Re: [RFC PATCH 1/3] dma-mapping: make overriding GFP_* flags arch customizable

[Christoph Hellwig]

On Mon, Sep 23, 2019 at 02:34:16PM +0200, Halil Pasic wrote:
> Before commit 57bf5a8963f8 ("dma-mapping: clear harmful GFP_* flags in
> common code") tweaking the client code supplied GFP_* flags used to be
> an issue handled in the architecture specific code. The commit message
> suggests, that fixing the client code would actually be a better way
> of dealing with this.
> 
> On s390 common I/O devices are generally capable of using the full 64
> bit address space for DMA I/O, but some chunks of the DMA memory need to
> be 31 bit addressable (in physical address space) because the
> instructions involved mandate it. Before switching to DMA API this used
> to be a non-issue, we used to allocate those chunks from ZONE_DMA.
> Currently our only option with the DMA API is to restrict the devices to
> (via dma_mask and dma_mask_coherent) to 31 bit, which is sub-optimal.
> 
> Thus s390 we would benefit form having control over what flags are
> dropped.

No way, sorry.  You need to express that using a dma mask instead of
overloading the GFP flags.