linux-kernel - Re: [PATCH 2/2] of/fdt: skip unflattening of disabled nodes

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Date:   Sun, 8 Oct 2017 15:57:51 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     Rob Herring <robh@...nel.org>, devicetree@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Nicolas Pitre <nicolas.pitre@...aro.org>
Subject: Re: [PATCH 2/2] of/fdt: skip unflattening of disabled nodes

On 10/03/17 09:18, Rob Herring wrote:
> For static DT usecases, we don't need the disabled nodes and can skip
> unflattening. This saves a significant amount of RAM in memory constrained
> cases. In one example on STM32F469, the RAM usage goes from 118K to 26K.
> 
> There are a few cases in the kernel that modify the status property
> dynamically. These all are changes from enabled to disabled, depend on
> OF_DYNAMIC or are not FDT based (PDT based).
> 
> Cc: Nicolas Pitre <nicolas.pitre@...aro.org>
> Cc: Frank Rowand <frowand.list@...il.com>
> Signed-off-by: Rob Herring <robh@...nel.org>
> ---
> For more background, see this presentation from Nico:
> 
> https://connect.linaro.org/resource/sfo17/sfo17-100/
> 
>  drivers/of/fdt.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index f8c39705418b..efe91c6856a0 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -396,6 +396,10 @@ static int unflatten_dt_nodes(const void *blob,
>  		if (WARN_ON_ONCE(depth >= FDT_MAX_DEPTH))
>  			continue;
>  
> +		if (!IS_ENABLED(CONFIG_OF_DYNAMIC) &&
> +		    !of_fdt_device_is_available(blob, offset))
> +			continue;
> +
>  		if (!populate_node(blob, offset, &mem, nps[depth],
>  				   &nps[depth+1], dryrun))
>  			return mem - base;
> 

Hi Rob,

I strongly support the idea of this patch, but there may be an
issue we have to resolve.  I'm pretty sure we had talked about
the issue a long time ago, and it has been sitting on my todo
list.

We have two sets of node traversal macros and functions.  One
set honors the status property, and the other ignores it.  If
I recall our previous discussion properly, we want the normal
usage to honor the status property and only a tiny (or maybe
non-existent) set of locations to be allowed to ignore the
status property.

A rough sense of how often the status property is honored or
not is:

   $ git grep for_each_child_of_node | wc -l
   293
   $ git grep of_get_next_child | wc -l
   103

   $ git grep for_each_available_child_of_node | wc -l
   106
   $ git grep of_get_next_available_child | wc -l
   20

Many of the cases where the status flag is ignored will not
actually encounter a node that is not available, so many of
the cases where status is not checked could currently be
checking status.

And just for completeness, there are a number of standalone
checks for whether a node is available:

   $ git grep of_device_is_available | wc -l
   128

It will be a pain to manually check all of the sites that
ignore the status property, but that task should be done.

In the mean time, maybe we could flush out the few cases
that currently depend on ignoring the status property by

   - making for_each_child_of_node() and of_get_next_child()
     actually check for valid status

   - provide a temporary (one or two kernel release)
     CONFIG option to allow the old behavior for
     for_each_child_of_node() and of_get_next_child()
     just in case we miss any locations that need to
     be fixed

   - fix up the few places in core device tree code that
     actually need to ignore status (if such places exist)

In the end, the *_available_*() interfaces should be
removed, because the normal behavior of node traversal
should be to only traverse nodes that are available.

-Frank