Re: [PATCH RFC v7 0/9] firmware: arm_scmi: vendors: Qualcomm Generic Vendor Extensions

[Pragnesh Papaniya]

On 16-Jun-26 1:57 PM, Sudeep Holla wrote:
> On Wed, Jun 10, 2026 at 02:21:27PM +0530, Pragnesh Papaniya wrote:
>> The QCOM SCMI vendor protocol provides a generic way of exposing a number of
>> Qualcomm SoC specific features (like memory bus scaling) through a mixture of
>> pre-determined algorithm strings and param_id pairs hosted on the SCMI
>> controller. On Qualcomm Glymur and Hamoa SoCs, the memlat governor and the
>> mechanism to control the various caches and RAM is hosted on the CPU Control
>> Processor (CPUCP) and the method to tweak and start the governor is exposed
>> through the QCOM SCMI Generic Extension Protocol.
>>
>> This series introduces the devfreq SCMI client driver that uses the MEMLAT
>> algorithm string hosted on the QCOM SCMI Generic Extension Protocol to detect
>> memory latency workloads and control frequency/level of the various memory
>> buses (DDR/LLCC/DDR_QOS). DDR/LLCC/DDR_QOS are modelled as devfreq devices
>> using the remote devfreq governor. This provides basic insight into device
>> operation via trans_stat and lets userspace further tweak the parameters of
>> the remote governor.
>>
>> trans_stat data for DDR/LLCC/DDR_QOS is now available with this series:
>>
>>      From  :   To
>>    315000000 479000000 545000000 725000000 840000000  959000000 1090000000 1211000000   time(ms)
>>    315000000:         0         3         6         6         6         7         0        30    143956
>>    479000000:         2         0         7         1         1         1         0         3       356
>>    545000000:         7         6         0         5         5         0         0        10      1200
>>    725000000:         3         0         5         0         6         1         0         6      2172
>>    840000000:         8         2         3         2         0         4         0        12      1188
>>    959000000:         3         0         1         2         2         0         0        13       272
>>   1090000000:         0         0         0         0         0         0         0         0         0
>>   1211000000:        35         4        11         5        11         8         0         0     21684
>> Total transition : 253
>>
>> QCOM SCMI Generic Vendor protocol background:
>> A lot of the vendor protocol numbers used internally were for
>> debug/internal development purposes that were either highly SoC-specific
>> or had to be disabled because some features were fused out during
>> production. This led to a large number of vendor protocol numbers being
>> quickly consumed and never released. Using a single generic vendor
>> protocol with functionality abstracted behind algorithm strings gives us
>> the flexibility of letting such functionality exist during initial
>> development/debugging while still being able to expose mature features
>> (like MEMLAT) once they have stabilised. The param_ids are expected to
>> act as ABI for algorithm strings like MEMLAT.
>>
> 
> Not sure if it was discussed in the previous versions or not, it would be
> good if you can capture why some of bus scaling doesn't work with the existing
> SCMI performance protocol and the monitors don't fit the MPAM mode.
> 
> Please capture them in 1/9 as a motivation for this vendor protocol. It will
> then help to understand it better as I am still struggling to. Sorry for that.

Thanks for the input!

SCMI perf protocol exports perf domains to kernel where kernel can set
the frequency but here the scaling governor runs on the SCP while kernel
just observes frequency changes made by remote governor. While MPAM is
not enabled/supported on all hardware (Hamoa). Here's the pseudo-code
for remote governor on CPUCP to help you understand more:

Barebone Memlat Pseudocode:
Every sample window, get snapshot of latest AMU counters from each CPU and scale all the memory according to the map_table:

For each CPU
    // Calculate IPM ( Instruction retired / cache miss count (L2 cache refills for LLCC voting and CPU RD miss counter for DDR))
    If (IPM < IPM_CEIL)
        Use CPU cycle counter to determine CPU frequency in the past N milliseconds

LLCC_freq = lookup_llcc_freq(cpu_freq_max)
DDR_freq = lookup_ddr_freq(cpu_freq_max)	
DDR_QOS_freq = lookup_ddr_qos_freq(cpu_freq_max)

// Scale all memories
Scale_freq(Memory); // LLCC/DDR/DDR_QOS

>