On Sat, 3 Mar 2018, Fredrik Noring wrote:

> Christoph, Alan,
> > If it is allocating / freeing this memory all the time in the hot path
> > it should really use a dma pool (see include/ilinux/dmapool.h).
> > The dma coherent APIs aren't really built for being called in the
> > hot path.
> hcd_buffer_free uses a combination of dma pools and dma coherent APIs:
>       ...
>       for (i = 0; i < HCD_BUFFER_POOLS; i++) {
>               if (size <= pool_max[i]) {
>                       dma_pool_free(hcd->pool[i], addr, dma);
>                       return;
>               }
>       }
>       dma_free_coherent(hcd->self.sysdev, size, addr, dma);
> Alan, can dma_free_coherent be delayed to a point when IRQs are enabled?

Yes, subject to the usual concerns about not being delayed for too 
long.  Also, some HCDs are highly memory-constrained.  I don't know if 
they use this API, but if they do then delaying a free could result in 
not enough memory being available when an allocation is needed.

Alan Stern

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