Re: [PATCH] drm/panfrost: Ignore core_mask for poweroff and sync interrupts

From: AngeloGioacchino Del Regno
Date: Thu Nov 23 2023 - 08:25:07 EST

Il 23/11/23 13:59, Boris Brezillon ha scritto:
On Thu, 23 Nov 2023 12:15:01 +0100
AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
wrote:

Il 23/11/23 11:35, Boris Brezillon ha scritto:
On Thu, 23 Nov 2023 10:53:20 +0100
AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
wrote:
Some SoCs may be equipped with a GPU containing two core groups
and this is exactly the case of Samsung's Exynos 5422 featuring
an ARM Mali-T628 MP6 GPU: the support for this GPU in Panfrost
is partial, as this driver currently supports using only one
core group and that's reflected on all parts of it, including
the power on (and power off, previously to this patch) function.

The issue with this is that even though executing the soft reset
operation should power off all cores unconditionally, on at least
one platform we're seeing a crash that seems to be happening due
to an interrupt firing which may be because we are calling power
transition only on the first core group, leaving the second one
unchanged, or because ISR execution was pending before entering
the panfrost_gpu_power_off() function and executed after powering
off the GPU cores, or all of the above.

Finally, solve this by changing the power off flow to
1. Mask and clear all interrupts: we don't need nor want any, as
we are polling PWRTRANS anyway;
2. Call synchronize_irq() after that to make sure that any pending
ISR is executed before powering off the GPU Shaders/Tilers/L2
hence avoiding unpowered registers R/W; and
3. Ignore the core_mask and ask the GPU to poweroff both core groups

Could we split that in two patches? 1+2 in one patch, and 3 in another.
These are two orthogonal fixes IMO.

My initial idea was exactly that, but I opted for one patch doing 'em all
because a "full fix" comprises all of 1+2+3: the third one without the
first two and vice-versa may not fully resolve the issue that was seen
on the HC1 board.

Guess it depends how you see it. I'd argue that these are 2 orthogonal
bugs, and the suspend fix might be worth backporting to older versions.


Yes, but older versions are not affected by this regression because the GPU
was never turned off for real... so this commit is really fixing just the
issues that came out *because* of the "Really power off" commit, which is
not worth backporting as we already saw one regression with that and doing
such thing would be mostly dangerous.

If your suggestion was to backport 1+2 instead, I disagree: there are no
suspend instabilities in older kernel versions, so we'd be fixing something
that anyway always worked fine for years.

This is to say: the "Really power off" code should be treated like a commit
that implements a new feature, rather than fixing an old one, *only* because
there is a possibility to create regressions somehow (again, we already had
one).

At least IMO...


So, while I agree that it'd be slightly more readable as a diff if those
were two different commits I do have reasons against splitting.....

If we just need a quick fix to avoid PWRTRANS interrupts from kicking
in when we power-off the cores, I think we'd be better off dropping
GPU_IRQ_POWER_CHANGED[_ALL] from the value we write to GPU_INT_MASK
at [re]initialization time, and then have a separate series that fixes
the problem more generically.


But that didn't work:
https://lore.kernel.org/all/d95259b8-10cf-4ded-866c-47cbd2a44f84@xxxxxxxxxx/


...while this "full" solution worked:
https://lore.kernel.org/all/39e9514b-087c-42eb-8d0e-f75dc620e954@xxxxxxxxxx/

https://lore.kernel.org/all/5b24cc73-23aa-4837-abb9-b6d138b46426@xxxxxxxxxx/


...so this *is* a "quick fix" already... :-)

+ gpu_write(pfdev, GPU_INT_MASK, 0);
+ gpu_write(pfdev, GPU_INT_CLEAR, GPU_IRQ_MASK_ALL);
+
+ /*
+ * Make sure that we don't have pending ISRs, otherwise we'll be
+ * reading and/or writing registers while the GPU is powered off
+ */
+ synchronize_irq(pfdev->irq);

Could we move that to a panfrost_gpu_suspend_irq() helper? I'm also not
sure making it part of panfrost_gpu_power_off() is a good idea. I'd
rather have this panfrost_gpu_suspend_irq() helper called from
panfrost_device_[runtime_]suspend(), along with
panfrost_{mmu,job}_suspend_irq().

Okay I will move that to a helper, but I still want to clarify:
- For JOB, we're checking if panfrost_job_is_idle() before trying
to runtime_suspend() (hence before trying to power off cores),
so implicitly no interrupt can fire I guess? Though there could
still be a pending ISR there too.... mmh. Brain ticking :-)

There's indeed no reason to see job interrupts if we're asked to enter
suspend, but it's mostly a matter of safety/correctness. If, as
expected, there's no pending interrupt, the write(_INT_MASK) +
synchronize_irq() should be relatively cheap.

