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Subject: Re: [Xen-devel] [PATCH v2 5/9] xen/gntdev: Allow mappings for DMA
 buffers
To:     Oleksandr Andrushchenko <andr2000@gmail.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org,
        jgross@suse.com, konrad.wilk@oracle.com
Cc:     daniel.vetter@intel.com, matthew.d.roper@intel.com,
        dongwon.kim@intel.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
References: <20180601114132.22596-1-andr2000@gmail.com>
 <20180601114132.22596-6-andr2000@gmail.com>
 <64facf05-0a51-c3d9-9d3b-780893248628@oracle.com>
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From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <a172746d-7a97-159f-71a7-511b2d239089@oracle.com>
Date:   Wed, 6 Jun 2018 17:19:51 -0400
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Sender: linux-kernel-owner@vger.kernel.org
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On 06/06/2018 04:14 AM, Oleksandr Andrushchenko wrote:
> On 06/04/2018 11:12 PM, Boris Ostrovsky wrote:
>> On 06/01/2018 07:41 AM, Oleksandr Andrushchenko wrote:

>> @@ -121,8 +146,27 @@ static void gntdev_free_map(struct grant_map *map)
>>       if (map == NULL)
>>           return;
>>   +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>> +    if (map->dma_vaddr) {
>> +        struct gnttab_dma_alloc_args args;
>> +
>> +        args.dev = map->dma_dev;
>> +        args.coherent = map->dma_flags & GNTDEV_DMA_FLAG_COHERENT;
>> +        args.nr_pages = map->count;
>> +        args.pages = map->pages;
>> +        args.frames = map->frames;
>> +        args.vaddr = map->dma_vaddr;
>> +        args.dev_bus_addr = map->dma_bus_addr;
>> +
>> +        gnttab_dma_free_pages(&args);
>> +    } else
>> +#endif
>>       if (map->pages)
>>           gnttab_free_pages(map->count, map->pages);
>> +
>> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>> +    kfree(map->frames);
>> +#endif
>>
>> Can this be done under if (map->dma_vaddr) ?
>
>>   In other words, is it
>> possible for dma_vaddr to be NULL and still have unallocated frames
>> pointer?
> It is possible to have vaddr == NULL and frames != NULL as we
> allocate frames outside of gnttab_dma_alloc_pages which
> may fail. Calling kfree on NULL pointer is safe,


I am not questioning safety of the code, I would like avoid another ifdef.


> so
> I see no reason to change this code.
>>
>>>       kfree(map->pages);
>>>       kfree(map->grants);
>>>       kfree(map->map_ops);
>>> @@ -132,7 +176,8 @@ static void gntdev_free_map(struct grant_map *map)
>>>       kfree(map);
>>>   }
>>>   -static struct grant_map *gntdev_alloc_map(struct gntdev_priv
>>> *priv, int count)
>>> +static struct grant_map *gntdev_alloc_map(struct gntdev_priv *priv,
>>> int count,
>>> +                      int dma_flags)
>>>   {
>>>       struct grant_map *add;
>>>       int i;
>>> @@ -155,6 +200,37 @@ static struct grant_map
>>> *gntdev_alloc_map(struct gntdev_priv *priv, int count)
>>>           NULL == add->pages)
>>>           goto err;
>>>   +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>>> +    add->dma_flags = dma_flags;
>>> +
>>> +    /*
>>> +     * Check if this mapping is requested to be backed
>>> +     * by a DMA buffer.
>>> +     */
>>> +    if (dma_flags & (GNTDEV_DMA_FLAG_WC | GNTDEV_DMA_FLAG_COHERENT)) {
>>> +        struct gnttab_dma_alloc_args args;
>>> +
>>> +        add->frames = kcalloc(count, sizeof(add->frames[0]),
>>> +                      GFP_KERNEL);
>>> +        if (!add->frames)
>>> +            goto err;
>>> +
>>> +        /* Remember the device, so we can free DMA memory. */
>>> +        add->dma_dev = priv->dma_dev;
>>> +
>>> +        args.dev = priv->dma_dev;
>>> +        args.coherent = dma_flags & GNTDEV_DMA_FLAG_COHERENT;
>>> +        args.nr_pages = count;
>>> +        args.pages = add->pages;
>>> +        args.frames = add->frames;
>>> +
>>> +        if (gnttab_dma_alloc_pages(&args))
>>> +            goto err;
>>> +
>>> +        add->dma_vaddr = args.vaddr;
>>> +        add->dma_bus_addr = args.dev_bus_addr;
>>> +    } else
>>> +#endif
>>>       if (gnttab_alloc_pages(count, add->pages))
>>>           goto err;
>>>   @@ -325,6 +401,14 @@ static int map_grant_pages(struct grant_map
>>> *map)
>>>           map->unmap_ops[i].handle = map->map_ops[i].handle;
>>>           if (use_ptemod)
>>>               map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
>>> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
>>> +        else if (map->dma_vaddr) {
>>> +            unsigned long mfn;
>>> +
>>> +            mfn = __pfn_to_mfn(page_to_pfn(map->pages[i]));
>>
>> Not pfn_to_mfn()?
> I'd love to, but pfn_to_mfn is only defined for x86, not ARM: [1] and [2]
> Thus,
>
> drivers/xen/gntdev.c:408:10: error: implicit declaration of function
> ‘pfn_to_mfn’ [-Werror=implicit-function-declaration]
>     mfn = pfn_to_mfn(page_to_pfn(map->pages[i]));
>
> So, I'll keep __pfn_to_mfn


How will this work on non-PV x86?

-boris