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

From: Andy Shevchenko
Date: Wed Apr 27 2016 - 04:25:33 EST


On Wed, Apr 27, 2016 at 9:43 AM, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxx>
>> ---
>> 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.

How?

>
> 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.

Yes, the property name is slightly different as meaning.

If we find data-width property driver will use it, otherwise it takes
old name and converts variable to be in bytes in the driver.

--
With Best Regards,
Andy Shevchenko