linux-kernel - Re: [PATCH v6 2/4] dmaengine: dw: revisit data

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Message-ID: <CAHp75Vc=EZ1gLHpackYEAQXuTs_WCqOP1bk0SBkLJHdje3zCug@mail.gmail.com>
Date:	Wed, 27 Apr 2016 11:25:23 +0300
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Viresh Kumar <vireshk@...nel.org>,
	Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dmaengine <dmaengine@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Brown <broonie@...nel.org>,
	Vineet Gupta <vgupta@...opsys.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v6 2/4] dmaengine: dw: revisit data_width property

On Wed, Apr 27, 2016 at 9:43 AM, Viresh Kumar <viresh.kumar@...aro.org> 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@...ux.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.

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