Re: [Xenomai-help] dma in rtdm

[Jan Kiszka <jan.kiszka@domain.hid>]

[-- Attachment #1: Type: text/plain, Size: 1591 bytes --]

Harkema, G.A. wrote:
> Hallo,
> 
>  
> 
> We are currently trying to develop a realtime usb2.0 stack based on
> xenomai rtdm. In non-realtime mode we use first the kmalloc() function
> to allocate a dma buffer and use the dma_map_single() function to get
> the bus address for the dma engine of the ehci hostcontroller. This
> works fine. For use in a realtime context we replaced the kmalloc()
> function by the rtdm_malloc() function. In our logs we see that the data

rtdm_malloc does not return memory guaranteed to be usable for DMA (we
should document this somewhere). Depending on the size of your system
heap, the memory /may/ come from a pre-allocated kmalloc'ed (heap <=
128K) or from a vmalloc'ed block.

> transfer take place, but the dma buffer was not filled with the
> (received)data. We don't know where the data in the memory is stored. Is
> this the right way for using dma in realtime context ? I guess not., but
> what is the right way. Who can help us with this problem.

I'm convinced that ahead-of-time allocation of URBs, transfer
descriptors, and other buffers is the way to go. The application /
high-level driver requesting some service from the UHCI controller
should take care of providing them. That way you will gain full
determinism, because even rtdm_malloc is not guaranteed to work always.
Low memory or fragmentation caused by other users of this service may
bite you in complex scenarios.

Jan


PS: Is your design now based on usb4rt or do you work on a separate
solution? I haven't heard any news from you on this.


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 250 bytes --]