linux-kernel - Re: [PATCH V1] i2c: tegra: fix tegra186 hw supported features

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Date:   Mon, 18 Feb 2019 15:50:02 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     Sowjanya Komatineni <skomatineni@...dia.com>, talho@...dia.com,
        wsa@...-dreams.de, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH V1] i2c: tegra: fix tegra186 hw supported features

On Mon, Feb 18, 2019 at 09:34:38AM +0000, Jon Hunter wrote:
> 
> On 18/02/2019 08:42, Thierry Reding wrote:
> > On Sat, Feb 16, 2019 at 08:33:07AM -0800, Sowjanya Komatineni wrote:
> >> Tegra186 does not support multi-master mode and also there is no
> >> master fifo control register.
> >>
> >> This patch fixes supported features of Tegra186 and prevents
> >> crashing during boot as master fifo control register are not
> >> present on Tegra186 and prior.
> >>
> >> Signed-off-by: Sowjanya Komatineni <skomatineni@...dia.com>
> >> ---
> >>  drivers/i2c/busses/i2c-tegra.c | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > We really should've caught this earlier. Jon, let's think about ways to
> > make it easier to catch these things in the future on our test system.
> 
> Indeed. This has fixed the boot regression I was seeing over the weekend
> on -next for Tegra186. However, I am bit confused here, because when I
> look at the Tegra186 TRM it states that the I2C supports multi-master
> mode which disagrees with this patch. Furthermore, it seems odd that
> Tegra210 and Tegra194 would support multi-master mode but Tegra186 does
> not. So is this really correct?

I would expect at least the missing master FIFO registers to cause a
crash on Tegra186. The internal architecture specification also says
that multi-master is supported.

Sowjanya: do you have other documents that suggest we don't support
multi-master on Tegra186 specifically? Or is this to work around an
issue specific to Tegra186?

As it is this contradicts documentation, so we need to either fix the
commit to remove only master FIFO support or we need to fix the docs
to reflect reality.

Thierry

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