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

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Date:	Fri, 23 Jan 2015 17:52:10 +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 16:27, Dmitry Osipenko пишет:
> 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.
>
Gaahh! I'm sure it wasn't working before! I'll make more testing and send v3 
without "val = 0", if all will be fine.

-- 
Dmitry
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