[U-Boot] [PATCH 2/9] arm64: Make full va map code more dynamic

[Stephen Warren]

On 02/23/2016 01:00 PM, Simon Glass wrote:
> Hi Stephen,
>
> On 23 February 2016 at 10:40, Stephen Warren <swarren at wwwdotorg.org> wrote:
>> On 02/23/2016 10:30 AM, Simon Glass wrote:
>>>
>>> Hi Stephen,
>>>
>>> On 23 February 2016 at 10:21, Stephen Warren <swarren at wwwdotorg.org>
>>> wrote:
>>>>
>>>> On 02/23/2016 06:17 AM, Simon Glass wrote:
>>>>>
>>>>>
>>>>> Hi Alex,
>>>>>
>>>>> On 21 February 2016 at 18:57, Alexander Graf <agraf at suse.de> wrote:
>>>>>>
>>>>>>
>>>>>> The idea to generate our pages tables from an array of memory ranges
>>>>>> is very sound. However, instead of hard coding the code to create up
>>>>>> to 2 levels of 64k granule page tables, we really should just create
>>>>>> normal 4k page tables that allow us to set caching attributes on 2M
>>>>>> or 4k level later on.
>>>>>>
>>>>>> So this patch moves the full_va mapping code to 4k page size and
>>>>>> makes it fully flexible to dynamically create as many levels as
>>>>>> necessary for a map (including dynamic 1G/2M pages). It also adds
>>>>>> support to dynamically split a large map into smaller ones when
>>>>>> some code wants to set dcache attributes.
>>>>>>
>>>>>> With all this in place, there is very little reason to create your
>>>>>> own page tables in board specific files.
>>>>
>>>>
>>>>
>>>>>>     static struct mm_region mem_map[] = CONFIG_SYS_MEM_MAP;
>>>>>
>>>>>
>>>>>
>>>>> I am not ken on the idea of using a big #define table on these boards.
>>>>> Is there not a device-tree binding for this that we can use? It is
>>>>> just a data table, We are moving to Kconfig and eventually want to
>>>>> drop the config files.
>>>>
>>>>
>>>>
>>>> I would strongly object to making the MMU setup depend on device tree
>>>> parsing. This is low-level system code that should be handled purely by
>>>> simple standalone C code.
>>>
>>>
>>> Because...?
>>
>>
>> There is literally zero benefit from putting the exact same content into DT,
>> and hence having to run significantly more code to parse DT and get back
>> exactly the same hard-coded table.
>
> We do this so that board-specific variations can be described in one
> place. In the board-specific case, there are benefits.

I'd like to see an explicit enumeration of the benefits; I'm not aware 
of any (either benefits, or such an enumeration). Board-specific data 
can just as easily (actually, more easily due to lack of need for 
parsing code) be stored in C data structures vs. stored in DT.

Or put another way, the simple fact that some data is board-specific 
does not in-and-of-itself mean there's a benefit to putting it into DT. 
To move something into DT, we should be able to enumerate some other 
benefit, such as:
- Speeds up boot time.
- Allows code to be simpler.
- Simplifies editing the data.

(Note that I don't believe any of those example potential benefits are 
actually true, but in fact are the opposite of the truth).

>> DT is not a goal in-and-of-itself. In some cases there are benefits to
>> placing configuration data outside a binary, and in those cases DT is an
>> acceptable mechanism to do that. However, any benefit from doing so derives
>> from arguments for separating the data out of the code, not because "use DT"
>> is itself a benefit.
>
> That's fine as far as it goes.
>
> The config file is not an acceptable means of providing per-board or
> per-arch configuration. If it is arch-specific and/or SoC-specific,
> but NOT board-specific then we can have it in a C table in a source
> file (not the config header) that is built into the binary. If it is
> board-specific, it must use the device tree.
>
> What category are we talking about here? Unfortunately it's not
> entirely clear from the patches and I lack the knowledge/background to
> figure it out.

I expect this data is SoC-specific. At least for Tegra in the codebase, 
that's certainly true. I believe it's true for other SoCs in the current 
codebase too. I don't expect this to change going forward, at the very 
least for Tegra.