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

[Halil Pasic]

On Mon, 23 Sep 2019 17:21:17 +0200
Christoph Hellwig <hch@xxxxxx> wrote:

> 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.

Thanks for your feedback and sorry for the delay. Can you help me figure
out how can I express that using a dma mask? 

IMHO what you ask from me is frankly impossible.

What I need is the ability to ask for  (considering the physical
address) 31 bit addressable DMA memory if the chunk is supposed to host
control-type data that needs to be 31 bit addressable because that is
how the architecture is, without affecting the normal data-path. So
normally 64 bit mask is fine but occasionally (control) we would need
a 31 bit mask.

Regards,
Halil