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

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

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?


-aneesh