linux-kernel - Re: [PATCH 07/15] dmaengine: dw: revisit data

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Date:	Mon, 25 Jan 2016 10:31:30 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Viresh Kumar <vireshk@...nel.org>,
	Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"dmaengine\@vger.kernel.org" <dmaengine@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	Dan Williams <dan.j.williams@...el.com>,
	"devicetree\@vger.kernel.org" <devicetree@...r.kernel.org>,
	"linux-snps-arc\@lists.infradead.org" 
	<linux-snps-arc@...ts.infradead.org>,
	"linux-arm-kernel\@lists.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 07/15] dmaengine: dw: revisit data_width property

Andy Shevchenko <andriy.shevchenko@...ux.intel.com> writes:

> On Mon, 2016-01-25 at 07:32 +0000, Vineet Gupta wrote:
>> On Monday 25 January 2016 12:55 AM, Mans Rullgard wrote:
>> > From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
>> > 
>> > 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@...ux.intel.com>
>> > Signed-off-by: Mans Rullgard <mans@...sr.com>
>> > ---
>> > 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).
>
> While this comment is good, I have still note that using non-power of 2
> values will not break anything. Least power of two number will be used
> in that case. So, means I would suggest to replace 'always' by 'better
> to be' or something like that.

Although the code rounds down, the hardware actually works in powers of
two, and it's better to document this.

-- 
Måns Rullgård