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

[Simon Glass]

Hi Stephen,

On 23 February 2016 at 13:33, Stephen Warren <swarren at wwwdotorg.org> wrote:
> 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).

Didn't this get discussed to death in the Linux mailing list with the
result that platform data was abolished in favour of device tree? From
my perspective:

- the relevant configuration is mostly in one place
- we can share it with Linux
- it is easier to maintain a few text files than dispersed platform data
- it permits easy run-time configuration, avoiding the need for
multiple builds for trivial differences
- it converts to platform data fairly easily at run-time, so most of
the code can still deal with that
- it is easy to have base SoC data that is expanded/overridden by board data
- the configuration can be listed and queried easily, by U-Boot at
run-time, or by build systems
- device tree is a well-understood format with robust tools

I suspect others have done a much more thoughtful and persuasive analysis.

If you want to pick up on these points I suggest starting a new thread!

>
>>> 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.

In that case a C structure seems fine to me.

Regards,
Simon