Re: [PATCH 8/8] WIP: add a dma_alloc_contiguous API

[Christoph Hellwig]

On Tue, Nov 10, 2020 at 06:50:32PM +0900, Tomasz Figa wrote:
> In what terms it doesn't actually work? Last time I checked some
> platforms actually defined CONFIG_DMA_NONCOHERENT, so those would
> instead use the kmalloc() + dma_map() path. I don't have any
> background on why that was added and whether it needs to be preserved,
> though. Kieran, Laurent, do you have any insight?

CONFIG_DMA_NONCOHERENT is set on sh and mips for platforms that may
support non-coherent DMA at compile time (but at least for mips that
doesn't actually means this gets used).  Using that ifdef to decide
on using usb_alloc_coherent vs letting the usb layer map the data
seems at best odd, and if we are unlucky papering over a bug somewhere.