linux-kernel - Re: [PATCH] of: cache phandle nodes to decrease cost of of_find_node_by

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Date:   Thu, 1 Feb 2018 08:26:57 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Chintan Pandya <cpandya@...eaurora.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] of: cache phandle nodes to decrease cost of of_find_node_by_phandle()

On Thu, Feb 1, 2018 at 8:24 AM, Rob Herring <robh+dt@...nel.org> wrote:
> On Wed, Jan 31, 2018 at 3:43 PM, Frank Rowand <frowand.list@...il.com> wrote:
>> On 01/31/18 12:05, frowand.list@...il.com wrote:
>>> From: Frank Rowand <frank.rowand@...y.com>
>>>
>>> Create a cache of the nodes that contain a phandle property.  Use this
>>> cache to find the node for a given phandle value instead of scanning
>>> the devicetree to find the node.  If the phandle value is not found
>>> in the cache, of_find_node_by_phandle() will fall back to the tree
>>> scan algorithm.
>>>
>>> The cache is initialized in of_core_init().
>>>
>>> The cache is freed via a late_initcall_sync().
>>>
>>> Signed-off-by: Frank Rowand <frank.rowand@...y.com>
>>> ---
>>>
>>> Some of_find_by_phandle() calls may occur before the cache is
>>> initialized or after it is freed.  For example, for the qualcomm
>>> qcom-apq8074-dragonboard, 11 calls occur before the initialization
>>> and 80 occur after the cache is freed (out of 516 total calls.)
>>>
>>>
>>>  drivers/of/base.c       | 85 ++++++++++++++++++++++++++++++++++++++++++++++---
>>>  drivers/of/of_private.h |  5 +++
>>>  drivers/of/resolver.c   | 21 ------------
>>>  3 files changed, 86 insertions(+), 25 deletions(-)
>>
>> Some observations....
>>
>> The size of the cache for a normal device tree would be a couple of
>> words of overhead for the cache, plus one pointer per devicetree node
>> that contains a phandle property.  This will be less space than
>> would be used by adding a hash field to each device node.  It is
>> also less space than was used by the older algorithm (long gone)
>> that added a linked list through the nodes that contained a
>> phandle property.
>>
>> This is assuming that the values of the phandle properties are
>> the default ones created by the dtc compiler.  In the case
>> where a very large phandle property value is hand-coded in
>> a devicetree source, the size of the cache is capped at one
>> entry per node.  In this case, a little bit of space will be
>> wasted -- but this is just a sanity fallback, it should not
>> be encountered, and can be fixed by fixing the devicetree
>> source.
>
> I don't think we should rely on how dtc allocates phandles. dtc is not
> the only source of DeviceTrees. If we could do that, then lets make
> them have some known flag in the upper byte so we have some hint for
> phandle values. 2^24 phandles should be enough for anyone.TM
>
> Your cache size is also going to balloon if the dtb was built with
> '-@'. Since you walk the tree for every phandle, it is conceivable
> that you could make things slower.

Disregard the 2nd statement. Only 1 walk is not going to register.