Re: [PATCH] Improve pci_alloc_consistent wrapper on preemptive kernels

[Jeff Garzik <jgarzik@pobox.com>]

Andi Kleen wrote:
> This is a minor optimization for the pci_alloc_consistent wrapper for
> the generic dma API. When the kernel is compiled preemptive the caller
> can decide if the allocation needs to be GFP_KERNEL or GFP_ATOMIC.
[...]
> +/* Would be better to move this out of line. It's already quite big. */
>  static inline void *
>  pci_alloc_consistent(struct pci_dev *hwdev, size_t size,
>  		     dma_addr_t *dma_handle)
>  {
> -	return dma_alloc_coherent(hwdev == NULL ? NULL : &hwdev->dev, size, dma_handle, GFP_ATOMIC);
> +	return dma_alloc_coherent(hwdev == NULL ? NULL : &hwdev->dev, size, dma_handle, preempt_atomic() ? GFP_ATOMIC : GFP_KERNEL);
>  }


I do agree with the patch, but I have two worries:

1) Changing from GFP_ATOMIC to <something else> may break code
2) Conversely from #1, I also worry why GFP_ATOMIC would be needed at 
all.  I code all my drivers to require that pci_alloc_consistent() be 
called from somewhere that is allowed to sleep.

	Jeff