Messages in this thread | | | Date | Wed, 7 Aug 2019 08:14:02 +0200 | From | Christoph Hellwig <> | Subject | Re: [PATCH] dma-mapping: fix page attributes for dma_mmap_* |
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On Tue, Aug 06, 2019 at 05:45:03PM +0100, Russell King - ARM Linux admin wrote: > We could have used a different approach, making all IO writes contain > a "drain write buffer" instruction, and map DMA memory as "buffered", > but as there were no Linux barriers defined to order memory accesses > to DMA memory (so, for example, ring buffers can be updated in the > correct order) back in those days, using the uncached/unbuffered mode > was the sanest and most reliable solution.
Absolutely makes sense so far.
> > > The other really weird things is that in arm32 > > > pgprot_dmacoherent incudes the L_PTE_XN bit, which from my understanding > > > is the no-execture bit, but pgprot_writecombine does not. This seems to > > > not very unintentional. So minus that the whole DMA_ATTR_WRITE_COMBІNE > > > seems to be about flagging old arm specific drivers as having the proper > > > barriers in places and otherwise is a no-op. > > > > I think it only matters for Armv7 CPUs, but yes, we should probably be > > setting L_PTE_XN for both of these memory types. > > Conventionally, pgprot_writecombine() has only been used to change > the memory type and not the permissions. Since writecombine memory > is still capable of being executed, I don't see any reason to set XN > for it. > > If the user wishes to mmap() using PROT_READ|PROT_EXEC, then is there > really a reason for writecombine to set XN overriding the user? > > That said, pgprot_writecombine() is mostly used for framebuffers, which > arguably shouldn't be executable anyway - but who'd want to mmap() the > framebuffer with PROT_EXEC?
Well, I was mostly taking about DMA_ATTR_WRITE_COMBINE, which really should include the NX bit even if pgprot_writecombine doesn't, right?
> > > - make DMA_ATTR_WRITE_COMBINE a no-op and schedule it for removal, > > > thus removing the last instances of arch_dma_mmap_pgprot > > > > All sounds good to me, although I suppose 32-bit Arm platforms without > > CONFIG_ARM_DMA_MEM_BUFFERABLE may run into issues if DMA_ATTR_WRITE_COMBINE > > disappears. Only one way to find out... > > Looking at the results of grep, I think only OMAP2+ and Exynos may be > affected.
As you mentioned later we also have the dma_alloc_wc wrapper, and a single instance of dma_alloc_writecombine.
Exynos looks like purely ARM v7 from Kconfig, so it shouldn't even be affected.
> However, removing writecombine support from the DMA API is going to > have a huge impact for framebuffers on earlier ARMs - that's where we > do expect framebuffers to be mapped "uncached/buffered" for performance > reasons and not "uncached/unbuffered". It's quite literally the > difference between console scrolling being usable and totally unusable. > > Given what I've said above, switching to using buffered mode for normal > DMA mappings is data-corrupting risky - as in your filesystem could get > fried. I don't think we should play fast and loose with people's data > by randomly changing that "because we'd like to", and I don't see that > screwing the console is really an option either.
Oh well. If we can't make dma_alloc_wc generally safe I fear we'll have to keep it, but maybe literally limit it to the pre ARM v6 platforms. While pretty much all callers seems platform specific, there actually are a decent number that can only work on ARM v7 or arm64.
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