mcpm_loopback(9)

💫 mcpm_loopback(9) — the kernel API manual, bestie edition 💫

okay so you found the mcpm_loopback(9) page, iconic. this is the MCPM one. MCPM stands for Multi-Cluster Power Management and honestly the page is basically "we're gonna soft reset the CPU and see what happens" but, like, in a controlled way. let's get into it 🫶

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✨ NAME ✨

mcpm_loopback — make a run through the MCPM low-level code 💅

like, you're literally doing a little lap through the MCPM machinery. a vibe check for the low-level code, if you will.

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🌸 SYNOPSIS 🌸

int mcpm_loopback (void (*cache_disable)(void) );

that's it. one function pointer, one return value, no drama. 🎀

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🦋 ARGUMENTS 🦋

• cache_disable — a pointer to function performing cache disabling. yes bestie, you have to bring your own cache-disabling function. it's giving "bring your own snacks to the party". 🍓

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🔥 DESCRIPTION 🔥

okay so here's the whole thing. mcpm_loopback exercises the MCPM machinery by soft resetting the CPU and branching to the MCPM low-level entry code before returning to the caller. so the CPU takes a little trip, hits the low-level MCPM entry code, and then comes right back to you like nothing happened. main character energy, honestly. 💀

the cache_disable function must do the necessary cache disabling to let the regular kernel init code turn it back on as if the CPU was hotplugged in. so like — you disable it, kernel init re-enables it, and everybody pretends the CPU just got plugged in fresh. no cap, that's the whole bit. 🔁

the MCPM state machine gets set as if the cluster was initialized, meaning the power_up_setup callback passed to mcpm_sync_init will be invoked for all affinity levels. all of them. every single affinity level. it's thorough like that. 👑

and why would you even want this? glad you asked:

• it may be useful to initialize some resources — for example, enabling the CCI that requires the cache to be off 👀
• or simply for testing purposes — sometimes you just gotta poke the machinery and see if it pokes back 🧚

⚠️ real talk: the cache has to be off for this to be correct. that's why cache_disable is your job and not the function's. don't skimp on it. 🙈

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🌈 SEE ALSO 🌈

the kernel file ./arch/arm/include/asm/mcpm.h — go read it, it's literally right there, iykyk 📖:

• mcpm_is_available(9)
• mcpm_cpu_power_up(9)
• mcpm_cpu_power_down(9)
• mcpm_wait_for_cpu_powerdown(9)
• mcpm_cpu_suspend(9)
• mcpm_cpu_powered_up(9)
• mcpm_platform_register(9)
• mcpm_sync_init(9)

the whole MCPM friend group fr fr 💖

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anyway that's the page!! slay 💅✨

this little love letter to the MCPM subsystem comes to you from ./arch/arm/include/asm, dated October 2026, and it's page mcpm_loopback(9) — go forth and loop back, bestie 🎀🦋💜