linux-kernel - Re: [PATCH V6 1/3] dma-mapping: Introduce device_is

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Message-ID: <5e792b21-1d63-9200-b36a-c423385e2a2e@arm.com>
Date:   Thu, 21 Sep 2017 11:47:30 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Huacai Chen <chenhc@...ote.com>, Christoph Hellwig <hch@....de>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Fuxin Zhang <zhangfx@...ote.com>, linux-kernel@...r.kernel.org,
        "James E . J . Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH V6 1/3] dma-mapping: Introduce device_is_coherent() as a
 helper

On 19/09/17 09:52, Huacai Chen wrote:
> We will use device_is_coherent() as a helper function, which will be
> used in the next patch.
> 
> There is a MIPS-specific plat_device_is_coherent(), but we need a more
> generic solution, so add and use a new function pointer in dma_map_ops.

I think we're heading in the right direction with the series, but I
still don't like this patch. I can pretty much guarantee that driver
authors *will* abuse a generic device_is_coherent() API to mean "I can
skip other DMA API calls and just use virt_to_phys()".

I think it would be far better to allow architectures to provide their
own override of dma_get_cache_alignment(), and let the coherency detail
remain internal to the relevant arch implementations.

[...]
> @@ -697,6 +698,15 @@ static inline void *dma_zalloc_coherent(struct device *dev, size_t size,
>  }
>  
>  #ifdef CONFIG_HAS_DMA
> +static inline int device_is_coherent(struct device *dev)
> +{
> +	const struct dma_map_ops *ops = get_dma_ops(dev);
> +	if (ops && ops->device_is_coherent)
> +		return ops->device_is_coherent(dev);
> +	else
> +		return 1;    /* compatible behavior */

That is also quite scary - if someone now adds a new
dma_get_cache_alignemnt() call and dutifully passes a non-NULL device,
they will now get back an alignment of 1 on all non-coherent platforms
except MIPS: hello data corruption.

Robin.

> +}
> +
>  static inline int dma_get_cache_alignment(void)
>  {
>  #ifdef ARCH_DMA_MINALIGN
>