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

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Date:	Fri, 23 Jan 2015 16:27:07 +0300
From:	Dmitry Osipenko <digetx@...il.com>
To:	Thierry Reding <thierry.reding@...il.com>
CC:	Alexandre Courbot <gnurou@...il.com>,
	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

23.01.2015 12:45, Thierry Reding пишет:
> On Thu, Jan 22, 2015 at 08:18:34PM +0300, Dmitry Osipenko wrote:
>> 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.
>
> What does I2C_FIFO_STATUS have to do with anything?
>
> My point was more that we already tell hardware how much data is to be
> transferred (via the packet header in tegra_i2c_xfer_msg()), hence the
> hardware shouldn't care whether the FIFO is padded with random data or
> zeros.
>
> Thierry
>
Got your point. I was thinking it's expected behavior, but now I'll elaborate 
this more.

-- 
Dmitry
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