[PATCH] fdt_support.c: Allow late kernel cmdline modification

[Niko Mauno]

On 2/22/21 10:21 PM, Tom Rini wrote:
> On Mon, Feb 22, 2021 at 07:18:51PM +0000, Niko Mauno wrote:
> 
>> By declaring board-specific board_fdt_chosen_bootargs() the kernel
>> command line arguments can be adjusted before injecting to flat dt
>> chosen node.
>>
>> Signed-off-by: Niko Mauno <niko.mauno at vaisala.com>
>> ---
>>   common/fdt_support.c  | 12 +++++++++++-
>>   include/fdt_support.h | 10 ++++++++++
>>   2 files changed, 21 insertions(+), 1 deletion(-)
>>
>> diff --git a/common/fdt_support.c b/common/fdt_support.c
>> index 08d540bfc8..4379507e30 100644
>> --- a/common/fdt_support.c
>> +++ b/common/fdt_support.c
>> @@ -269,6 +269,15 @@ int fdt_initrd(void *fdt, ulong initrd_start, ulong initrd_end)
>>   	return 0;
>>   }
>>   
>> +/**
>> + * board_fdt_chosen_bootargs - boards may override this function to use
>> + *                             alternative kernel command line arguments
>> + */
>> +__weak char *board_fdt_chosen_bootargs(void)
>> +{
>> +	return env_get("bootargs");
>> +}
>> +
>>   int fdt_chosen(void *fdt)
>>   {
>>   	int   nodeoffset;
>> @@ -286,7 +295,8 @@ int fdt_chosen(void *fdt)
>>   	if (nodeoffset < 0)
>>   		return nodeoffset;
>>   
>> -	str = env_get("bootargs");
>> +	str = board_fdt_chosen_bootargs();
>> +
>>   	if (str) {
>>   		err = fdt_setprop(fdt, nodeoffset, "bootargs", str,
>>   				  strlen(str) + 1);
> 
> What's this look like in practice?  I'm very wary about changing "set
> bootargs to what you pass to the kernel" logic that's been there for
> forever and a constant of every platform.  Thanks!
> 
Hi Tom,

    Effectively this change as such should introduce no changes to 
prevalent "bootargs" logic when an overriding 
board_fdt_chosen_bootargs() is not defined. Our motivation for proposing 
this change is that it provides us with a means to introduce an entry 
point where we can append rootfs specific dm-verity kernel command line 
metrics (such as described in 
https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html) 
by reading a property from within a kernel tier FIT image header, in 
order to facilitate secure boot process all the way to root filesystem.

-Niko