linux-kernel - Re: [PATCH] spi: tegra: avoid unsigned->signed->unsigned promotion

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Message-ID: <xa1tli038ktc.fsf@mina86.com>
Date:	Mon, 02 Dec 2013 14:05:35 +0100
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Laxman Dewangan <ldewangan@...dia.com>,
	Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
	linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] spi: tegra: avoid unsigned->signed->unsigned promotion

> On Fri, Nov 29, 2013 at 06:12:44PM +0100, Michal Nazarewicz wrote:
>> u8 type, which is unsigned, is promoted to int, which is singned, when
>> doing a binary shift.  Then, on 64-bit machines, it is further promoted
>> to unsigned long which may lead to more significant half of the value
>> to be all ones.  To avoid this, explicitly promote to an unsigned type.

On Fri, Nov 29 2013, Thierry Reding wrote:
> But I don't think there's a real issue here. Even on 64-bit platforms,
> writel() will only write 32-bit registers, and therefore discard the
> upper 32 bits.
>
> I think in general perhaps a more proper fix would be to use only u32
> instead of unsigned long for these cases, since then the problem goes
> away. u32 is also the type of the value used by writel() so it makes
> perfect sense to use it.

Yes, you're absolutely right.  I dunno why I missed the fact it's used
in writel only.  I'll prepare a new patch soon(ish).

-- 
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.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
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