linux-kernel - Re: [PATCH 1/7] k3dma: Fix hisi burst clipping

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Message-ID: <CALAqxLW0vr5mAyyrozooL=arPn=gs4m69_k=7-kYopN4K7_1xw@mail.gmail.com>
Date:	Thu, 4 Aug 2016 10:36:32 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andy Green <andy.green@...aro.org>,
	Zhangfei Gao <zhangfei.gao@...aro.org>,
	Jingoo Han <jg1.han@...sung.com>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Mark Brown <broonie@...nel.org>, Andy Green <andy@...mcat.com>
Subject: Re: [PATCH 1/7] k3dma: Fix hisi burst clipping

On Thu, Aug 4, 2016 at 6:08 AM, Vinod Koul <vinod.koul@...el.com> wrote:
> On Fri, Jul 29, 2016 at 03:40:46PM -0700, John Stultz wrote:
>> On Sun, Jul 24, 2016 at 12:25 AM, Vinod Koul <vinod.koul@...el.com> wrote:
>> > On Wed, Jul 20, 2016 at 08:53:03PM -0700, John Stultz wrote:
>> >> From: Andy Green <andy.green@...aro.org>
>> >>
>> >> Max burst len is a 4-bit field, but at the moment it's clipped with
>> >> a 5-bit constant... reduce it to that which can be expressed
>> >
>> > Maybe we should GENMASK() etc to avoid these errors..
>>
>> Not sure I follow what you're thinking here... can you clarify a bit?
>
> I am assuming the 4bit field was a mistake by orignal author. Using GENMASK,
> BIT etc macro helps you to avoid those as one would look at datasheet and
> say this is specfied as bit 5 thru 9, so let me say GENMASK(5, 0) rather
> than a typo which missed 5th bit.
>
> But ofcourse if the error was due to some other reason then this one doesnt
> help.

It was more that in taking the cfg->src/dst_maxburst value and setting
it as a value that can be represented in 4 bits for the hardware:
        if ((maxburst == 0) || (maxburst > 16))
                val = 16;
        else
                val = maxburst - 1;

However, the logic error is trying to set the value to 16 if maxburst
is larger then 16, when 16 is 5 bits, when 15 is the largest value we
can express in 4 bits.

So I'm not sure how GENMASK() would be particularly useful (I guess
using val = GENMASK(4,0) instead of val = 15?). But I'm not sure that
makes the code any clearer.

thanks
-john