Re: [PATCH] firmware: smccc: Pass only the required device probe arguments

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 11/10/2026 14:25, Aneesh Kumar K.V wrote:
> Suzuki K Poulose <suzuki.poulose@arm.com> writes:
> 
>> Hi Aneesh
>>
>> On 10/10/2026 11:02, Aneesh Kumar K.V (Arm) wrote:
>>> The device probe passes zeroes for all seven argument registers to
>>> arm_smccc_1_1_invoke(). On ARM32 Thumb-2, this binds an input to r7 and
>>> GCC 8.5 reports "r7 cannot be used in asm here".
>>>
>>> The current smccc_devices entries need at most one argument after the
>>> function ID: TRNG_VERSION has none and RSI_ABI_VERSION takes a requested
>>> version. Pass zero for that argument and omit the unused arguments,
>>> avoiding the r7 constraint. Check argument requirements when adding new
>>> probe functions to smccc_devices.
>>
>> Is it safer to use x1-x3 to 0 as they could be the results that we
>> get back from SMCCC v1.0 onwards ?
>>
> 
> 
> I'm not sure I follow. Passing additional zero arguments only
> initializes the input registers; it should not affect how return values
> are handled. arm_smccc_1_1_invoke() already declares x0-x3 as outputs.
> Since these probe functions require no inputs in x2 or x3 and we only
> inspect x0 on return, why do we need to initialize them to zero?

Agree, I meant, passing 0s for x0-x3 future proofs the new calls.
We know that x0-x3 could get overwritten by these calls, and is
safe to be zero'd on a call.

But agree, for the existing users just x1 is enough.

Either way,

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>


Cheers
Suzuki

> 
> 
> -aneesh