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Date:   Mon, 18 Feb 2019 17:28:24 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Sowjanya Komatineni <skomatineni@...dia.com>
Cc:     Jonathan Hunter <jonathanh@...dia.com>,
        Timo Alho <talho@...dia.com>,
        "wsa@...-dreams.de" <wsa@...-dreams.de>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i2c@...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 04:13:46PM +0000, Sowjanya Komatineni 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
> 
> Thierry/Jonathan,
> 
> Design wise we support but due to known hw bugs we defeatured
> multi-master mode. TRM docs also says no multi master support for
> T210, T186 and multi-master is supported only on T194.
> 
> T210 also has multi master enabled in driver. We should disable that
> also.

Okay, I think it makes sense to split this into two patches:

1) remove master FIFO support on Tegra186 because that leads to a
   crash

2) remove multi-master support on Tegra210 and Tegra186 with a commit
   message that explains that it was defeatured because of known bugs

Thierry

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