[PATCH] rpi: always set fdt_addr with firmware-provided FDT address

[Mauro Salvini]

Hi Matthias,

On 29/09/21 13:41, Matthias Brugger wrote:
> Hi Mauro,
> 
> On 29/09/2021 12:14, Mauro Salvini wrote:
>> Hi Matthias
>>
>> On 15/09/21 13:16, mbrugger at suse.com (Matthias Brugger) wrote:
>>> Hi Mauro,
>>>
>>> On 07/06/2021 11:27, Mauro Salvini wrote:
>>>> On 12/05/21 14:39, Mauro Salvini wrote:
>>>>> Raspberry firmware prepares the FDT blob in memory at an address
>>>>> that depends on both the memory size and the blob size [1].
>>>>> After commit ade243a211d6 ("rpi: passthrough of the firmware 
>>>>> provided FDT
>>>>> blob") this FDT is passed to kernel through fdt_addr environment 
>>>>> variable,
>>>>> handled in set_fdt_addr() function in board file.
>>>>>
>>>>> When u-boot environment is persistently saved, if a change happens
>>>>> in loaded FDT (e.g. for a new overlay applied), firmware produces a 
>>>>> FDT
>>>>> address different from the saved one, but u-boot still use the saved
>>>>> one because set_fdt_addr() function does not overwrite the fdt_addr
>>>>> variable. So, for example, if there is a script that uses fdt 
>>>>> commands for
>>>>> e.g. manipulate the bootargs, boot hangs with error
>>>>>
>>>>> libfdt fdt_check_header(): FDT_ERR_BADMAGIC
>>>>>
>>>>> Removing the fdt_addr variable in saved environment allows to boot.
>>>>>
>>>>> With this patch set_fdt_addr() function always overwrite fdt_addr 
>>>>> value.
>>>>>
>>>>> [1] https://www.raspberrypi.org/forums//viewtopic.php?f=107&t=134018
>>>>>
>>>
>>> First of all sorry for the very late reply.
>>> I'm hesitant to apply this patch, basically because it can break 
>>> other setups
>>> where people load a custom DTB to fdt_addr.
>>>
>>> I wonder why you can't erase fdt_addr from your persistent storage. 
>>> There is a
>>> command called eraseenv that should to the job.
>>
>> Sorry me too for the late reply.
>>
>> So your suggestion is to erase the fdt_addr variable from the 
>> environment each time one needs to "refresh" it (one example could be 
>> the situation that I ponted out).
>>
>> Yes, this could be the solution, but the need to delete the fdt_addr 
>> variable when e.g. one changes the dtb loaded by rpi firmware should 
>> be documented somewhere.
>>
> 
> Hm, maybe I didn't understand the problem. My understanding is, that 
> when you save the environment with saveenv, you also save the fdt_addr. 
> And that's a problem because if you add a overlay later, the fdt_addr 
> changed, which will not be reflected. So my question was if you couldn't 
> just delete the fdt_addr variable from you saved environment so that 
> when lateron you load overlays, you won't hit the problem. >
> My understanding was that you are setting a custom environment for your 
> boards, but later your customers might add a overlay via e.g. config.txt 
> and that will break booting the system.
> 
> But from your response it seems thats not what you are experiencing. Or 
> do you change the DTB loaded from FW in the U-Boot shell?


Maybe I wasn't too clear in my explanation ;-)

At every boot, u-boot executes set_fdt_addr() function. At the very 
first boot, if nobody has changed the default u-boot environment built 
in u-boot, adding a custom fdt_addr variable, this function saves the 
fdt_addr variable in persistent u-boot env. Variable contains the 
address where RPI firmware has loaded the dtb, passed to u-boot by RPI 
firmware.
This variable is not changed anymore by u-boot itself (to change it, you 
have to use u-boot cli). If you still use the same dtb, loaded by the 
RPI firmware, it's not a problem, because the fdt address computed by 
RPI firmware is always the same at every boot, and equal to the one 
saved in persistent env. If you delete the variable from saved env, at 
next boot it will be re-created in persistent env, with the value from 
RPI firmware.

Suppose that after some time one changes the config.txt and adds an overlay.

On the next boot, RPI firmware applies the overlay, and the new fdt 
address could be different from the one computed until last boot (why it 
changes, I don't know).
But set_fdt_addr() function does not update the value in env, because 
fdt_addr is set yet.
Then, u-boot uses the fdt from a wrong address, and, if it executes a 
script to manupulate e.g. bootargs, you got the error.

Applying the patch, the variable will be saved at every boot, the the 
value saved is always the right one. Conversely, as you pointed out, a 
fdt_addr variable set in the persistent environment by the user will be 
overwritten.

Then, the best option should be to write somewhere in the documentation 
that, if one loads the dtb using RPI firmware instead of u-boot, after 
changing config.txt or the dtb itself, the fdt_addr variable in u-boot 
environment must be deleted.

Feel free to require more clarifications if needed.

Thank you

Mauro

> 
> Regards,
> Matthias
> 
>> Thanks, regards
>> Mauro
>>
>>>
>>> Regards,
>>> Matthias
>>>
>>>>> Signed-off-by: Mauro Salvini <m.salvini at koansoftware.com>
>>>>> Cc: C?dric Schieli <cschieli at gmail.com>
>>>>> Cc: Matthias Brugger <mbrugger at suse.com>
>>>>> ---
>>>>> ? board/raspberrypi/rpi/rpi.c | 3 ---
>>>>> ? 1 file changed, 3 deletions(-)
>>>>>
>>>>> diff --git a/board/raspberrypi/rpi/rpi.c b/board/raspberrypi/rpi/rpi.c
>>>>> index df52a4689f..611013471e 100644
>>>>> --- a/board/raspberrypi/rpi/rpi.c
>>>>> +++ b/board/raspberrypi/rpi/rpi.c
>>>>> @@ -318,9 +318,6 @@ static void set_fdtfile(void)
>>>>> ?? */
>>>>> ? static void set_fdt_addr(void)
>>>>> ? {
>>>>> -??? if (env_get("fdt_addr"))
>>>>> -??????? return;
>>>>> -
>>>>> ????? if (fdt_magic(fw_dtb_pointer) != FDT_MAGIC)
>>>>> ????????? return;
>>>>>
>>>>
>>>>
>>>> Hi all,
>>>>
>>>> kind ping.
>>>>
>>>> Regards
>>>>
>>
>>
>