[RFC/RFT][PATCH v2] dma-mapping: set default segment_boundary_mask to ULONG_MAX

[Nicolin Chen]

Hi Robin/Christoph,

This v2 was sent a while ago. I know that we had a concern,
yet will we have a closure whether merging it or not?

Thanks!
Nic

On Mon, Apr 06, 2020 at 02:06:43PM -0700, Nicolin Chen wrote:
> The default segment_boundary_mask was set to DMA_BIT_MAKS(32)
> a decade ago by referencing SCSI/block subsystem, as a 32-bit
> mask was good enough for most of the devices.
> 
> Now more and more drivers set dma_masks above DMA_BIT_MAKS(32)
> while only a handful of them call dma_set_seg_boundary(). This
> means that most drivers have a 4GB segmention boundary because
> DMA API returns a 32-bit default value, though they might not
> really have such a limit.
> 
> The default segment_boundary_mask should mean "no limit" since
> the device doesn't explicitly set the mask. But a 32-bit mask
> certainly limits those devices capable of 32+ bits addressing.
> 
> So this patch sets default segment_boundary_mask to ULONG_MAX.
> 
> Signed-off-by: Nicolin Chen <nicoleotsuka at gmail.com>
> ---
> Changelog:
> v1->v2
>  * Followed Robin's comments to revise the commit message by
>    dropping one paragraph of not-entirely-true justification
>    (no git-diff level change, so please ack if you tested v1)
> 
>  include/linux/dma-mapping.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 330ad58fbf4d..ff8cefe85f30 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -736,7 +736,7 @@ static inline unsigned long dma_get_seg_boundary(struct device *dev)
>  {
>  	if (dev->dma_parms && dev->dma_parms->segment_boundary_mask)
>  		return dev->dma_parms->segment_boundary_mask;
> -	return DMA_BIT_MASK(32);
> +	return ULONG_MAX;
>  }
>  
>  static inline int dma_set_seg_boundary(struct device *dev, unsigned long mask)
> -- 
> 2.17.1
>