On Thu, Apr 26, 2018 at 10:35:56PM -0700, Christoph Hellwig wrote: > On Thu, Apr 26, 2018 at 09:54:06PM +0000, Luis R. Rodriguez wrote: > > In practice if you don't have a floppy device on x86, you don't need ZONE_DMA, > > I call BS on that, I did not explain though that it was not me who claimed this though. The list displayed below is the result of trying to confirm/deny this, and what could be done, and also evaluating if there is *any* gain about doing something about it. But curious, on a standard qemu x86_x64 KVM guest, which of the drivers do we know for certain *are* being used from the ones listed? What about Xen guests, I wonder? > and you actually explain later why it it BS due > to some drivers using it more explicitly. Or implicitly. The list I showed is the work to show that the users of GFP_DMA on x86 is *much* more wide spread than expected from the above claim. I however did not also answer the above qemu x86_64 question, but would be good to know. Note I stated that the claim was *in practice*. > But even more importantly > we have plenty driver using it through dma_alloc_* and a small DMA > mask, and they are in use Do we have a list of users for x86 with a small DMA mask? Or, given that I'm not aware of a tool to be able to look for this in an easy way, would it be good to find out which x86 drivers do have a small mask? > - we actually had a 4.16 regression due to them. Ah what commit was the culprit? Is that fixed already? If so what commit? > > SCSI is *severely* affected: > > Not really. We have unchecked_isa_dma to support about 4 drivers, Ah very neat: * CONFIG_CHR_DEV_OSST - "SCSI OnStream SC-x0 tape support" * CONFIG_SCSI_ADVANSYS - "AdvanSys SCSI support" * CONFIG_SCSI_AHA1542 - "Adaptec AHA1542 support" * CONFIG_SCSI_ESAS2R - "ATTO Technology's ExpressSAS RAID adapter driver" > and less than a hand ful of drivers doing stupid things, which can > be fixed easily, and just need a volunteer. Care to list what needs to be done? Can an eager beaver student do it? > > That's the end of the review of all current explicit callers on x86. > > > > # dma_alloc_coherent_gfp_flags() and dma_generic_alloc_coherent() > > > > dma_alloc_coherent_gfp_flags() and dma_generic_alloc_coherent() set > > GFP_DMA if if (dma_mask <= DMA_BIT_MASK(24)) > > All that code is long gone and replaced with dma-direct. Which still > uses GFP_DMA based on the dma mask, though - see above. And that's mostly IOMMU code, on the alloc() dma_map_ops. Luis
