Messages in this thread | | | Subject | Re: "Consolidate get_dma_ops" breaks Xen on ARM | From | Julien Grall <> | Date | Thu, 13 Apr 2017 15:04:19 +0100 |
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Hi Stefano,
Sorry for the late answer.
On 12/04/17 00:39, Stefano Stabellini wrote: > On Tue, 11 Apr 2017, Catalin Marinas wrote: >> On Tue, Apr 11, 2017 at 01:43:28PM +0100, Julien Grall wrote: >>> On 11/04/17 02:14, Bart Van Assche wrote: >>>> On 04/10/17 17:31, Stefano Stabellini wrote: >>>>> I think the reason is that, as you can see, if (dev && dev->dma_ops), >>>>> dev->dma_ops is returned, while before this changes, xen_dma_ops was >>>>> returned on Xen on ARM. >>>>> >>>>> Unfortunately DMA cannot work properly without using the appropriate >>>>> xen_dma_ops. See drivers/xen/swiotlb-xen.c and arch/arm/xen/mm.c for >>>>> more details. (The problem is easy to spot, but I wasn't CC'ed on the >>>>> patch.) >>>>> >>>>> I don't know how to solve this problem without introducing some sort of >>>>> if (xen()) in include/linux/dma-mapping.h. >>>> >>>> Sorry but I don't have access to an ARM development system. Does your >>>> comment apply to dev == NULL only, dev != NULL only or perhaps to both? >>>> If your comment applies to dev != NULL only, can you check whether >>>> adding something like set_dma_ops(dev, get_arch_dma_ops(NULL)) to the >>>> appropriate ARM arch_setup_dma_ops() function is sufficient? >>> >>> If I understand correctly, set_dma_ops will replace dev->dma_ops with >>> Xen DMA ops. >>> >>> However, Xen DMA ops will need in some places to call the device >>> specific DMA ops (see __generic_dma_ops(...)). So I think replacing >>> dev->dma_ops is not a solution here. >>> >>> The hackish patch below is fixing the problem for both ARM64 and ARM32. >>> >>> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h >>> index 0977317c6835..43a73ddeec7a 100644 >>> --- a/include/linux/dma-mapping.h >>> +++ b/include/linux/dma-mapping.h >>> @@ -174,6 +174,8 @@ int dma_mmap_from_coherent(struct device *dev, struct vm_area_struct *vma, >>> #include <asm/dma-mapping.h> >>> static inline const struct dma_map_ops *get_dma_ops(struct device *dev) >>> { >>> + if (xen_initial_domain()) >>> + return xen_dma_ops; >>> if (dev && dev->dma_ops) >>> return dev->dma_ops; >>> return get_arch_dma_ops(dev ? dev->bus : NULL); >> >> If we do this, I guess there is no need to check for >> xen_initial_domain() in the get_arch_dma_ops() function. Anyway, this >> hunk would break the other architectures since xen_dma_ops is only >> defined for arm and arm64. >> >>> It is not nice as this is common code, but I can't find a better solution >>> so far. Any opinions? >> >> A different hack would be to avoid the generic get_dma_ops >> implementation on arm with some #ifdef hacks above. >> >> Yet another way would be for dom0 to always set dev->dma_ops to >> xen_dma_ops and preserve the real dma_ops somewhere under dev->archdata. >> You could intercept the arch_setup_dma_ops() function for this or use >> bus_register_notifier() (though I think the former is easier). The Xen >> code making use of the real dma_ops would have to dig them out from >> dev->archdata. > > This is a good suggestion, Catalin. Thank you. See below. Is that what > you have in mind? Julien could you test it, please? If it is the right > approach, I'll submit the patch properly and rename __generic_dma_ops to > xen_generic_dma_ops or something.
This patch is fixing the bug I encountered.
Cheers,
-- Julien Grall
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