[PATCH v2 2/2] treewide: Add the __GFP_PACKED flag to several non-DMA kmalloc() allocations

[Christoph Hellwig]

On Thu, Oct 27, 2022 at 11:29:48PM +0100, Catalin Marinas wrote:
> The driver is not necessarily broken. The same small kmalloc() in a USB
> driver can work fine on a fully coherent platform but if that chip ends
> up on a SoC that doesn't support coherent DMA, it needs bigger kmalloc()
> alignment. The driver could check if it's coherent but that's more of an
> arch detail that the driver shouldn't care about. If we define a new API
> like dma_alloc() and drivers don't use it, that's when we can claim they
> are broken.
> 
> A further optimisation would be for dma_alloc() to take a struct device
> pointer and check dev_is_dma_coherent() before deciding to align the
> size, though this doesn't work when the allocation place cannot tell the
> destination device (e.g. alloc_skb(), though these buffers are
> cacheline-aligned already).

The other thing would be to check the dma mask so that we don't end
up bounce buffering again.