Re: [PATCH] firmware: stratix10-svc: build only on 64-bit ARM

[Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>]

On 21/03/2021 22:09, Arnd Bergmann wrote:
> On Sun, Mar 21, 2021 at 7:46 PM Krzysztof Kozlowski
> <krzysztof.kozlowski@canonical.com> wrote:
>>
>> The Stratix10 service layer and RCU drivers are useful only on
>> Stratix10, so on ARMv8.  Compile testing the RCU driver on 32-bit ARM
>> fails:
>>
>>   drivers/firmware/stratix10-rsu.c: In function 'rsu_status_callback':
>>   include/linux/compiler_types.h:320:38: error: call to '__compiletime_assert_179'
>>     declared with attribute error: FIELD_GET: type of reg too small for mask
>>     _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>>   ...
>>   drivers/firmware/stratix10-rsu.c:96:26: note: in expansion of macro 'FIELD_GET'
>>     priv->status.version = FIELD_GET(RSU_VERSION_MASK,
>>
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
>> Reported-by: kernel test robot <lkp@intel.com>
> 
> While I agree that one shouldn't run 32-bit kernels on this, we should also try
> to write drivers portably, and in theory any SoC that can run a 64-bit
> Arm kernel
> should also be able to run a 32-bit kernel if you include the same drivers.
> 
> It seems that the problem here is in the smccc definition
> 
> struct arm_smccc_res {
>         unsigned long a0;
>         unsigned long a1;
>         unsigned long a2;
>         unsigned long a3;
> };
> 
> so the result of
> 
> #define RSU_VERSION_MASK                GENMASK_ULL(63, 32)
>           priv->status.version = FIELD_GET(RSU_VERSION_MASK, res->a2);
> 
> tries to access bits that are just not returned by the firmware here,
> which indicates that it probably won't work in this case.
> 
> What I'm not entirely sure about is whether this is a problem in
> the Intel firmware implementation requiring the smccc caller to
> run in a 64-bit context, or if it's a mistake in the way the driver
> extracts the information if the firmware can actually pass it down
> correctly.

The SMC has two calling conventions - SMC32/HVC32 and SMC64/HVC64. The
Stratix 10 driver uses the 64-bit calling convention (see
INTEL_SIP_SMC_FAST_CALL_VAL in
include/linux/firmware/intel/stratix10-smc.h), so it should not run in
aarch32 (regardless of type of hardware).

I think that my patch limiting the support to 64-bit makes sense.

Best regards,
Krzysztof