- For MMU, we're not checking anything, but I guess that if there
is no job, the mmu can't be doing anything at all?
...but then you also gave me the doubt about that one as well.

Same here, if we've properly flushed all jobs, and handled all pending
interrupts, we shouldn't end up with pending MMU irqs when we're asked
to suspend. But the extra mask+synchronize_irq() buys us extra safety.


What I think that would be sensible to do is to get this commit as
a "clear" fix for the "Really power off" one, then have one or more
additional commit(s) without any fixes tag to improve the IRQ suspend
with the new mmu/job irq suspend helpers.

If you need a self-contained fix to avoid power transition interrupts
messing up with suspend, then, as I suggested, it might make more sense
to drop GPU_IRQ_POWER_CHANGED[_ALL] when writing GPU_INT_MASK, which
you want anyway, to avoid being interrupted when you do power
transitions.


Of course *this* commit would introduce the panfrost_gpu_suspend_irq()
helper directly, instead of moving the logic to a helper in a later one.

Any reason against? :-)

+
+ /* Now it's safe to request poweroff for Shaders, Tilers and L2 */

It was safe before too, it's just that we probably don't want to be

In theory yes, in practice the Odroid HC1 board crashed :-P

Just to be clear, it's not the accesses to the PWROFF/PWRTRANS
registers in this function that were causing the crash, it's the fact we
were writing to those regs, and leaving the corresponding interrupt
unprocessed before returning from panfrost_device[_runtime]_suspend().


P.S.: Joking! I understand what you're saying :-)

Okay, but the comment was still inaccurate :P.


Hahaha! Fair! :-D


interrupted, if all we do is ignore the interrupts we receive, hence
the suggestion to not use GPU_IRQ_MASK_ALL, and only enable the
IRQs we care about instead.
+ gpu_write(pfdev, SHADER_PWROFF_LO, pfdev->features.shader_present);
ret = readl_relaxed_poll_timeout(pfdev->iomem + SHADER_PWRTRANS_LO,
val, !val, 1, 1000);
if (ret)
@@ -441,7 +451,7 @@ void panfrost_gpu_power_off(struct panfrost_device *pfdev)
if (ret)
dev_err(pfdev->dev, "tiler power transition timeout");
- gpu_write(pfdev, L2_PWROFF_LO, pfdev->features.l2_present & core_mask);
+ gpu_write(pfdev, L2_PWROFF_LO, pfdev->features.l2_present);

I really think we should have a helper doing the 'write(PWROFF_{LO,HI} +
poll(PWRTRANS_{LO,HI})' sequence, similar to what's done here [1][2],
such that, once we got it right for one block, we have it working for
all of them. And if there's a fix to apply, it automatically applies
to all blocks without having to fix the same bug in each copy.

...technically yes, but practically this driver doesn't currently support any
GPU that even fills the _LO registers.

Once we have a solution that works for all use cases, it can work for
the limited set we support at the moment :P.


I guess that we can do that later?

Sure, that was more of a follow-up patch suggestion.


Thanks for that.


That's just to (paranoidly) avoid any risk to introduce other regressions in
this merge window, since we're fixing one that shouldn't have happened in the
first place...

Agreed, but if that's the goal, then I'd go for the simpler change I
suggested above (dropping GPU_IRQ_POWER_CHANGED[_ALL] from the
interrupt mask altogether). This way you don't have to worry about
receiving power transition interrupts in the first place, and you also
save interrupt context switching time when you power on the various
blocks.


...but again, that wouldn't fully work, as per krzk's test.


Note that these panthor_gpu_block_power_{on,off}() helpers also handle
the case where power transitions are already in progress when you ask a
new power transition, which I don't think is checked in
panfrost_gpu_power_{off,on}().

I admit I didn't analyze the panthor driver - but here, the only power transitions
that may happen are either because of panfrost_gpu_power_on(), or because of
panfrost_gpu_power_off(), and both are polling and blocking... so from what I
understand, there's no possibility to have "another" power transition happening
while calling poweron, or poweroff.

Well yes, in theory there's no reason to have more than one transition
happening at a given time (that's assuming power transition never time
out, or if they do, the system gets back to an idle state before we try
to do the next power transition). It's just that, if it ever happens,
for any reason, we'd be safe against this unexpected situation, for a
cost that's relatively low (just an extra register read if things are in
the expected idle state).

Definitely not saying we should do that in this patch, but I think we
should do anything we can do to improve robustness, assuming the cost
of these extra checks is acceptable (we're not really in a fastpath
here).

There's a lot of room for improvement in Panfrost, on that I agree.

Cheers,
Angelo