Am Donnerstag, 5. Juni 2003 21:53 schrieb Alan Stern:
> On Thu, 5 Jun 2003, Oliver Neukum wrote:
> > > That helps. So a buffer used for device input should be allocated
> > > separately from anything written by the CPU, whereas a buffer used for
> > > device output can be shared with other data in a buffer allocated by
> > > kmalloc, but not on the stack.
> >
> > Judging by this implementation from mips Doing so would be quite
> > inefficient.
> >
> > extern inline dma_addr_t pci_map_single(struct pci_dev *hwdev, void *ptr,
> > size_t size, int direction)
> > {
> > if (direction == PCI_DMA_NONE)
> > BUG();
> >
> > #ifndef CONFIG_COHERENT_IO
> > dma_cache_wback_inv((unsigned long)ptr, size);
> > #endif
> >
> > return virt_to_bus(ptr);
> > }
> >
> > It is probably best to allocate all buffers seperately.
>
> I don't follow you. What's inefficient about this? It looks like the
> decision of whether or not to call dma_cache_wback_inv() has nothing to do
> with what other data is stored in the same memory block with the output
> buffer. And there's nothing that indicates dma_cache_wback_inv() is less
> efficient when ptr isn't aligned on a cacheline.
No, I meant sharing it. You'd get a full hard cache miss.
Regards
Oliver
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