Nouveau - Apr 2018 - [PATCH v2 5/5] ARM: Unconditionally enable ARM_DMA_USE_IOMMU

On Wed, Apr 25, 2018 at 12:10:51PM +0200, Thierry Reding
wrote:
> From: Thierry Reding <treding at nvidia.com>
> 
> The ARM_DMA_USE_IOMMU Kconfig option has side-effects that drivers can
> not opt into but have to explicitly opt out of. This can lead to subtle
> bugs that are difficult to track down and not immediately obvious to be
> related to this Kconfig option.
> 
> To avoid this confusion, always enable the option to expose any lurking
> bugs once and allow any regressions introduced by the DMA/IOMMU code to
> be caught more quickly in the future.
> 
> Note that some drivers still use the Kconfig symbol to provide different
> code paths depending on what architecture the code runs on (e.g. 32-bit
> ARM vs. 64-bit ARM which have different and incompatible implementations
> of the DMA/IOMMU integration code), so leave the symbol in place to keep
> those drivers working.
> 
> For the long term, it is preferable to transition everyone to the
> generic DMA/IOMMU integration code in drivers/iommu/dma-iommu.c.
> 
> Signed-off-by: Thierry Reding <treding at nvidia.com>
> ---
> Changes in v2:
> - new patch
> 
>  arch/arm/Kconfig               |  2 +-
>  arch/arm/include/asm/device.h  |  6 ------
>  arch/arm/mm/dma-mapping.c      | 18 ------------------
>  drivers/iommu/Kconfig          |  7 -------
>  drivers/media/platform/Kconfig |  1 -
>  5 files changed, 1 insertion(+), 33 deletions(-)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index fa0b190f8a38..3c91de78535a 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -124,7 +124,7 @@ config NEED_SG_DMA_LENGTH
>  	bool
>  
>  config ARM_DMA_USE_IOMMU
> -	bool
> +	def_bool y
>  	select ARM_HAS_SG_CHAIN
>  	select NEED_SG_DMA_LENGTH
This doesn't work - as has recently been discussed with hch, we can't
globally enable NEED_SG_DMA_LENGTH on ARM - the ARM architecture
pre-dates the addition of the DMA length member in the scatterlist,
and not every machine supports the splitting of the DMA length from
the non-DMA length member.  Hence, this will cause a regression,
sorry.

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On Wed, Apr 25, 2018 at 11:25:11AM +0100, Russell King - ARM Linux
wrote:
> >  config ARM_DMA_USE_IOMMU
> > -	bool
> > +	def_bool y
> >  	select ARM_HAS_SG_CHAIN
> >  	select NEED_SG_DMA_LENGTH
> 
> This doesn't work - as has recently been discussed with hch, we
can't
> globally enable NEED_SG_DMA_LENGTH on ARM - the ARM architecture
> pre-dates the addition of the DMA length member in the scatterlist,
> and not every machine supports the splitting of the DMA length from
> the non-DMA length member.  Hence, this will cause a regression,
> sorry.
Agreed as-is.  However supporting it everywhere should not be much work,
I'll take a look at it.