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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [RFC PATCH] xen/gntdev: Stop abusing DT of_dma_configure API
On 9/26/19 1:46 PM, Robin Murphy wrote: > On 2019-09-26 11:17 am, Oleksandr Andrushchenko wrote: >> >> On 9/26/19 12:49 PM, Julien Grall wrote: >>> Hi Rob, >>> >>> >>> On 9/25/19 10:50 PM, Rob Herring wrote: >>>> As the comment says, this isn't a DT based device. of_dma_configure() >>>> is going to stop allowing a NULL DT node, so this needs to be fixed. >>> >>> And this can't work on arch not selecting CONFIG_OF and can select >>> CONFIG_XEN_GRANT_DMA_ALLOC. >>> >>> We are lucky enough on x86 because, AFAICT, arch_setup_dma_ops is just >>> a nop. >>> >> No luck is needed as [1] does nothing for those platforms not using >> CONFIG_OF >>>> >>>> Not sure exactly what setup besides arch_setup_dma_ops is needed... >>> >>> We probably want to update dma_mask, coherent_dma_mask and >>> dma_pfn_offset. >>> >>> Also, while look at of_configure_dma, I noticed that we consider the >>> DMA will not be coherent for the grant-table. Oleksandr, do you know >>> why they can't be coherent? >> The main and the only reason to use of_configure_dma is that if we don't >> then we >> are about to stay with dma_dummy_ops [2]. It effectively means that >> operations on dma-bufs >> will end up returning errors, like [3], [4], thus not making it possible >> for Xen PV DRM and DMA >> part of gntdev driver to do what we need (dma-bufs in our use-cases >> allow zero-copying >> while using graphics buffers and many more). >> >> I didn't find any better way of achieving that, but of_configure_dma... >> If there is any better solution which will not break the existing >> functionality then >> I will definitely change the drivers so we do not abuse DT ) >> Before that, please keep in mind that merging this RFC will break Xen PV >> DRM + >> DMA buf support in gntdev... >> Hope we can work out some acceptable solution, so everyone is happy > > As I mentioned elsewhere, the recent dma-direct rework means that > dma_dummy_ops are now only explicitly installed for the ACPI error > case, so - much as I may dislike it - you should get regular > (direct/SWIOTLB) ops by default again. Ah, my bad, I missed that change. So, if no dummy dma ops are to be used then I believe we can apply both changes, e.g. remove of_dma_configure from both of the drivers. > > Coherency is trickier - if the guest is allocating buffers for the PV > device, which may be shared directly with hardware by the host driver, > then the coherency of the PV device should really reflect that of the > underlying hardware to avoid potential problems. There are some cases > where the stage 2 attributes alone wouldn't be enough to correct a > mismatch. > > Robin. Thank you, Oleksandr _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel
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