On Mon, 28 May 2007, Lorenz Kolb wrote:
On Sat, 26 May 2007, Joachim F?rster wrote:
First question: To implement mmap on such kind of device IO memory, I have to use snd_pcm_lib_mmap_iomem() as mmap() callback - like it is used in the rme32 driver, right?
Yes. rme32 is quite similar.
Second question: Do the following parameters make sense regarding the size of the HW ring buffer and period sizes:
.buffer_bytes_max=16*1024; .period_bytes_max=8*1024; .period_bytes_min=?? resonable value ??
32 or 64
.periods_min=2; .periods_max=?? resonable value ??
16
Furthermore, the sound controller will fire an interrupt after one period is played. Are there any arguments against these values? Any comments?
Yes, I think that the access through a PCI I/O window to a PCI device is much slower than PCI DMA transfers with bursts. I would really consider to design hardware with real DMA. Of course, if it's only experimental stuff, then don't worry.
Hi Jaroslav,
First of all: thanks for Your answer. Second: We do not have a PCI Bus. It is an embedded system with an OCP-Bus (aka OPB).
The ML403 holds a SoC with PPC and FPGA.
So the only difference between DMA with bursts and a dedicated memory (as far as we do see it) is the amount of time the bus gets occupied. With dedicated memory the system will (hopefully ;-)) do a burst write each period. So what happens in DMA case (except the more complex hardware as the DMA controller has to be written (in VHDL) as well and even more memory is located at an more complex bus (PLB - processor local bus): the bus will be occupied twice: (1) CPU writes data to memory. --> start DMA --> (2) Controller fetches data from memory. While (2) is in progress the poor CPU is "disconnected" from the bus. In an embedded system with a not too powerful CPU (PPC 405 @300MHz) this might be quite inefficient. Or did we miss something?
Probably not :-) I was speaking about standard PCI architecture in PC. Your design is quite different. Anyway, I wonder for what purpose you need such embedded platform with Linux? Perhaps, I'm too curious ;-)
Jaroslav
----- Jaroslav Kysela perex@suse.cz Linux Kernel Sound Maintainer ALSA Project, SUSE Labs