Re: [Xen-devel] [PATCH 42/44] powerpc/cell: use the dma_supported method for ops switching

[Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>]

On Fri, 2017-06-16 at 20:10 +0200, Christoph Hellwig wrote:
> Besides removing the last instance of the set_dma_mask method this also
> reduced the code duplication.

What is your rationale here ? (I have missed patch 0 it seems).

dma_supported() was supposed to be pretty much a "const" function
simply informing whether a given setup is possible. Having it perform
an actual switch of ops seems to be pushing it...

What if a driver wants to test various dma masks and then pick one ?

Where does the API documents that if a driver calls dma_supported() it
then *must* set the corresponding mask and use that ?

I don't like a function that is a "boolean query" like this one to have
such a major side effect.

From an API standpoint, dma_set_mask() is when the mask is established,
and thus when the ops switch should happen.

Ben.

> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  arch/powerpc/platforms/cell/iommu.c | 25 +++++++++----------------
>  1 file changed, 9 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/cell/iommu.c 
> b/arch/powerpc/platforms/cell/iommu.c
> index 497bfbdbd967..29d4f96ed33e 100644
> --- a/arch/powerpc/platforms/cell/iommu.c
> +++ b/arch/powerpc/platforms/cell/iommu.c
> @@ -644,20 +644,14 @@ static void dma_fixed_unmap_sg(struct device *dev, 
> struct scatterlist *sg,
>                                  direction, attrs);
>  }
>  
> -static int dma_fixed_dma_supported(struct device *dev, u64 mask)
> -{
> -     return mask == DMA_BIT_MASK(64);
> -}
> -
> -static int dma_set_mask_and_switch(struct device *dev, u64 dma_mask);
> +static int dma_suported_and_switch(struct device *dev, u64 dma_mask);
>  
>  static const struct dma_map_ops dma_iommu_fixed_ops = {
>       .alloc          = dma_fixed_alloc_coherent,
>       .free           = dma_fixed_free_coherent,
>       .map_sg         = dma_fixed_map_sg,
>       .unmap_sg       = dma_fixed_unmap_sg,
> -     .dma_supported  = dma_fixed_dma_supported,
> -     .set_dma_mask   = dma_set_mask_and_switch,
> +     .dma_supported  = dma_suported_and_switch,
>       .map_page       = dma_fixed_map_page,
>       .unmap_page     = dma_fixed_unmap_page,
>       .mapping_error  = dma_iommu_mapping_error,
> @@ -952,11 +946,8 @@ static u64 cell_iommu_get_fixed_address(struct device 
> *dev)
>       return dev_addr;
>  }
>  
> -static int dma_set_mask_and_switch(struct device *dev, u64 dma_mask)
> +static int dma_suported_and_switch(struct device *dev, u64 dma_mask)
>  {
> -     if (!dev->dma_mask || !dma_supported(dev, dma_mask))
> -             return -EIO;
> -
>       if (dma_mask == DMA_BIT_MASK(64) &&
>           cell_iommu_get_fixed_address(dev) != OF_BAD_ADDR) {
>               u64 addr = cell_iommu_get_fixed_address(dev) +
> @@ -965,14 +956,16 @@ static int dma_set_mask_and_switch(struct device *dev, 
> u64 dma_mask)
>               dev_dbg(dev, "iommu: fixed addr = %llx\n", addr);
>               set_dma_ops(dev, &dma_iommu_fixed_ops);
>               set_dma_offset(dev, addr);
> -     } else {
> +             return 1;
> +     }
> +
> +     if (dma_iommu_dma_supported(dev, dma_mask)) {
>               dev_dbg(dev, "iommu: not 64-bit, using default ops\n");
>               set_dma_ops(dev, get_pci_dma_ops());
>               cell_dma_dev_setup(dev);
> +             return 1;
>       }
>  
> -     *dev->dma_mask = dma_mask;
> -
>       return 0;
>  }
>  
> @@ -1127,7 +1120,7 @@ static int __init cell_iommu_fixed_mapping_init(void)
>               cell_iommu_setup_window(iommu, np, dbase, dsize, 0);
>       }
>  
> -     dma_iommu_ops.set_dma_mask = dma_set_mask_and_switch;
> +     dma_iommu_ops.dma_supported = dma_suported_and_switch;
>       set_pci_dma_ops(&dma_iommu_ops);
>  
>       return 0;

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