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Subject: Re: [PATCH 1/3] of/fdt: Scan the root node properties earlier
To:     Rob Herring <robh@kernel.org>
Cc:     devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <b1b9e5bc-42b7-8359-bae2-78c5cca4e6ab@gmail.com>
Date:   Thu, 6 Sep 2018 14:03:53 -0700
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Hi Rob,

On 09/05/18 14:31, Rob Herring wrote:
> On Wed, Sep 5, 2018 at 4:10 PM Frank Rowand <frowand.list@gmail.com> wrote:
>>
>> On 09/05/18 13:06, Rob Herring wrote:
>>> On Wed, Sep 5, 2018 at 1:19 PM Frank Rowand <frowand.list@gmail.com> wrote:
>>>>
>>>> On 09/05/18 04:51, Rob Herring wrote:
>>>>> On Tue, Sep 4, 2018 at 8:49 PM Frank Rowand <frowand.list@gmail.com> wrote:
>>>>>>
>>>>>> On 08/30/18 12:05, Rob Herring wrote:
>>>>>>> Scan the root node properties (#{size,address}-cells) earlier,
>>>>>>
>>>>>>                                                         ^^^^^^^
>>>>>>                              before mdesc->dt_fixup() is called
>>>>>>
>>>>>>> so that
>>>>>>> the dt_root_addr_cells and dt_root_size_cells variables are initialized
>>>>>>> and can be used.
>>>>>>                  by mdesc->dt_fixup()
>>>>>
>>>>> That's an ARM specific detail. Granted, ARM is the only caller.
>>>>
>>>> The dt_root_addr_cells and dt_root_size_cells variables are being
>>>> initialized earlier in this patch series so that of_fdt_limit_memory()
>>>> can use them.  The only caller of of_fdt_limit_memory() is
>>>> exynos_dt_fixup(), which is an mdesc->dt_fixup() function.
>>>>
>>>>
>>>>>
>>>>>>>
>>>>>>> Cc: Frank Rowand <frowand.list@gmail.com>
>>>>>>> Signed-off-by: Rob Herring <robh@kernel.org>
>>>>>>> ---
>>>>>>>  drivers/of/fdt.c | 7 ++++---
>>>>>>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>>>>>>
>>>>>> Moving early_init_dt_scan_root() to inside early_init_dt_verify()
>>>>>> puts something that has nothing to do with verifying the fdt
>>>>>> into a function whose purpose is the verify.  It hides the side
>>>>>> effect of initializing the dt_root_addr_cells and dt_root_size_cells
>>>>>> variables.
>>>>>
>>>>> It already has the side effect of setting initial_boot_params which
>>>>> every subsequent function needs.
>>>>
>>>> And that side effect should probably also be moved.
>>>
>>> So 2 functions? One to set the blob and one to verify it. Then we can
>>
>> No, I would not add yet another function.  All of these side effects are
>> an argument in favor of a single setup_machine_fdt(), as I suggested below.
>> Then all of these side effects could be in setup_machine_fdt() instead
>> of hiding them in sub-functions that are called by all of the different
>> architectures.
>>
>>
>>> just let arches decide if they want to do any verification or not.
>>>
>>> Perhaps it should be called fdt_init(blob) and then it is vague enough
>>> I can do whatever I want.
>>>
>>>>>> I suggest creating a new function early_init_dt_scan_init_pre_dt_fixup(),
>>>>>> move the chunk of code there instead of to early_init_dt_scan_nodes(),
>>>>>> and call the new function from setup_machine_fdt(), just before
>>>>>> calling  mdesc->dt_fixup().  This would be a little bit more code,
>>>>>> but more clearly showing the intent.
>>>>>
>>>>> I'm trying to reduce the number of functions arches call
>>>>
>>>> I like that goal.
>>>>
>>>>
>>>>> and renaming
>>>>> would need a bunch of arch changes. This change will also let me make
>>>>> early_init_dt_scan_root private as powerpc is the only user. I need to
>>>>> dust off a patch for that.
>>>>>
>>>>> I'd be more inclined to push exynos to remove this altogether. After
>>>>
>>>> Not a bad idea.
>>>>
>>>>> all, if they claim their bindings are unstable, they can't really
>>>>> claim their bootloader is stable/fixed.
>>>>
>>>> It seems that this series is showing us that maybe the three architecture
>>>> specific (arc, arm, arm64) versions of setup_machine_fdt() should be
>>>> consolidated so that we have consistent behavior for FDT.
>>>>
>>>> If we had a single setup_machine_fdt() then some of he hidden side
>>>> effects of functions called by setup_machine_fdt() could instead
>>>> be hoisted into setup_machine_fdt().
>>>
>>> Those functions are all quite a bit different. ARM matches the machine
>>> desc while arm64 doesn't have any such thing. How the DTB gets mapped
>>> into virtual space also varies.
>>
>> I argue that they _should be_ made to be more alike than different.  You
>> have only pointed out two differences.  Of those, the mapping could be
>> cleanly handled by an mdesc-> callback.  (I would have to look at the
>> match to see if that could be handled easily, but I would expect so.)
> 
> The machine desc is in no way common and only used on a few arches
> (and not even common across those arches). So there's no way the core
> DT code can just call a mdesc callback without addressing making that
> common first. And callbacks are just another way to call arch specific
> functions which are another thing I'm trying to remove.
> 
>> On the other hand, in a previous reply you considered removing
>> of_fdt_limit_memory(), which is only used for an exynos fixup.  If
>> you do that, then patch 1 disappears, and we can continue to
>> sweep under the rug the side effects that you reminded me of
>> with patch 1.
> 
> I'm inclined to just drop the patch. Seemed like a simple clean-up and
> I'm not interested in doing more right now (did you look at the stack
> of stuff in dt/testing branch). Maybe someone else will care (spoiler:
> they won't).

I would agree with just dropping patch 1 and 2.  Patch 3 is still fine.

> 
> Rob
>