linux-kernel - Re: CONFIG_DMA_NOOP

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Date:   Mon, 16 Oct 2017 09:12:26 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Marian Mihailescu <marian.mihailescu@...laide.edu.au>,
        Randy Dunlap <rdunlap@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Bart Van Assche <bart.vanassche@...disk.com>,
        iommu@...ts.linux-foundation.org, Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: CONFIG_DMA_NOOP_OPS breaks ARM arch

+ Robin and Christoph

On 16/10/17 06:27, Marian Mihailescu wrote:
> I am using 4.14-rc4 with a patch on top that includes
> arch/arm/include/asm/dma-mapping.h in a module.
> 
> I have MMU enabled, so
> select DMA_NOOP_OPS if !MMU
> does nothing for me, and I get a compile error because dma_noop_ops is unknown.

Can you post an error message here, please?

> 
> Maybe I should include linux/dma-mapping.h?

Where to include? In your driver or what?

> 
> Thanks for the quick reply.

with CONFIG_MMU compiler should optimise out dma_noop_ops in:

return IS_ENABLED(CONFIG_MMU) ? &arm_dma_ops : &dma_noop_ops;

What toolchain are you using?

Cheers
Vladimir

> 
> 
> On Mon, Oct 16, 2017 at 2:28 PM, Randy Dunlap <rdunlap@...radead.org> wrote:
>> On 10/15/17 20:29, Randy Dunlap wrote:
>>> On 10/15/17 20:27, Randy Dunlap wrote:
>>>> On 10/15/17 19:27, Marian Mihailescu wrote:
>>>>> After commit 7844572c633964c864d9f32dc3f2a8ffe5d70371, dma_noop_ops
>>>>> are built only for architectures that use it.
>>>>>
>>>>> For ARM architecture, CONFIG_DMA_NOOP_OPS is not selected, and cannot
>>>>> be selected.
>>
>> What kernel version are you looking at?
>> I see that it is selected:
>>
>> --- a/arch/arm/Kconfig
>> +++ b/arch/arm/Kconfig
>> @@ -22,6 +22,7 @@ config ARM
>>         select CLONE_BACKWARDS
>>         select CPU_PM if (SUSPEND || CPU_IDLE)
>>         select DCACHE_WORD_ACCESS if HAVE_EFFICIENT_UNALIGNED_ACCESS
>> +       select DMA_NOOP_OPS if !MMU
>>         select EDAC_SUPPORT
>>         select EDAC_ATOMIC_SCRUB
>>         select GENERIC_ALLOCATOR
>>
>>
>> That's in commit ID 1c51c429f30ea10428337f3a33c12059ba59f668 from May 24, 2017.
>>
>>>>> However, arch/arm/include/asm/dma-mapping.h is referencing dma_noop_ops:
>>>>>
>>>>> static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
>>>>> {
>>>>> return IS_ENABLED(CONFIG_MMU) ? &arm_dma_ops : &dma_noop_ops;
>>>>> }
>>>>>
>>>>> I will let a maintainer suggest the best resolution for this :)
>>>>>
>>>>
>>>> add Bart and iommu mailing list.
>>>>
>>>
>>> and add Vladimir.
>>>
>>>
>>
>>
>> --
>> ~Randy