Re: revert dma direct internals abuse

[Thomas Hellstrom <thellstrom@vmware.com>]

Christoph,


On Mon, 2019-04-08 at 12:55 +0200, Christoph Hellwig wrote:
> Hi Linus,
> 
> unfortunately it took less than a merge window for the:
> 
> /*
>  * All the dma_direct_* declarations are here just for the indirect
> call bypass,
>  * and must not be used directly drivers!
>  */
> 
> warning in dma-mapping.h to be ignored.  This series reverts the
> offender
> to keep our API clean.
> 
> Thomas: please talk me first about your needs and I'll be happy to
> figure out
> a proper API for what you want to do.

We HAVE discussed our needs, although admittedly some of my emails
ended up unanswered.

We've as you're well aware of had a discussion with the other
subsystems doing user-space DMA-buffers wanting this functionality from
the dma api (AMD graphics and RDMA people IIRC). that is a bool that
tells us whether streaming dma mappings are coherent, and I described
in detail why we couldn't use the dma_sync_* API and
dma_alloc_coherent().

The other option we have is to just fail miserably without messages if
streaming DMA is not coherent, which I think the other drivers might
do... That's all I'm trying to avoid here. I'd much prefer to have the
dma API export this bool.

/Thomas