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Subject: Re: [PATCH v17 02/10] of: Add a common kexec FDT setup function
To:     Rob Herring <robh@kernel.org>
Cc:     Thiago Jung Bauermann <bauerman@linux.ibm.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        "AKASHI, Takahiro" <takahiro.akashi@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Will Deacon <will@kernel.org>, Joe Perches <joe@perches.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        James Morse <james.morse@arm.com>,
        Sasha Levin <sashal@kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Frank Rowand <frowand.list@gmail.com>,
        vincenzo.frascino@arm.com, Mark Rutland <mark.rutland@arm.com>,
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        "Serge E. Hallyn" <serge@hallyn.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Allison Randal <allison@lohutok.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
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        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
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From:   Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Message-ID: <11aff288-feaa-8e43-fcda-12bc12fbf4cf@linux.microsoft.com>
Date:   Fri, 12 Feb 2021 10:27:56 -0800
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On 2/12/21 10:24 AM, Rob Herring wrote:
> On Fri, Feb 12, 2021 at 11:19 AM Lakshmi Ramasubramanian
> <nramas@linux.microsoft.com> wrote:
>>
>> On 2/12/21 6:38 AM, Rob Herring wrote:
>>> On Thu, Feb 11, 2021 at 7:17 PM Lakshmi Ramasubramanian
>>> <nramas@linux.microsoft.com> wrote:
>>>>
>>>> On 2/11/21 5:09 PM, Thiago Jung Bauermann wrote:
>>>>>
>>>>> There's actually a complication that I just noticed and needs to be
>>>>> addressed. More below.
>>>>>
>>>>
>>>> <...>
>>>>
>>>>>> +
>>>>>> +/*
>>>>>> + * of_kexec_alloc_and_setup_fdt - Alloc and setup a new Flattened Device Tree
>>>>>> + *
>>>>>> + * @image:          kexec image being loaded.
>>>>>> + * @initrd_load_addr:       Address where the next initrd will be loaded.
>>>>>> + * @initrd_len:             Size of the next initrd, or 0 if there will be none.
>>>>>> + * @cmdline:                Command line for the next kernel, or NULL if there will
>>>>>> + *                  be none.
>>>>>> + *
>>>>>> + * Return: fdt on success, or NULL errno on error.
>>>>>> + */
>>>>>> +void *of_kexec_alloc_and_setup_fdt(const struct kimage *image,
>>>>>> +                               unsigned long initrd_load_addr,
>>>>>> +                               unsigned long initrd_len,
>>>>>> +                               const char *cmdline)
>>>>>> +{
>>>>>> +    void *fdt;
>>>>>> +    int ret, chosen_node;
>>>>>> +    const void *prop;
>>>>>> +    unsigned long fdt_size;
>>>>>> +
>>>>>> +    fdt_size = fdt_totalsize(initial_boot_params) +
>>>>>> +               (cmdline ? strlen(cmdline) : 0) +
>>>>>> +               FDT_EXTRA_SPACE;
>>>>>
>>>>> Just adding 4 KB to initial_boot_params won't be enough for crash
>>>>> kernels on ppc64. The current powerpc code doubles the size of
>>>>> initial_boot_params (which is normally larger than 4 KB) and even that
>>>>> isn't enough. A patch was added to powerpc/next today which uses a more
>>>>> precise (but arch-specific) formula:
>>>>>
>>>>> https://lore.kernel.org/linuxppc-dev/161243826811.119001.14083048209224609814.stgit@hbathini/
>>>>>
>>>>> So I believe we need a hook here where architectures can provide their
>>>>> own specific calculation for the size of the fdt. Perhaps a weakly
>>>>> defined function providing a default implementation which an
>>>>> arch-specific file can override (a la arch_kexec_kernel_image_load())?
>>>>>
>>>>> Then the powerpc specific hook would be the kexec_fdt_totalsize_ppc64()
>>>>> function from the patch I linked above.
>>>>>
>>>>
>>>> Do you think it'd better to add "fdt_size" parameter to
>>>> of_kexec_alloc_and_setup_fdt() so that the caller can provide the
>>>> desired FDT buffer size?
>>>
>>> Yes, I guess so. But please define the param as extra size, not total
>>> size. The kernel command line size addition can be in the common code.
>>
>> Will do. Just to clarify -
>>
>> The common code will do:
>>
>> fdt_totalsize(initial_boot_params) + strlen(cmdline) + extra_fdt_size
>>
>> The caller will pass "extra_fdt_size"
>> ARM64 => 4KB
>> PPC64 => fdt_totalsize(initial_boot_params) - which will be updated when
>> the patch Thiago had referred to is merged.
> 
> Yes, I'd leave the 4KB in there by default and arm64 use 0.
> 

Sounds good.

common:
fdt_totalsize(initial_boot_params) + strlen(cmdline) + 0x1000 + extra

arm64 => 0 for extra
ppc => fdt_totalsize(initial_boot_params) for extra.

  -lakshmi