Re: [PATCH] drm/msm: Disable frequency clamping on a630

From: Caleb Connolly
Date: Mon Aug 09 2021 - 16:35:10 EST

On 09/08/2021 18:58, Rob Clark wrote:
On Mon, Aug 9, 2021 at 10:28 AM Akhil P Oommen <akhilpo@xxxxxxxxxxxxxx> wrote:

On 8/9/2021 9:48 PM, Caleb Connolly wrote:

On 09/08/2021 17:12, Rob Clark wrote:
On Mon, Aug 9, 2021 at 7:52 AM Akhil P Oommen <akhilpo@xxxxxxxxxxxxxx>

On 8/8/2021 10:22 PM, Rob Clark wrote:
On Sun, Aug 8, 2021 at 7:33 AM Caleb Connolly
<caleb.connolly@xxxxxxxxxx> wrote:

On 07/08/2021 21:04, Rob Clark wrote:
On Sat, Aug 7, 2021 at 12:21 PM Caleb Connolly
<caleb.connolly@xxxxxxxxxx> wrote:

Hi Rob, Akhil,

On 29/07/2021 21:53, Rob Clark wrote:
On Thu, Jul 29, 2021 at 1:28 PM Caleb Connolly
<caleb.connolly@xxxxxxxxxx> wrote:

On 29/07/2021 21:24, Rob Clark wrote:
On Thu, Jul 29, 2021 at 1:06 PM Caleb Connolly
<caleb.connolly@xxxxxxxxxx> wrote:

Hi Rob,

I've done some more testing! It looks like before that patch
("drm/msm: Devfreq tuning") the GPU would never get above
the second frequency in the OPP table (342MHz) (at least, not
in glxgears). With the patch applied it would more
aggressively jump up to the max frequency which seems to be
unstable at the default regulator voltages.

*ohh*, yeah, ok, that would explain it

Hacking the pm8005 s1 regulator (which provides VDD_GFX) up
to 0.988v (instead of the stock 0.516v) makes the GPU stable
at the higher frequencies.

Applying this patch reverts the behaviour, and the GPU never
goes above 342MHz in glxgears, losing ~30% performance in

I think (?) that enabling CPR support would be the proper
solution to this - that would ensure that the regulators run
at the voltage the hardware needs to be stable.

Is hacking the voltage higher (although ideally not quite
that high) an acceptable short term solution until we have
CPR? Or would it be safer to just not make use of the higher
frequencies on a630 for now?

tbh, I'm not sure about the regulator stuff and CPR.. Bjorn is
on CC and I added sboyd, maybe one of them knows better.

In the short term, removing the higher problematic OPPs from
dts might
be a better option than this patch (which I'm dropping), since
is nothing stopping other workloads from hitting higher OPPs.
Oh yeah that sounds like a more sensible workaround than mine .

I'm slightly curious why I didn't have problems at higher OPPs
on my
c630 laptop (sdm850)
Perhaps you won the sillicon lottery - iirc sdm850 is binned
for higher clocks as is out of the factory.

Would it be best to drop the OPPs for all devices? Or just
those affected? I guess it's possible another c630 might
crash where yours doesn't?

I've not heard any reports of similar issues from the handful of
folks with c630's on #aarch64-laptops.. but I can't really say
if that
is luck or not.
It looks like this affects at least the OnePlus 6 and PocoPhone
F1, I've done some more poking and the following diff
seems to fix the stability issues completely, it seems the delay
is required to let the update propagate.

This doesn't feel like the right fix, but hopefully it's enough
to come up with a better solution than disabling the new
devfreq behaviour on a630.

diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
index d7cec7f0dde0..69e2a5e84dae 100644
--- a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
+++ b/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
@@ -139,6 +139,10 @@ void a6xx_gmu_set_freq(struct msm_gpu *gpu,
struct dev_pm_opp *opp)

+ dev_pm_opp_set_opp(&gpu->pdev->dev, opp);
+ usleep_range(300, 500);

I am a bit confused. We don't define a power domain for gpu in dt,
correct? Then what exactly set_opp do here? Do you think this usleep is
what is helping here somehow to mask the issue?
The power domains (for cx and gx) are defined in the GMU DT, the OPPs in
the GPU DT. For the sake of simplicity I'll refer to the lowest
frequency (257000000) and OPP level (RPMH_REGULATOR_LEVEL_LOW_SVS) as
the "min" state, and the highest frequency (710000000) and OPP level
(RPMH_REGULATOR_LEVEL_TURBO_L1) as the "max" state. These are defined in
sdm845.dtsi under the gpu node.

