linux-kernel - Re: "Consolidate get_dma

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Date:   Tue, 11 Apr 2017 13:43:28 +0100
From:   Julien Grall <julien.grall@....com>
To:     Bart Van Assche <Bart.VanAssche@...disk.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Cc:     "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: "Consolidate get_dma_ops" breaks Xen on ARM

Hi Bart,

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);

It is not nice as this is common code, but I can't find a better solution
so far. Any opinions?

Cheers,

-- 
Julien Grall