Re: [PATCH v7 6/6] drm/msm: iommu: Replace runtime calls with runtime suppliers
From: Robin Murphy
Date: Wed Feb 14 2018 - 11:03:50 EST
On 14/02/18 10:33, Vivek Gautam wrote:
On Wed, Feb 14, 2018 at 2:46 PM, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
Adding Jordan to this thread as well.
On Wed, Feb 14, 2018 at 6:13 PM, Vivek Gautam
<vivek.gautam@xxxxxxxxxxxxxx> wrote:
Hi Tomasz,
On Wed, Feb 14, 2018 at 11:08 AM, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
On Wed, Feb 14, 2018 at 1:17 PM, Vivek Gautam
<vivek.gautam@xxxxxxxxxxxxxx> wrote:
Hi Tomasz,
On Wed, Feb 14, 2018 at 8:31 AM, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
On Wed, Feb 14, 2018 at 11:13 AM, Rob Clark <robdclark@xxxxxxxxx> wrote:
On Tue, Feb 13, 2018 at 8:59 PM, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
On Wed, Feb 14, 2018 at 3:03 AM, Rob Clark <robdclark@xxxxxxxxx> wrote:
On Tue, Feb 13, 2018 at 4:10 AM, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
Hi Vivek,
Thanks for the patch. Please see my comments inline.
On Wed, Feb 7, 2018 at 7:31 PM, Vivek Gautam
<vivek.gautam@xxxxxxxxxxxxxx> wrote:
While handling the concerned iommu, there should not be a
need to power control the drm devices from iommu interface.
If these drm devices need to be powered around this time,
the respective drivers should take care of this.
Replace the pm_runtime_get/put_sync(<drm_device>) with
pm_runtime_get/put_suppliers(<drm_device>) calls, to power-up
the connected iommu through the device link interface.
In case the device link is not setup these get/put_suppliers()
calls will be a no-op, and the iommu driver should take care of
powering on its devices accordingly.
Signed-off-by: Vivek Gautam <vivek.gautam@xxxxxxxxxxxxxx>
---
drivers/gpu/drm/msm/msm_iommu.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)
diff --git a/drivers/gpu/drm/msm/msm_iommu.c b/drivers/gpu/drm/msm/msm_iommu.c
index b23d33622f37..1ab629bbee69 100644
--- a/drivers/gpu/drm/msm/msm_iommu.c
+++ b/drivers/gpu/drm/msm/msm_iommu.c
@@ -40,9 +40,9 @@ static int msm_iommu_attach(struct msm_mmu *mmu, const char * const *names,
struct msm_iommu *iommu = to_msm_iommu(mmu);
int ret;
- pm_runtime_get_sync(mmu->dev);
+ pm_runtime_get_suppliers(mmu->dev);
ret = iommu_attach_device(iommu->domain, mmu->dev);
- pm_runtime_put_sync(mmu->dev);
+ pm_runtime_put_suppliers(mmu->dev);
For me, it looks like a wrong place to handle runtime PM of IOMMU
here. iommu_attach_device() calls into IOMMU driver's attach_device()
callback and that's where necessary runtime PM gets should happen, if
any. In other words, driver A (MSM DRM driver) shouldn't be dealing
with power state of device controlled by driver B (ARM SMMU).
Note that we end up having to do the same, because of iommu_unmap()
while DRM driver is powered off.. it might be cleaner if it was all
self contained in the iommu driver, but that would make it so other
drivers couldn't call iommu_unmap() from an irq handler, which is
apparently something that some of them want to do..
I'd assume that runtime PM status is already guaranteed to be active
when the IRQ handler is running, by some other means (e.g.
pm_runtime_get_sync() called earlier, when queuing some work to the
hardware). Otherwise, I'm not sure how a powered down device could
trigger an IRQ.
So, if the master device power is already on, suppliers should be
powered on as well, thanks to device links.
umm, that is kindof the inverse of the problem.. the problem is
things like gpu driver (and v4l2 drivers that import dma-buf's,
afaict).. they will potentially call iommu->unmap() when device is not
active (due to userspace or things beyond the control of the driver)..
so *they* would want iommu to do pm get/put calls.
Which is fine and which is actually already done by one of the patches
in this series, not for map/unmap, but probe, add_device,
remove_device. Having parts of the API doing it inside the callback
and other parts outside sounds at least inconsistent.
But other drivers
trying to unmap from irq ctx would not. Which is the contradictory
requirement that lead to the idea of iommu user powering up iommu for
unmap.
Sorry, maybe I wasn't clear. My last message was supposed to show that
it's not contradictory at all, because "other drivers trying to unmap
from irq ctx" would already have called pm_runtime_get_*() earlier
from a non-irq ctx, which would have also done the same on all the
linked suppliers, including the IOMMU. The ultimate result would be
that the map/unmap() of the IOMMU driver calling pm_runtime_get_sync()
would do nothing besides incrementing the reference count.
The entire point was to avoid the slowpath that pm_runtime_get/put_sync()
would add in map/unmap. It would not be correct to add a slowpath in irq_ctx
for taking care of non-irq_ctx and for the situations where master is already
powered-off.
Correct me if I'm wrong, but I believe that with what I'm proposing
there wouldn't be any slow path.
Yea, but only when the power domain is irq-safe? And not all platforms
enable irq-safe power domains. For instance, msm doesn't enable its
gdsc power domains as irq-safe.
Is it something i am missing?
irq-safe would matter if there would exist a case when the call is
done from IRQ context and the power is off. As I explained in a), it
shouldn't happen.
Hi Robin, Will
Does adding pm_runtime_get() in map/unmap sounds good to you?
Given that we spent significant effort last year removing as much
locking as we possibly could from the map/unmap path to minimise the
significant performance impact it was having on networking/storage/etc.
workloads, I really don't want to introduce more for the sake of one
specific use-case, so no.
Robin.