The new devfreq behaviour unmasks what I think is a driver bug, it
inadvertently puts much more strain on the GPU regulators than they
usually get. With the new behaviour the GPU jumps from it's min state to
the max state and back again extremely rapidly under workloads as small
as refreshing UI. Where previously the GPU would rarely if ever go above
342MHz when interacting with the device, it now jumps between min and
max many times per second.

If my understanding is correct, the current implementation of the GMU
set freq is the following:
- Get OPP for frequency to set
- Push the frequency to the GMU - immediately updating the core clock
- Call dev_pm_opp_set_opp() which triggers a notify chain, this winds
up somewhere in power management code and causes the gx regulator level
to be updated

Nope. dev_pm_opp_set_opp() sets the bandwidth for gpu and nothing else.
We were using a different api earlier which got deprecated -
Huh ok, thanks for the correction. So it's the GMU writes in this function which cause the regulator to be adjusted?

Hmm, ok, if this is just setting icc vote, the order shouldn't be too important.

I guess GMU then is the one that is controlling the regulator(s) to
ensure adequate voltage for the requested freq?

But the GMU fw should be the same for a618 and a630, md5sum of what
I'm using (from linux-firmware):

ab20135f7adf48e0f344282a37da80e4 a630_gmu.bin
Same here.

The regulator will then take some time to reach it's new voltage level
and stabilise. I believe that rapid transitions between min and max
state - in combination with the increased current load from the GPU core
- lead to the regulator becoming unstable (e.g. when it's requested to
transition from it's lowest to highest levels immediately after
transitioning down), the unstable voltage causes the GPU to crash.

Sillicon lottery will of course play a role here - this is very much an
edge case and would definitely be different on a per-device and even
per-unit basis.

Hmm, I thought "opp-level = RPMH_REGULATOR_LEVEL_*" did *something*,
but tbh I'm not sure exactly what..

I feel we should just leave the new dcvs feature (shall we call it NAP?)
disabled for a630 (and 10ms devfreq interval), until this is root
I believe this hacky workaround expresses the root cause of the issue
quite clearly, by setting the OPP first and allowing the gx regulator to
become stable before telling the GPU to change clock speeds, we avoid
the edge case and prevent the crashes.

I took some rough measurements by adding logging to msm_devfreq_idle and
causing UI updates for ~20 seconds and that function is being called
about 30 times per second, this means the GPU is transitioning between
min (idle) state and max (active / boost) state at that frequency and
causing the issue I described above. It's likely that the usleep is
helping to mask this behaviour.

I hope this serves as a slightly better explanation of what I perceive
to be the issue, I realise my previous explanations were not very
adequate, I apologise for all the noise.

I suppose "NAP" is a reasonable name.

But I think that reverting to previous behavior would not be enough,
there is nothing stopping devfreq from jumping from min to max freq,
which AFAIU should be enough to trigger this. I guess that there just
hasn't been enough testing with different game workloads on those
phones to trigger this.

That said, I haven't seen similar issues on my sdm850 laptop, where I
defn have triggered mix->max freq transitions.. I guess it would be
interesting to know if this issue could be reproduced on db845c, or if
it really is board specific?
My db845c arrives this week, I'll definitely try and reproduce this.

