Re: [PATCH v2 3/3] drm/panfrost: Synchronize and disable interrupts before powering off

From: AngeloGioacchino Del Regno
Date: Tue Nov 28 2023 - 10:42:38 EST

Il 28/11/23 16:38, Boris Brezillon ha scritto:
On Tue, 28 Nov 2023 16:10:43 +0100
AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
wrote:

Il 28/11/23 15:06, Boris Brezillon ha scritto:
On Tue, 28 Nov 2023 13:45:10 +0100
AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
wrote:
To make sure that we don't unintentionally perform any unclocked and/or
unpowered R/W operation on GPU registers, before turning off clocks and
regulators we must make sure that no GPU, JOB or MMU ISR execution is
pending: doing that required to add a mechanism to synchronize the
interrupts on suspend.

Add functions panfrost_{gpu,job,mmu}_suspend_irq() which will perform
interrupts masking and ISR execution synchronization, and then call
those in the panfrost_device_runtime_suspend() handler in the exact
sequence of job (may require mmu!) -> mmu -> gpu.

As a side note, JOB and MMU suspend_irq functions needed some special
treatment: as their interrupt handlers will unmask interrupts, it was
necessary to add a bitmap for "is_suspending" which is used to address
the possible corner case of unintentional IRQ unmasking because of ISR
execution after a call to synchronize_irq().

Of course, unmasking the interrupts is being done as part of the reset
happening during runtime_resume(): since we're anyway resuming all of
GPU, JOB, MMU, the only additional action is to zero out the newly
introduced `is_suspending` bitmap directly in the resume handler, as
to avoid adding panfrost_{job,mmu}_resume_irq() function just for
clearing own bits, especially because it currently makes way more sense
to just zero out the bitmap.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
---
drivers/gpu/drm/panfrost/panfrost_device.c | 4 ++++
drivers/gpu/drm/panfrost/panfrost_device.h | 7 +++++++
drivers/gpu/drm/panfrost/panfrost_gpu.c | 7 +++++++
drivers/gpu/drm/panfrost/panfrost_gpu.h | 1 +
drivers/gpu/drm/panfrost/panfrost_job.c | 18 +++++++++++++++---
drivers/gpu/drm/panfrost/panfrost_job.h | 1 +
drivers/gpu/drm/panfrost/panfrost_mmu.c | 17 ++++++++++++++---
drivers/gpu/drm/panfrost/panfrost_mmu.h | 1 +
8 files changed, 50 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/panfrost/panfrost_device.c b/drivers/gpu/drm/panfrost/panfrost_device.c
index c90ad5ee34e7..ed34aa55a7da 100644
--- a/drivers/gpu/drm/panfrost/panfrost_device.c
+++ b/drivers/gpu/drm/panfrost/panfrost_device.c
@@ -407,6 +407,7 @@ static int panfrost_device_runtime_resume(struct device *dev)
{
struct panfrost_device *pfdev = dev_get_drvdata(dev);
+ bitmap_zero(pfdev->is_suspending, PANFROST_COMP_BIT_MAX);

I would let each sub-block clear their bit in the reset path, since
that's where the IRQs are effectively unmasked.


Honestly I wouldn't like seeing that: the reason is that this is something that
is done *for* suspend/resume and only for that, while reset may be called out of
the suspend/resume handlers.

I find clearing the suspend bits in the HW reset path a bit confusing, especially
when it is possible to avoid doing it there...

Well, I do think it's preferable to keep the irq_is_no_longer_suspended
state update where the interrupt is effectively unmasked. Note that
when you do a reset, the IRQ is silently suspended just after the
reset happens, because the xxx_INT_MASKs are restored to their default
value, so I do consider that clearing this bit in the reset path makes
sense.


Okay then, I can move it, no problem.


panfrost_device_reset(pfdev);
panfrost_devfreq_resume(pfdev);
@@ -421,6 +422,9 @@ static int panfrost_device_runtime_suspend(struct device *dev)
return -EBUSY;
panfrost_devfreq_suspend(pfdev);
+ panfrost_job_suspend_irq(pfdev);
+ panfrost_mmu_suspend_irq(pfdev);
+ panfrost_gpu_suspend_irq(pfdev);
panfrost_gpu_power_off(pfdev);
return 0;
diff --git a/drivers/gpu/drm/panfrost/panfrost_device.h b/drivers/gpu/drm/panfrost/panfrost_device.h
index 54a8aad54259..29f89f2d3679 100644
--- a/drivers/gpu/drm/panfrost/panfrost_device.h
+++ b/drivers/gpu/drm/panfrost/panfrost_device.h
@@ -25,6 +25,12 @@ struct panfrost_perfcnt;
#define NUM_JOB_SLOTS 3
#define MAX_PM_DOMAINS 5
+enum panfrost_drv_comp_bits {
+ PANFROST_COMP_BIT_MMU,
+ PANFROST_COMP_BIT_JOB,
+ PANFROST_COMP_BIT_MAX
+};
+
/**
* enum panfrost_gpu_pm - Supported kernel power management features
* @GPU_PM_CLK_DIS: Allow disabling clocks during system suspend
@@ -109,6 +115,7 @@ struct panfrost_device {
struct panfrost_features features;
const struct panfrost_compatible *comp;
+ DECLARE_BITMAP(is_suspending, PANFROST_COMP_BIT_MAX);

nit: Maybe s/is_suspending/suspended_irqs/, given the state remains
until the device is resumed.

If we keep the `is_suspending` name, we can use this one more generically in
case we ever need to, what do you think?

I'm lost. Why would we want to reserve a name for something we don't
know about? My comment was mostly relating to the fact this bitmap
doesn't reflect the is_suspending state, but rather is_suspended,
because it remains set until the device is resumed. And we actually want
it to reflect the is_suspended state, so we can catch interrupts that
are not for us without reading regs in the hard irq handler, when the
GPU is suspended.

`is_suspended` (fun story: that's the first name I gave it) looks good to me,
the doubt I raised was about calling it `suspended_irqs` instead, as I would
prefer to keep names "more generic", but that's just personal preference at
this point anyway.

Cheers,
Angelo