Re: [PATCH 07/15] dmaengine: dw: revisit data_width property

[Vineet Gupta]

On Monday 25 January 2016 12:55 AM, Mans Rullgard wrote:
> From: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
>
> There are several changes are done here:
>
>  - Convert the property to be in bytes
>
>    Much more convenient than keeping encoded value.
>
>  - Use one value for all AHB masters for now
>
>    It seems in practice we have no controllers where masters have different
>    data bus width, we still might return to distinct values when there is a use
>    case.
>
>  - Rename data_width to data-width in the device tree bindings.
>
>  - While here, replace dwc_fast_ffs() by __ffs().
>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Signed-off-by: Mans Rullgard <mans@xxxxxxxxx>
> ---
> This patch changes the DT binding, so it should probably be amended for
> compatibility with old device trees.  I've included it as is since I think
> the change as such is good.
> ---
>  Documentation/devicetree/bindings/dma/snps-dma.txt |  5 ++-
>  arch/arc/boot/dts/abilis_tb10x.dtsi                |  2 +-
>  arch/arm/boot/dts/spear13xx.dtsi                   |  4 +--
>  drivers/dma/dw/core.c                              | 40 +++-------------------
>  drivers/dma/dw/platform.c                          |  8 ++---
>  drivers/dma/dw/regs.h                              |  2 +-
>  include/linux/platform_data/dma-dw.h               |  5 ++-
>  7 files changed, 16 insertions(+), 50 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/dma/snps-dma.txt b/Documentation/devicetree/bindings/dma/snps-dma.txt
> index c99c1ffac199..fe7f7710a6b4 100644
> --- a/Documentation/devicetree/bindings/dma/snps-dma.txt
> +++ b/Documentation/devicetree/bindings/dma/snps-dma.txt
> @@ -13,8 +13,7 @@ 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 (in bytes)

To the reader this suggests a value truely byte granular, but code uses ffs
implying that it is still power of 2.
Can you mention this here (....in bytes, always power of 2).

> ...
> @@ -726,10 +710,7 @@ dwc_prep_dma_memcpy(struct dma_chan *chan, dma_addr_t dest, dma_addr_t src,
>  
>  	dwc->direction = DMA_MEM_TO_MEM;
>  
> -	data_width = dw->data_width[dwc->m_master];
> -
> -	src_width = dst_width = min_t(unsigned int, data_width,
> -				      dwc_fast_ffs(src | dest | len));
> +	src_width = dst_width = __ffs(dw->data_width | src | dest | len);
> ...

-Vineet