linux-kernel - Re: [PATCH 03/32 v3] dmaengine: ste_dma40: Use the BIT macro to replace ugly '(1 << x)'s

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Date:	Wed, 24 Apr 2013 21:24:05 +0200
From:	Rabin Vincent <rabin@....in>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	arnd@...db.de, linus.walleij@...ricsson.com,
	Vinod Koul <vinod.koul@...el.com>, Dan Williams <djbw@...com>,
	Per Forlin <per.forlin@...ricsson.com>
Subject: Re: [PATCH 03/32 v3] dmaengine: ste_dma40: Use the BIT macro to
 replace ugly '(1 << x)'s

2013/4/24 Lee Jones <lee.jones@...aro.org>:
> The aim is to make the code that little more readable.
>
> Acked-by: Vinod Koul <vnod.koul@...el.com>
> Acked-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Lee Jones <lee.jones@...aro.org>

Please pay closer attention to the semantics of each usage instead of
just replacing all x left shifts of 1 by BIT(x) for "readability".

>         if (seg_max > STEDMA40_MAX_SEG_SIZE)
> -               seg_max -= (1 << max_w);
> +               seg_max -= BIT(max_w);
>
> -       if (!IS_ALIGNED(size, 1 << max_w))
> +       if (!IS_ALIGNED(size, BIT(max_w)))
>                 return -EINVAL;

Here and in all other places where the values are from cfg->data_width,
the semantic purpose of the shift is not for setting a particular bit
but instead for converting the data_width field into the data width
value in bytes.  You should not change these usages to BIT().

It would be instead better to just make the cfg->data_width as the
number of bytes and convert them to the appropriate hardware field
values when the descriptors are constructed.  That of course should be
in another patch.
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