Re: [PATCH v4 09/14] of: reserved-memory: Fix using wrong number of cells to get property 'alignment'

[Zijun Hu <quic_zijuhu@quicinc.com>]

On 2/25/2025 9:18 AM, William McVicker wrote:
> Hi Zijun and Rob,
> 
> On 01/13/2025, Rob Herring wrote:
>> On Thu, Jan 09, 2025 at 09:27:00PM +0800, Zijun Hu wrote:
>>> From: Zijun Hu <quic_zijuhu@quicinc.com>
>>>
>>> According to DT spec, size of property 'alignment' is based on parent
>>> node’s #size-cells property.
>>>
>>> But __reserved_mem_alloc_size() wrongly uses @dt_root_addr_cells to get
>>> the property obviously.
>>>
>>> Fix by using @dt_root_size_cells instead of @dt_root_addr_cells.
>>
>> I wonder if changing this might break someone. It's been this way for 
>> a long time. It might be better to change the spec or just read 
>> 'alignment' as whatever size it happens to be (len / 4). It's not really 
>> the kernel's job to validate the DT. We should first have some 
>> validation in place to *know* if there are any current .dts files that 
>> would break. That would probably be easier to implement in dtc than 
>> dtschema. Cases of #address-cells != #size-cells should be pretty rare, 
>> but that was the default for OpenFirmware.
>>
>> As the alignment is the base address alignment, it can be argued that 
>> "#address-cells" makes more sense to use than "#size-cells". So maybe 
>> the spec was a copy-n-paste error.
> 
> Yes, this breaks our Pixel downstream DT :( Also, the upstream Pixel 6 device
> tree has cases where #address-cells != #size-cells.
> 

it seems upstream upstream Pixel 6 has no property 'alignment'
git grep alignment arch/arm64/boot/dts/exynos/google/
so it should not be broken.

> I would prefer to not have this change, but if that's not possible, could we
> not backport it to all the stable branches? That way we can just force new
> devices to fix this instead of existing devices on older LTS kernels?
> 

the fix have stable and fix tags. not sure if we can control its
backporting. the fix has been backported to 6.1/6.6/6.12/6.13 automatically.


> Thanks,
> Will
> 
>>
>>>
>>> Fixes: 3f0c82066448 ("drivers: of: add initialization code for dynamic reserved memory")
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
>>> ---
>>>  drivers/of/of_reserved_mem.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/of/of_reserved_mem.c b/drivers/of/of_reserved_mem.c
>>> index 45517b9e57b1add36bdf2109227ebbf7df631a66..d2753756d7c30adcbd52f57338e281c16d821488 100644
>>> --- a/drivers/of/of_reserved_mem.c
>>> +++ b/drivers/of/of_reserved_mem.c
>>> @@ -409,12 +409,12 @@ static int __init __reserved_mem_alloc_size(unsigned long node, const char *unam
>>>  
>>>  	prop = of_get_flat_dt_prop(node, "alignment", &len);
>>>  	if (prop) {
>>> -		if (len != dt_root_addr_cells * sizeof(__be32)) {
>>> +		if (len != dt_root_size_cells * sizeof(__be32)) {
>>>  			pr_err("invalid alignment property in '%s' node.\n",
>>>  				uname);
>>>  			return -EINVAL;
>>>  		}
>>> -		align = dt_mem_next_cell(dt_root_addr_cells, &prop);
>>> +		align = dt_mem_next_cell(dt_root_size_cells, &prop);
>>>  	}
>>>  
>>>  	nomap = of_get_flat_dt_prop(node, "no-map", NULL) != NULL;
>>>
>>> -- 
>>> 2.34.1
>>>