens