netdev - Re: [RFC PATCH 1/3] dma-mapping: introduce a new dma api dma_addr_to_phys

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Date:   Tue, 22 Oct 2019 14:25:45 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Laurentiu Tudor <laurentiu.tudor@....com>,
        "hch@....de" <hch@....de>, "joro@...tes.org" <joro@...tes.org>,
        Ioana Ciocoi Radulescu <ruxandra.radulescu@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Ioana Ciornei <ioana.ciornei@....com>
Cc:     Leo Li <leoyang.li@....com>,
        Diana Madalina Craciun <diana.craciun@....com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Madalin-cristian Bucur <madalin.bucur@....com>
Subject: Re: [RFC PATCH 1/3] dma-mapping: introduce a new dma api
 dma_addr_to_phys_addr()

On 22/10/2019 13:55, Laurentiu Tudor wrote:
> From: Laurentiu Tudor <laurentiu.tudor@....com>
> 
> Introduce a new dma map op called dma_addr_to_phys_addr() that converts
> a dma address to the physical address backing it up and add wrapper for
> it.

I'd really love it if there was a name which could encapsulate that this 
is *only* for extreme special cases of constrained descriptors/pagetable 
entries/etc. where there's simply no practical way to keep track of a 
CPU address alongside the DMA address, and the only option is this 
potentially-arbitrarily-complex operation (I mean, on some systems it 
may end up taking locks and poking hardware).

Either way it's tricky - much as I don't like adding an interface which 
is ripe for drivers to misuse, I also really don't want hacks like 
bdf95923086f shoved into other APIs to compensate, so on balance I'd 
probably consider this proposal ever so slightly the lesser evil.

> Signed-off-by: Laurentiu Tudor <laurentiu.tudor@....com>
> ---
>   include/linux/dma-mapping.h | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 4a1c4fca475a..5965d159c9a9 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -132,6 +132,8 @@ struct dma_map_ops {
>   	u64 (*get_required_mask)(struct device *dev);
>   	size_t (*max_mapping_size)(struct device *dev);
>   	unsigned long (*get_merge_boundary)(struct device *dev);
> +	phys_addr_t (*dma_addr_to_phys_addr)(struct device *dev,
> +					     dma_addr_t dma_handle);

I'd be inclined to name the internal callback something a bit snappier 
like .get_phys_addr.

>   };
>   
>   #define DMA_MAPPING_ERROR		(~(dma_addr_t)0)
> @@ -442,6 +444,19 @@ static inline int dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
>   	return 0;
>   }
>   
> +static inline phys_addr_t dma_addr_to_phys_addr(struct device *dev,
> +						dma_addr_t dma_handle)
> +{
> +	const struct dma_map_ops *ops = get_dma_ops(dev);
> +
> +	if (dma_is_direct(ops))
> +		return (phys_addr_t)dma_handle;

Well that's not right, is it - remember why you had that namespace 
collision? ;)

Robin.

> +	else if (ops->dma_addr_to_phys_addr)
> +		return ops->dma_addr_to_phys_addr(dev, dma_handle);
> +
> +	return 0;
> +}
> +
>   void *dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
>   		gfp_t flag, unsigned long attrs);
>   void dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
> @@ -578,6 +593,12 @@ static inline unsigned long dma_get_merge_boundary(struct device *dev)
>   {
>   	return 0;
>   }
> +
> +static inline phys_addr_t dma_addr_to_phys_addr(struct device *dev,
> +						dma_addr_t dma_handle)
> +{
> +	return 0;
> +}
>   #endif /* CONFIG_HAS_DMA */
>   
>   static inline dma_addr_t dma_map_single_attrs(struct device *dev, void *ptr,
>