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.
