Re: [PATCH] ArmPkg/ArmScmiDxe: Fix the calculation of RequiredArraySize in ClockDescribeRates()

["PierreGondois" <pierre.gondois@arm.com>]

Hello Sudeep,

On 2/2/23 15:30, Sudeep Holla wrote:
> As per the SCMI specification, section CLOCK_DESCRIBE_RATES mentions that
> the value of num_rates_flags[11:0] in the response must be 3 if the return
> format is the triplet. Due to the buggy firmware, this was not noticed for
> long time. The firmware is now fixed resulting in ClockDescribeRates() to
> fail with "Buffer Too Small" error as the RequiredArraySize gets miscalculated
> as 72 instead of 24.
> 
> Fix the issue by reusing the logic for both the return format which must
> work if num_rates_flags has correct value as expected from the specification.
> 
> Cc: Pierre Gondois <Pierre.Gondois@arm.com>
> Cc: Girish Pathak <girish.pathak@arm.com>
> Cc: Jeff Brasen <jbrasen@nvidia.com>
> Reported-by: Sami Mujawar <sami.mujawar@arm.com>
> Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
> ---
>   ArmPkg/Drivers/ArmScmiDxe/ScmiClockProtocol.c | 7 +------
>   1 file changed, 1 insertion(+), 6 deletions(-)
> 
> Hi All,
> 
> Sorry for the resend, I hadn't subscribed before so resending after the
> subscription.
> 
> Regards,
> Sudeep
> 
> diff --git a/ArmPkg/Drivers/ArmScmiDxe/ScmiClockProtocol.c b/ArmPkg/Drivers/ArmScmiDxe/ScmiClockProtocol.c
> index 12a7e6df5d43..1412717bc227 100644
> --- a/ArmPkg/Drivers/ArmScmiDxe/ScmiClockProtocol.c
> +++ b/ArmPkg/Drivers/ArmScmiDxe/ScmiClockProtocol.c
> @@ -236,12 +236,7 @@ ClockDescribeRates (
>         *TotalRates = NUM_RATES (DescribeRates->NumRatesFlags)
>                       + NUM_REMAIN_RATES (DescribeRates->NumRatesFlags);
>   
> -      if (*Format == ScmiClockRateFormatDiscrete) {
> -        RequiredArraySize = (*TotalRates) * sizeof (UINT64);
> -      } else {
> -        // We need to return triplet of 64 bit value for each rate
> -        RequiredArraySize = (*TotalRates) * 3 * sizeof (UINT64);
> -      }
> +      RequiredArraySize = (*TotalRates) * sizeof (UINT64);
>   
>         if (RequiredArraySize > (*RateArraySize)) {
>           *RateArraySize = RequiredArraySize;

I think the following also needs to be modified:
"""
for (RateNo = 0; RateNo < NUM_RATES (DescribeRates->NumRatesFlags); RateNo++) {
   // Linear clock rates from minimum to maximum in steps
   // Minimum clock rate.
   Rate                                    = &DescribeRates->Rates[RateOffset++];
   RateArray[RateIndex].ContinuousRate.Min =
	ConvertTo64Bit (Rate->Low, Rate->High);

   Rate = &DescribeRates->Rates[RateOffset++];
   // Maximum clock rate.
   RateArray[RateIndex].ContinuousRate.Max =
	ConvertTo64Bit (Rate->Low, Rate->High);

   Rate = &DescribeRates->Rates[RateOffset++];
   // Step.
   RateArray[RateIndex++].ContinuousRate.Step =
	ConvertTo64Bit (Rate->Low, Rate->High);
}
"""

If the clock format is continuous, then per spec:
   NUM_RATES (DescribeRates->NumRatesFlags) == 3
However 3 rates are read per loop here. So I think the
loop needs to be removed aswell.

Without this additional change, I get:

"""
Synchronous Exception at 0xA2D0E6EBD20F8604

Synchronous Exception at 0xA2D0E6EBD20F8604
PC 0xA2D0E6EBD20F8604

Recursive exception occurred while dumping the CPU state
"""

With the above loop removed:
Reviewed-by: Pierre Gondois <pierre.gondois@arm.com>
Tested-by: Pierre Gondois <pierre.gondois@arm.com>

Regards,
Pierre