Re: [PATCH v6 2/4] dmaengine: dw: revisit data_width property

[Viresh Kumar <viresh.kumar@linaro.org>]

On 25-04-16, 15:35, Andy Shevchenko wrote:
> There are several changes are done here:
> 
> - Convert the property to be in bytes
> 
> Besides this is common practice for such property the use of a value in bytes
> much more convenient than handling the encoded value.
> 
> - Rename data_width to data-width in the device tree bindings
> 
> - While here, replace dwc_fast_ffs() by __ffs()
> 
> The change leaves the support for old format as well just in case someone will
> use a newer kernel with an old device tree blob.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
>  Documentation/devicetree/bindings/dma/snps-dma.txt |  6 ++--
>  arch/arc/boot/dts/abilis_tb10x.dtsi                |  2 +-
>  arch/arm/boot/dts/spear13xx.dtsi                   |  4 +--
>  drivers/dma/dw/core.c                              | 42 ++++++----------------
>  drivers/dma/dw/platform.c                          |  5 ++-
>  include/linux/platform_data/dma-dw.h               |  2 +-
>  6 files changed, 21 insertions(+), 40 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/dma/snps-dma.txt b/Documentation/devicetree/bindings/dma/snps-dma.txt
> index c99c1ff..544b9b9 100644
> --- a/Documentation/devicetree/bindings/dma/snps-dma.txt
> +++ b/Documentation/devicetree/bindings/dma/snps-dma.txt
> @@ -13,8 +13,8 @@ Required properties:
>  - chan_priority: priority of channels. 0 (default): increase from chan 0->n, 1:
>    increase from chan n->0
>  - block_size: Maximum block size supported by the controller
> -- data_width: Maximum data width supported by hardware per AHB master
> -  (0 - 8bits, 1 - 16bits, ..., 5 - 256bits)
> +- data-width: Maximum data width supported by hardware per AHB master
> +  (in bytes, power of 2)
>  
>  
>  Optional properties:
> @@ -38,7 +38,7 @@ Example:
>  		chan_allocation_order = <1>;
>  		chan_priority = <1>;
>  		block_size = <0xfff>;
> -		data_width = <3 3>;
> +		data-width = <8 8>;
>  	};

You broke backward compatibility with earlier DTs.

What's backward compatibility ?

Consider that the DT from an earlier version of kernel is part of the bootrom of
a SoC. Now that bootrom should work just fine with any new kernel version. i.e.
old DT + new kernels should always work.

-- 
viresh