linux-kernel - Re: [PATCH 15/15] dma-mapping: bypass indirect calls for dma-direct

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Date:   Fri, 14 Dec 2018 15:32:43 +0100
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        Robin Murphy <robin.murphy@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Keith Busch <keith.busch@...el.com>,
        Jonathan Derrick <jonathan.derrick@...el.com>,
        linux-pci@...r.kernel.org, linux-ia64@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/15] dma-mapping: bypass indirect calls for dma-direct

Hi Christoph,

On 2018-12-14 15:24, Christoph Hellwig wrote:
> On Fri, Dec 14, 2018 at 03:11:37PM +0100, Marek Szyprowski wrote:
>> Hi Christoph,
>>
>> On 2018-12-07 20:07, Christoph Hellwig wrote:
>>> Avoid expensive indirect calls in the fast path DMA mapping
>>> operations by directly calling the dma_direct_* ops if we are using
>>> the directly mapped DMA operations.
>>>
>>> Signed-off-by: Christoph Hellwig <hch@....de>
>> This breaks direct DMA on ARM64 (also todays linux-next). NULL
>> dev->dma_ops fallbacks to get_arch_dma_ops(), which in turn returns
>> non-functional &dma_dummy_ops on ARM64...
> Yeah, fallback from direct (NULL) dev->dma_ops to something else won't
> work with NULL as the indicator.
>
> Fortunately we shouldn't even need that thanks to the patch from Robin
> that explicitly set the dummy ops where needed.
>
> Can you try the patch below?

Yes, it fixes the problem.

> diff --git a/arch/arm64/include/asm/dma-mapping.h b/arch/arm64/include/asm/dma-mapping.h
> index 273e778f7de2..95dbf3ef735a 100644
> --- a/arch/arm64/include/asm/dma-mapping.h
> +++ b/arch/arm64/include/asm/dma-mapping.h
> @@ -26,11 +26,7 @@
>  
>  static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
>  {
> -	/*
> -	 * We expect no ISA devices, and all other DMA masters are expected to
> -	 * have someone call arch_setup_dma_ops at device creation time.
> -	 */
> -	return &dma_dummy_ops;
> +	return NULL;
>  }
>  
>  void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
>
>
Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland