On Wed, Oct 26, 2022 at 10:46:46AM -0700, Linus Torvalds wrote: > I think we should just stop bending over backwards over this, and say > "if your DMA isn't coherent, it's on your driver to mark its > allocations". [...] > That hardware may then be one of the one-off strange cases, but those > people with their masochistic tendencies can take the pain of "oh, now > I need to mark my broken driver with dma_alloc()". 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). Reading up on coccinelle to see if I can make this transition easier. If not, I'll probably go back to bouncing. -- Catalin
