linux-kernel - Re: [PATCH 1/2] i2c: tegra: Maintain CPU endianness

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Message-ID: <54C130EA.2050505@gmail.com>
Date:	Thu, 22 Jan 2015 20:18:34 +0300
From:	Dmitry Osipenko <digetx@...il.com>
To:	Alexandre Courbot <gnurou@...il.com>,
	Thierry Reding <thierry.reding@...il.com>
CC:	Wolfram Sang <wsa@...-dreams.de>,
	Stephen Warren <swarren@...dotorg.org>,
	Laxman Dewangan <ldewangan@...dia.com>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Bob Mottram <bob.mottram@...ethink.co.uk>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] i2c: tegra: Maintain CPU endianness

22.01.2015 19:06, Dmitry Osipenko пишет:
> 22.01.2015 18:22, Dmitry Osipenko пишет:
>> 22.01.2015 10:55, Alexandre Courbot пишет:
>>> On Thu, Jan 22, 2015 at 4:40 PM, Thierry Reding
>>> <thierry.reding@...il.com> wrote:
>>>>
>>>> Should this not technically be le32_to_cpu() since the data originates
>>>> from the I2C controller?
>>
>> No, i2c_readl returns value in CPU endianness, so it's correct. But for
>> i2c_writel should be used le32_to_cpu(), since it takes value in CPU endianness.
>> It's my overlook, V2 is coming.
>>
>>>>
>>>> Why does this have to be initialized to 0 now?
>>>
>>> I suspect this is because we are going to memcpy less than 4 bytes
>>> into it, but I cannot figure out how that memcpy if guaranteed to
>>> produce the expected result for both endiannesses.
>>>
>> That's correct. Memcpy is working with bytes, so it doesn't care about
>> endianness and produces expected result, since I2C message is char array.
>>
> I'll spend some more time reviewing, to see if nullifying should go as separate
> patch.
>
Well, I2C_FIFO_STATUS returns 8-bit value. The rest of bits very likely to be 
RAZ, however I don't see anything on it in documentation. In that case it won't 
cause any problems with LE value and nullifying is only needed for BE mode.

-- 
Dmitry
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