[U-Boot] [PATCH] serial: ns16550: Fix serial output on Tegra186

[Thierry Reding <thierry.reding@gmail.com>]

On Fri, Sep 30, 2016 at 10:47:38AM +0100, Paul Burton wrote:
> On Friday, 30 September 2016 17:53:38 BST Alexandre Courbot wrote:
> > On 09/30/2016 05:46 PM, Thierry Reding wrote:
> > > From: Thierry Reding <treding@nvidia.com>
> > > 
> > > For Tegra186 there are currently no UART clocks wired up in device tree.
> > > This exposes a regression introduced in commit 50fce1d5d874 ("serial:
> > > ns16550: Support clocks via phandle"), which causes the p2771-0000-500
> > > board (and probably any Tegra186-based board as well) to fail to boot.
> > > 
> > > The reason is that if no clocks property exists, then clk_get_by_index()
> > > returns -ENOENT (via fdtdec_parse_phandle_with_args()) rather than
> > > -ENODEV as the above-mentioned commit expects.
> > > 
> > > Fix this by checking for the right error code.
> > 
> > Tested-by: Alexandre Courbot <acourbot@nvidia.com>
> > 
> > I sent a similar patch ~10 minutes before this one, but Thierry's commit
> > message is clearer than mine (and his handling of -ENODEV probably more
> > correct as well), so let's go with this version!
> 
> 
> Hi Thierry & Alexandre,
> 
> Apologies for the breakage!
> 
> If a DT contains a clock & the UART node references it by phandle then I 
> believe clk_get_by_index() could return -ENODEV via 
> uclass_get_device_by_of_offset & uclass_find_device_by_of_offset. So we 
> probably need to handle both -ENOENT for the "no clocks property" case & -
> ENODEV for the "clocks property but no driver" case, as Alexandre's patch 
> does?

Indeed. I thought I had checked all possible return values, but I've
obviously been blind. Alex's patch looks more correct, though I'm
beginning to wonder if there's a better way to achieve this than keep
adding whatever error codes happen to be special cases.

How about we just leave out the else completely? At this point it seems
like any failure to obtain a valid clock would best be dealt with by
falling back to clock-frequency or the one specified in the config.

Are there even other errors besides -ENODEV, -ENOENT and -ENOSYS that we
could get here?

Thierry
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