[U-Boot] [RFC] ARM: rmobile: create DT memory nodes for R8A7795 3.0 and newer

[Marek Vasut]

On 06/19/2018 09:11 AM, Laurent Pinchart wrote:
> Hi Geert,
> 
> On Tuesday, 19 June 2018 09:58:59 EEST Geert Uytterhoeven wrote:
>> On Tue, Jun 19, 2018 at 4:15 AM Laurent Pinchart wrote:
>>> On Sunday, 17 June 2018 03:08:02 EEST Marek Vasut wrote:
>>>> On 06/16/2018 05:44 PM, Laurent Pinchart wrote:
>>>>> On Saturday, 16 June 2018 02:42:30 EEST Marek Vasut wrote:
>>>>>> On 06/16/2018 01:21 AM, Laurent Pinchart wrote:
>>>>>>> On Friday, 15 June 2018 15:00:31 EEST Marek Vasut wrote:
>>>>>>>> On 06/15/2018 01:43 PM, Marek Vasut wrote:
> 
> [snip]
> 
>>>>>>>>>>> Obvious design question is -- since you're adding new SMC call
>>>>>>>>>>> anyway, can't the call just return the memory layout table
>>>>>>>>>>> itself, so that it won't be duplicated both in U-Boot and ATF ?
>>>>>>>>>>
>>>>>>>>>> My gut feeling was to go with the smallest interface possible.
>>>>>>>>>
>>>>>>>>> But this doesn't scale. The API here uses some ad-hoc constants to
>>>>>>>>> identify memory layout tables which have to be encoded both in ATF
>>>>>>>>> and U-Boot, both of which must be kept in sync.
>>>>>>>>>
>>>>>>>>> The ATF already has those memory layout tables, it's only a matter
>>>>>>>>> of passing them to U-Boot. If you do just that, the ad-hoc
>>>>>>>>> constants and encoding of tables into U-Boot goes away and in fact
>>>>>>>>> simplifies the design.
>>>>>>>>>
>>>>>>>>> Yet, I have to wonder if ATF doesn't already contain some sort of
>>>>>>>>> standard SMC call to get memory topology. It surprises me that it
>>>>>>>>> wouldn't.
>>>>>>>>
>>>>>>>> In fact, Laurent (CCed) was solving some similar issue with lossy
>>>>>>>> decomp and I think this involved some passing of memory layout
>>>>>>>> information from ATF to U-Boot too, or am I mistaken ?
>>>>>>>
>>>>>>> That's correct, ATF stores information about the memory layout at a
>>>>>>> fixed address in system memory, and U-Boot can read it.
>>>>>>
>>>>>> Well, that sounds good ! Maybe we can avoid adding SMC call
>>>>>> altogether then? :)
>>>>>
>>>>> I'd prefer that, yes.
>>>>>
>>>>> Let's not duplicate the mechanism used to pass FCNL information from
>>>>> ATF to U- Boot, but instead create a data table format that can store
>>>>> all the information we need, in an easily extensible way.
>>>>>
>>>>> To see how the mechanism is implemented for FCNL, search for 47FD7000
>>>>> in the Renesas ATF sources
>>>>> (git://github.com/renesas-rcar/arm-trusted-firmware.git).
>>>>
>>>> For everyone involved, can you explain what FCNL is ? ;-)
>>>
>>> FCNL is Frame Compression Near Lossless. It's a way to reduce memory
>>> bandwidth by transparent compression and decompression of video frames.
>>> Compression is handled by an IP core called FCP, and decompression is
>>> handled by the DRAM controller. ATF programs the DRAM controller with
>>> ranges of memory addresses that will be dynamically decompressed. The
>>> registers containing those ranges are accessible in secure mode only, so
>>> neither U-Boot nor Linux can read them. That's why ATF has to pass the
>>> information to U-Boot, in order to add the ranges as reserved memory in
>>> DT.
>>>
>>>> Any yes, I agree this sounds good. I had a discussion on the U-Boot IRC
>>>> about passing the memory configuration around and the result is
>>>> basically the same -- pass a table from ATF to U-Boot. If there's
>>>> already something, great.
>>
>> Pass a "table"? Or an FDT containing the /memory nodes?
>> The latter allows for easier future extension.
> 
> ATF passes a table to U-Boot, and U-Boot then updates the FDT accordingly 
> before starting Linux.

At this point, U-Boot does not parse any such table. It could be some
fork does that, but mainline does not. But that code could (and probably
should) be added.

I don't think ATF has FDT support, but I might be wrong. And if ATF can
pass DT fragment or some simple DT, that'd be neat.

-- 
Best regards,
Marek Vasut