To workaround, I think we'd need to implement some way to limit that
maximum frequency jump (and then use delayed work to continue ramping
up the freq over time until we hit the target).. which seems like a
lot of work if this is just a board(s) specific workaround and isn't
needed once CPR is supported
Based on my reasoning above, I came up with the following: reducing
thrashing by preventing rapid idle/active transitions. The minimum
active time of 30ms was just used for testing, I think some number
between 2 and 4 frames would be a sensible choice - the higher the safer.

diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
index d7cec7f0dde0..87f2d1085c3e 100644
--- a/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
+++ b/drivers/gpu/drm/msm/adreno/a6xx_gmu.c
@@ -139,6 +139,8 @@ void a6xx_gmu_set_freq(struct msm_gpu *gpu, struct
dev_pm_opp *opp)

+ dev_pm_opp_set_opp(&gpu->pdev->dev, opp);
gmu_write(gmu, REG_A6XX_GMU_DCVS_ACK_OPTION, 0);

@@ -158,7 +160,6 @@ void a6xx_gmu_set_freq(struct msm_gpu *gpu, struct
dev_pm_opp *opp)
if (ret)
dev_err(gmu->dev, "GMU set GPU frequency error: %d\n",

- dev_pm_opp_set_opp(&gpu->pdev->dev, opp);

diff --git a/drivers/gpu/drm/msm/msm_gpu.h b/drivers/gpu/drm/msm/msm_gpu.h
index 0e4b45bff2e6..0e2293bcb46d 100644
--- a/drivers/gpu/drm/msm/msm_gpu.h
+++ b/drivers/gpu/drm/msm/msm_gpu.h
@@ -99,8 +99,8 @@ struct msm_gpu_devfreq {
/** time: Time of last sampling period. */
ktime_t time;

- /** idle_time: Time of last transition to idle: */
- ktime_t idle_time;
+ /** transition_time: Time of last transition between
idle/active: */
+ ktime_t transition_time;

* idle_freq:
diff --git a/drivers/gpu/drm/msm/msm_gpu_devfreq.c
index 0a1ee20296a2..774a7be33e7a 100644
--- a/drivers/gpu/drm/msm/msm_gpu_devfreq.c
+++ b/drivers/gpu/drm/msm/msm_gpu_devfreq.c
@@ -157,7 +157,7 @@ void msm_devfreq_active(struct msm_gpu *gpu)

- idle_time = ktime_to_ms(ktime_sub(ktime_get(), df->idle_time));
+ idle_time = ktime_to_ms(ktime_sub(ktime_get(),

* If we've been idle for a significant fraction of a polling
@@ -168,7 +168,7 @@ void msm_devfreq_active(struct msm_gpu *gpu)
target_freq *= 2;

- df->idle_freq = 0;
+ df->transition_time = ktime_get();;

msm_devfreq_target(&gpu->pdev->dev, &target_freq, 0);

@@ -185,6 +185,16 @@ void msm_devfreq_idle(struct msm_gpu *gpu)
struct msm_gpu_devfreq *df = &gpu->devfreq;
unsigned long idle_freq, target_freq = 0;
+ unsigned int active_time;
+ active_time = ktime_to_ms(ktime_sub(ktime_get(),
+ /*
+ * Don't go back to idle unless we've been active for at least 30ms
+ * to avoid thrashing.

This basically defeats the purpose of this feature! At least, we should
keep this '30' gpu specific. Does a Kconfig makes sense here?? BTW, if
300us was helping you earlier why do you want it to be 30ms now?
Previously I thought that the issue was related to specifically the transition from idle/active, hence sleeping to let the regulator catch up, whilst that masked the issue it didn't *fix* it, I now think it's actually due to the repeated transition between idle and active states.

Enforcing that the GPU stay active for at least two frames should still give the intended goal of reducing latency and more reliably fixes the issue.

AFAIU from reading the commit description, the goal of the devfreq tuning is to reduce latency by quickly bursting up when there's user activity, by telling the GPU to stay active for longer we shouldn't impede this behaviour at all.

Let's not kconfig, a single kernel should be able to work on multiple devices.

What I'm less sure about is whether this is a630 specific (in which
case something in the gpulist table would work) or board specific (in
which case I guess it needs to somehow come from the dtb)


Kind Regards,
Caleb (they/them)