On Thu, 2015-09-03 at 10:14 +0200, Clemens Ladisch wrote:
Koro Chen wrote:
On Thu, 2015-09-03 at 09:08 +0200, Takashi Iwai wrote:
On Thu, 03 Sep 2015 05:20:54 +0200, Koro Chen wrote:
When we use a ping-ping buffer in capture, and if hw_ptr reported at IRQ is a little smaller than period_size:
|xxxxxxxxxxxxxxxxxxxxxxxxxxxx--|-----------------------------| hw_ptr < period_size
How this happens? The period size is the size where irq (or wakeup) wakes up for read/write. Why the driver wakes up even if there is no enough data?
Yes it is odd to what we would normally expect. Due to our HW design, when irq comes, audio HW actually has collected a full period of data, but there is a buffer between the audio HW and memory, so at that moment some samples are still in the buffer, not on the memory.
Please note that ALSA (both the kernel and the userspace API) requires that after a period interrupt, the _complete_ period has been read from/ written to the memory buffer. This is needed because the interrupt is the mechanism that synchronizes software and the DMA controller. (Except when using the NO_PERIOD_WAKUP flag, but you cannot rely on the software using it.)
Typically, any buffering is done inside the DMA controller, which also issues interrupts, so this problem should not happen with correctly working hardware.
(On PCI systems, writes to system memory can be buffered, but if the interrupt handler does a read from a device register, the PCI memory ordering rules ensure that all DMA accesses started before the interrupt are finished before the read.)
How does your hardware work? I guess that whatever component does the buffering is independent of the component that generates interrupts, and it does not enforce any memory ordering either? And there isn't a mechanism to flush the buffer?
Yes, your guess is correct...our IRQ hardware is just a separated timer which is not related to any memory control. I am not sure if there is any way to manipulate the buffer and I will check it. Thanks for your advice!
Regards, Clemens _______________________________________________ Alsa-devel mailing list Alsa-devel@alsa-project.org http://mailman.alsa-project.org/mailman/listinfo/alsa-devel