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

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Message-ID: <20150123094552.GD3835@ulmo>
Date:	Fri, 23 Jan 2015 10:45:53 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Dmitry Osipenko <digetx@...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

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

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