[PATCH] soc: tegra: Add Tegra132 SoC to the DT list of Tegra SoCs

Mark Rutland mark.rutland at arm.com
Fri Jan 16 07:07:49 PST 2015

Hi Paul,

/me opens a delicious can of Lumbricus terrestris

My comments below aren't so much about this patch alone, but more about
the entire strategy with drivers/soc and the kind of code living there.

On Fri, Jan 16, 2015 at 11:39:13AM +0000, Paul Walmsley wrote:
> Add "nvidia,tegra132" as a compatible string that denotes a Tegra SoC.
> Signed-off-by: Paul Walmsley <paul at pwsan.com>
> Cc: Paul Walmsley <pwalmsley at nvidia.com>
> Cc: Stephen Warren <swarren at wwwdotorg.org>
> Cc: Thierry Reding <thierry.reding at gmail.com>
> Cc: Alexandre Courbot <gnurou at gmail.com>
> ---
>  drivers/soc/tegra/common.c | 1 +
>  1 file changed, 1 insertion(+)
> diff --git a/drivers/soc/tegra/common.c b/drivers/soc/tegra/common.c
> index a71cb74..a952986 100644
> --- a/drivers/soc/tegra/common.c
> +++ b/drivers/soc/tegra/common.c
> @@ -15,6 +15,7 @@ static const struct of_device_id tegra_machine_match[] = {
>  	{ .compatible = "nvidia,tegra30", },
>  	{ .compatible = "nvidia,tegra114", },
>  	{ .compatible = "nvidia,tegra124", },
> +	{ .compatible = "nvidia,tegra132", },
>  	{ }
>  };

I'm rather worried by this.

The only user of this table is soc_is_tegra(), which seems to guard
several probe paths in drivers which are DT driven. Given that, I don't
see why this is necessary at all, because the presence of the
appropriate nodes should be sufficient to handle the early return from
an initcall.

More worryingly, if soc_is_tegra() returns true but the node isn't
found, the drivers decide to do things. If tegra_pmc_early_init passes
soc_is_tegra() (because we happened to match "nvidia,tegra132"), but
doesn't find a DTB entry for the pmc, it then pokes an assumed
hard-coded physical address.

Assuming things because of the _lack_ of a node has been a major pain
point on 32-bit as DTBs advanced and code was changed, resulting in boot
breaking or DTB compatibility breaking over time as old and new kernels
made conflicting assumptions.

I don't want to see arm64 kernels making assumptions in that manner (nor
would I like to see it in new code for 32-bit ARM); it always leads to
boot breakage, and it's completely avoidable. The only thing we can know
is what is described in the DTB.

So NAK to this addition while soc_is_tegra is in use in that way, and
NAK to any DTS patches adding any of the strings that would cause
soc_is_tegra to return true on arm64.

There's also the worrying pattern of initcalls that look for particular
compatible strings -- why can't these use the existing DT probing
infrastructure rather than rolling their own? It would be nice to not
accumulate initcalls that only make sense on one platform.


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