linux-kernel - Re: [PATCH 1/1] base: power: runtime: Export pm_runtime_get/put

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Date:   Fri, 8 Dec 2017 15:07:19 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Vivek Gautam <vivek.gautam@...eaurora.org>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Linux PM <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Rob Clark <robdclark@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Will Deacon <will.deacon@....com>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH 1/1] base: power: runtime: Export pm_runtime_get/put_suppliers

On Fri, Dec 8, 2017 at 2:39 PM, Vivek Gautam
<vivek.gautam@...eaurora.org> wrote:
> Hi Greg,
>
>
> On Fri, Dec 8, 2017 at 6:51 PM, Greg KH <gregkh@...uxfoundation.org> wrote:
>> On Fri, Dec 08, 2017 at 06:00:47PM +0530, Vivek Gautam wrote:
>>> The device link allows the pm framework to tie the supplier and
>>> consumer. So, whenever the consumer is powered-on, the supplier
>>> is powered-on first.
>>>
>>> There are however cases in which the consumer wants to power-on
>>> the supplier, but not itself.
>>> E.g., A Graphics or multimedia driver wants to power-on the SMMU
>>> to unmap a buffer and finish the TLB operations without powering
>>> on itself. Some of these unmap requests are coming from the
>>> user space when the controller itself is not powered-up, and it
>>> can be huge penalty in terms of power and latency to power-up
>>> the graphics/mm controllers.
>>> There can be an argument that the supplier should handle this case
>>> on its own and there should not be a need for the consumer to
>>> power-on the supplier. But as discussed on the thread [1] about
>>> ARM-SMMU runtime pm, we don't want to introduce runtime pm calls
>>> in atomic paths, such as in arm_smmu_unmap.
>>>
>>> [1] https://patchwork.kernel.org/patch/9827825/
>>>
>>> Signed-off-by: Vivek Gautam <vivek.gautam@...eaurora.org>
>>> ---
>>>  drivers/base/power/runtime.c | 2 ++
>>>  1 file changed, 2 insertions(+)
>>>
>>> diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
>>> index 027d159ac381..af169304ca13 100644
>>> --- a/drivers/base/power/runtime.c
>>> +++ b/drivers/base/power/runtime.c
>>> @@ -1578,6 +1578,7 @@ void pm_runtime_get_suppliers(struct device *dev)
>>>
>>>       device_links_read_unlock(idx);
>>>  }
>>> +EXPORT_SYMBOL_GPL(pm_runtime_get_suppliers);
>>
>> We do not export symbols unless there are in-kernel users of them.
>> Where is the patch that adds a user for these functions?
>
> My apologies for not putting the changes for the user of these APIs.
> I will be sending a patch for the user (which would be:
> "drivers/gpu/drm/msm/msm_iommu.c"). The patch will be included
> with the arm-smmu runtime patch series. Right now I am facing issues
> with the use of clk_bulk_*() APIs on 4.15-rc kernel.
>
> But, I wanted to get opinions about this change since we had been
> discussing about this in the arm-smmu runtime patch thread [1].
>
> [1] https://patchwork.kernel.org/patch/9827825/
>
>
> P.S.: A snippet of the change in the user of these APIs:
>
>  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
>  @@ -76,9 +76,9 @@ static int msm_iommu_unmap(struct msm_mmu *mmu,
> uint64_t iova,
>   {
>          struct msm_iommu *iommu = to_msm_iommu(mmu);
>
>  -       pm_runtime_get_sync(mmu->dev);
>  +       pm_runtime_get_suppliers(mmu->dev);
>          iommu_unmap(iommu->domain, iova, len);
>  -       pm_runtime_put_sync(mmu->dev);
>  +       pm_runtime_put_suppliers(mmu->dev);
>
>          return 0;
>   }
>

Well, pm_runtime_get/put_suppliers() were not designed to be used
outside of the runtime PM core code.  I need to have a deeper look
into things at this point, so give me some time.

Thanks,
Rafael