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