linux-kernel - Re: [PATCH 10/16] dma-mapping: Add flags to dma_map

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Date:   Sun, 2 May 2021 17:32:02 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        <linux-kernel@...r.kernel.org>, <linux-nvme@...ts.infradead.org>,
        <linux-block@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        <linux-mm@...ck.org>, <iommu@...ts.linux-foundation.org>
CC:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 10/16] dma-mapping: Add flags to dma_map_ops to indicate
 PCI P2PDMA support

On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
> Add a flags member to the dma_map_ops structure with one flag to
> indicate support for PCI P2PDMA.
> 
> Also, add a helper to check if a device supports PCI P2PDMA.
> 
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> ---
>   include/linux/dma-map-ops.h |  3 +++
>   include/linux/dma-mapping.h |  5 +++++
>   kernel/dma/mapping.c        | 18 ++++++++++++++++++
>   3 files changed, 26 insertions(+)
> 
> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
> index 51872e736e7b..481892822104 100644
> --- a/include/linux/dma-map-ops.h
> +++ b/include/linux/dma-map-ops.h
> @@ -12,6 +12,9 @@
>   struct cma;
>   
>   struct dma_map_ops {
> +	unsigned int flags;
> +#define DMA_F_PCI_P2PDMA_SUPPORTED     (1 << 0)
> +

Can we move this up and out of the struct member area, so that it looks
more like this:

/*
  * Values for struct dma_map_ops.flags:
  *
  * DMA_F_PCI_P2PDMA_SUPPORTED: <documentation here...this is a good place to
  * explain exactly what this flag is for.>
  */
#define DMA_F_PCI_P2PDMA_SUPPORTED     (1 << 0)

struct dma_map_ops {
	unsigned int flags;


>   	void *(*alloc)(struct device *dev, size_t size,
>   			dma_addr_t *dma_handle, gfp_t gfp,
>   			unsigned long attrs);
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 50b8f586cf59..c31980ecca62 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -146,6 +146,7 @@ int dma_mmap_attrs(struct device *dev, struct vm_area_struct *vma,
>   		unsigned long attrs);
>   bool dma_can_mmap(struct device *dev);
>   int dma_supported(struct device *dev, u64 mask);
> +bool dma_pci_p2pdma_supported(struct device *dev);
>   int dma_set_mask(struct device *dev, u64 mask);
>   int dma_set_coherent_mask(struct device *dev, u64 mask);
>   u64 dma_get_required_mask(struct device *dev);
> @@ -247,6 +248,10 @@ static inline int dma_supported(struct device *dev, u64 mask)
>   {
>   	return 0;
>   }
> +static inline bool dma_pci_p2pdma_supported(struct device *dev)
> +{
> +	return 0;

Should be:
	
	return false;

> +}
>   static inline int dma_set_mask(struct device *dev, u64 mask)
>   {
>   	return -EIO;
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 923089c4267b..ce44a0fcc4e5 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -573,6 +573,24 @@ int dma_supported(struct device *dev, u64 mask)
>   }
>   EXPORT_SYMBOL(dma_supported);
>   
> +bool dma_pci_p2pdma_supported(struct device *dev)
> +{
> +	const struct dma_map_ops *ops = get_dma_ops(dev);
> +
> +	/* if ops is not set, dma direct will be used which supports P2PDMA */
> +	if (!ops)
> +		return true;
> +
> +	/*
> +	 * Note: dma_ops_bypass is not checked here because P2PDMA should
> +	 * not be used with dma mapping ops that do not have support even
> +	 * if the specific device is bypassing them.
> +	 */
> +
> +	return ops->flags & DMA_F_PCI_P2PDMA_SUPPORTED;

Wow, rather unusual combination of things in order decide this. It feels
a bit over-complicated to have flags and ops and a bool function all
dealing with the same 1-bit answer, but there is no caller shown here,
so I'll have to come back to this after reviewing subsequent patches.

thanks,
-- 
John Hubbard
NVIDIA

> +}
> +EXPORT_SYMBOL_GPL(dma_pci_p2pdma_supported);
> +
>   #ifdef CONFIG_ARCH_HAS_DMA_SET_MASK
>   void arch_dma_set_mask(struct device *dev, u64 mask);
>   #else
>