Re: [PATCH v7 6/6] drm/msm: iommu: Replace runtime calls with runtime suppliers

[Vivek Gautam]

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?

>
> a) For IRQ context, the master is already powered on and so the SMMU
> is also powered on, through respective device link.
> pm_runtime_get_sync() would ultimately just increment the runtime PM
> usage count.
>
> b) For a case when the master is already powered off (which wouldn't
> be IRQ context, for the reason stated in a)), powering on the SMMU is
> unavoidable, if the SMMU hardware really needs to be accessed (i.e.
> some TLBs need to be invalidated, if their state is preserved despite
> master being powered down).
>
>>
>>>
>>>>
>>>> There has already been some discussion about this on various earlier
>>>> permutations of this patchset.  I think we have exhausted all other
>>>> options.
>>>
>>> I guess I should have read those. Let me do that now.
>> Yea, i point to the thread in cover letter and [PATCH 1/6].
>> Thanks.
>
> I read through all the links in the cover letter and I could see other
> attempts not working out indeed, but they were different from what I'm
> proposing.
>
> There was also a point raised that __pm_runtime_resume() called from
> pm_runtime_get_sync() would grab dev->power_lock spinlock, which is
> true, except that if the device is already active, it would do it only
> for the time of checking device state, so I doubt it would really be a
> significant point of contention.
>
> Best regards,
> Tomasz
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