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.
Alan Stern
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