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From:   Thiago Jung Bauermann <bauerman@linux.ibm.com>
To:     Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Cc:     Rob Herring <robh@kernel.org>, Mimi Zohar <zohar@linux.ibm.com>,
        "AKASHI, Takahiro" <takahiro.akashi@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        Catalin Marinas <catalin.marinas@arm.com>,
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Subject: Re: [PATCH v17 02/10] of: Add a common kexec FDT setup function
In-reply-to: <11aff288-feaa-8e43-fcda-12bc12fbf4cf@linux.microsoft.com>
Date:   Fri, 12 Feb 2021 16:39:39 -0300
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Lakshmi Ramasubramanian <nramas@linux.microsoft.com> writes:

> 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.

Looks good to me.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center