Re: [PATCH v2 2/6] iommu/arm-smmu: Add interconnect bandwidth voting support

[Konrad Dybcio]

On 6/17/26 4:26 PM, Bibek Kumar Patro wrote:
> 
> 
> On 6/16/2026 5:51 AM, Dmitry Baryshkov wrote:
>> On Mon, Jun 15, 2026 at 06:36:51PM +0530, Bibek Kumar Patro wrote:
>>>
>>>
>>> On 6/8/2026 7:25 PM, Dmitry Baryshkov wrote:
>>>> On Tue, May 26, 2026 at 08:12:03PM +0530, Bibek Kumar Patro wrote:
>>>>> On some SoCs the SMMU registers require an active interconnect
>>>>> bandwidth vote to be accessible. While other clients typically
>>>>> satisfy this requirement implicitly, certain corner cases (e.g.
>>>>> during sleep/wakeup transitions) can leave the SMMU without a
>>>>> vote, causing intermittent register access failures.
>>>>>
>>>>> Add support for an optional interconnect path to the arm-smmu
>>>>> driver and vote for bandwidth while the SMMU is active. The path
>>>>> is acquired from DT if present and ignored otherwise.
>>>>>
>>>>> The bandwidth vote is enabled before accessing SMMU registers
>>>>> during probe and runtime resume, and released during runtime
>>>>> suspend and on error paths.
>>>>>
>>>>> Generally, from an architectural perspective, GEM_NOC and DDR are
>>>>> expected to have an active vote whenever the adreno_smmu block is
>>>>> powered on. In most common use cases, this requirement is implicitly
>>>>> satisfied because other GPU-related clients (for example, the GMU
>>>>> device) already hold a GEM_NOC vote when adreno_smmu is enabled.
>>>>>
>>>>> However, there are certain corner cases, such as during sleep/wakeup
>>>>> transitions, where the GEM_NOC vote can be removed before adreno_smmu
>>>>> is powered down. If adreno_smmu is then accessed while the interconnect
>>>>> vote is missing, it can lead to the observed failures. Because of the
>>>>> precise ordering involved, this scenario is difficult to reproduce
>>>>> consistently.
>>>>> (also GDSC is involved in adreno usecases can have an independent vote)
>>>>>
>>>>> Signed-off-by: Bibek Kumar Patro <bibek.patro@xxxxxxxxxxxxxxxx>
>>>>> ---
>>>>>    drivers/iommu/arm/arm-smmu/arm-smmu.c | 57 +++++++++++++++++++++++++++++++++--
>>>>>    drivers/iommu/arm/arm-smmu/arm-smmu.h |  2 ++
>>>>>    2 files changed, 57 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/drivers/iommu/arm/arm-smmu/arm-smmu.c
>>>>> index 0bd21d206eb3e75c3b9fb1364cdc92e82c5aa499..07c7e44ec6a5bd1488f00f87d859a20495e46601 100644
>>>>> --- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
>>>>> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
>>>>> @@ -53,6 +53,11 @@
>>>>>    #define MSI_IOVA_BASE            0x8000000
>>>>>    #define MSI_IOVA_LENGTH            0x100000
>>>>> +/* Interconnect bandwidth vote values for the SMMU register access path */
>>>>> +#define ARM_SMMU_ICC_AVG_BW        0
>>>>> +#define ARM_SMMU_ICC_PEAK_BW_HIGH    1000
>>>>
>>>> totally random numbers, which might be different for non-Qualcomm platform.
>>>>
>>>
>>> Ideally, any non-zero value would be enough to keep the path active.
>>
>> This is true for Qualcomm devices. However, you are adding this to a
>> generic code.
>>
>>> Here 1 Would be enough to keep the path active, but might be too small to
>>> reliably keep the bus active.
>>> Other is UINT_MAX, which will reliably keep the bus active but might cause a
>>> power penalty.
>>>
>>> #define ARM_SMMU_ICC_PEAK_BW_HIGH    UINT_MAX
>>>
>>> seems to be suitable here to reliably keep the bus active by BCM
>>> for both Qualcomm and non-Qualcomm platforms (with some power penalty).
>>>
>>> LMK, if you feel otherwise.
>>
>> Shift it to the qcom instance or provide platform-specific values? (My
>> preference would be towards the first solution).
>>
> 
> 
> To support platform-specific values, we may need to introduce a LUT-based approach in the driver. (Bandwidth voting values cannot be placed in device-tree property IIRC ?)
> 
> Currently, all Qualcomm platforms use 0x1000 for SMMU ICC voting. I

(you used decimal 1000)

> can evaluate if this could be moved to a Qualcomm-specific
> implementation.

Add a vendor hook to arm_smmu_runtime_suspend/resume and handle it within
the QC driver

Konrad