Re: [PATCH] of/kexec: save pa of initial_boot_params for arm64 and use it at kexec

[Usama Arif <usamaarif642@gmail.com>]

On 23/10/2024 14:40, Rob Herring wrote:
> On Mon, Oct 7, 2024 at 10:30 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>
>>
>>
>> On 07/10/2024 15:39, Rob Herring wrote:
>>> On Mon, Oct 7, 2024 at 9:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>>>
>>>>
>>>>
>>>> On 04/10/2024 01:03, Rob Herring wrote:
>>>>> On Thu, Oct 03, 2024 at 12:38:40PM +0100, Usama Arif wrote:
>>>>>>  __pa() is only intended to be used for linear map addresses and using
>>>>>> it for initial_boot_params which is in fixmap for arm64 will give an
>>>>>> incorrect value. Hence stash the physical address when it is known at
>>>>>> boot time and use it at kexec time instead of converting the virtual
>>>>>> address using __pa().
>>>>>>
>>>>>> Reported-by: Breno Leitao <leitao@debian.org>
>>>>>> Suggested-by: Mark Rutland <mark.rutland@arm.com>
>>>>>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>>>>>> Fixes: ac10be5cdbfa ("arm64: Use common of_kexec_alloc_and_setup_fdt()")
>>>>>> ---
>>>>>>  arch/arm64/kernel/setup.c | 8 ++++++++
>>>>>>  drivers/of/fdt.c          | 6 ++++++
>>>>>>  drivers/of/kexec.c        | 8 ++++++--
>>>>>>  include/linux/of_fdt.h    | 2 ++
>>>>>>  4 files changed, 22 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
>>>>>> index b22d28ec8028..a4d96f5e2e05 100644
>>>>>> --- a/arch/arm64/kernel/setup.c
>>>>>> +++ b/arch/arm64/kernel/setup.c
>>>>>> @@ -194,6 +194,14 @@ static void __init setup_machine_fdt(phys_addr_t dt_phys)
>>>>>>      /* Early fixups are done, map the FDT as read-only now */
>>>>>>      fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL_RO);
>>>>>>
>>>>>> +    /*
>>>>>> +     * Save dt_phys address so that it can be used later for kexec. This
>>>>>> +     * is done as __pa() is only intended to be used for linear map addresses
>>>>>> +     * and using it for initial_boot_params which is in fixmap will give an
>>>>>> +     * incorrect value.
>>>>>> +     */
>>>>>> +    set_initial_boot_params_pa(dt_phys);
>>>>>
>>>>> No new arch->dt functions please. If we need to save off the PA, then do
>>>>> that when we set initial_boot_params.
>>>>>
>>>>> Rob
>>>>
>>>>
>>>> initial_boot_params is set in early_init_dt_verify, called by early_init_dt_scan.
>>>> This is done in setup_machine_fdt in arm64 where the PA is available,
>>>> but in other functions in other architectures, where the PA is not available.
>>>
>>> Doesn't __pa() work for all the other architectures? That's what your
>>> patch indicates.
>>>
>>
>> Yes, __pa() works for all other architectures.
>>
>> But we would need to add initial_boot_params_pa of type phys_addr_t
>> as an argument for early_init_dt_scan, which is called by all other archs,
>> and we technically cant use 0 as an invalid value.
>>
>> We could convert initial_boot_params_pa to void *, and pass NULL for all
>> other archs. But again, I don't really think we should be changing the
>> early_init_dt_scan(dt_virt) call in all other archs to
>> early_init_dt_scan(dt_virt, NULL) just to save initial_boot_params_pa
>> in arm64?
>>
>>>> So it makes it quite messy to set it in the same place as initial_boot_params.
>>>> Its only needed for arm64 and making a change in all archs probably isnt a good idea?
>>>>
>>>> Any reason to not add a new function to make arch -> of/fdt call?
>>>
>>> Yes. It is the opposite direction I have reworked the interfaces to.
>>> We don't want each arch calling various early DT functions at random
>>> times and order. That's fragile when the DT functions make assumptions
>>> about when they are called or what's been initialized already.
>>>
>>> Another option is to make arm64 copy the DT as some arches do.
>>>
>>> Rob
>>
>> Ah maybe I didn't understand this properly, but isnt early_init_dt_scan an
>> arch -> of/fdt interfaces. set_initial_boot_params_pa is a similar interface
>> to early_init_dt_scan?
> 
> Yes, and I don't want more APIs if they can be avoided. When is
> set_initial_boot_params_pa() supposed to be called? Is it before or
> after early_init_dt_scan()? 

Its only needed in arm64, and can be either before or after, as long as its
somewhere in setup_machine_fdt, where dt_phys is available.

> Can subsequent OF functions assume the PA
> is valid?

After set_initial_boot_params_pa has been called, yes.

> If an arch doesn't call set_initial_boot_params_pa() is
> __pa() valid or did they just forget to call it? 

Only arm64 seems to do the fixmap as discussed in
https://lore.kernel.org/all/1ea5538f-7e96-4034-9af9-e2d5fd72e069@gmail.com/,
so __pa should work in others.

Requiring the PA to
> be set at the same time as initial_boot_params avoids all those issues
> with any period of time having the PA incorrect.
> 

Are you recommending I send a patch which changes all archs to call
early_init_dt_scan(dt_virt, NULL)?
or maybe early_init_dt_scan(dt_virt, __pa(dt_virt))?
and arm to call early_init_dt_scan(dt_virt, dt_phys).

Happy to do send a v2 with that if its the way forward, although I feel
set_initial_boot_params_pa() in just one arch might be better than
changing this for all archs.

Thanks
Usama

> Rob