Re: [PATCH v1 03/10] drm/panthor: Introduce framework for architecture-specific features

[Karunika Choo <karunika.choo@arm.com>]

On 24/10/2025 07:43, Boris Brezillon wrote:
> On Tue, 14 Oct 2025 10:43:30 +0100
> Karunika Choo <karunika.choo@arm.com> wrote:
> 
>> Add a framework to support architecture-specific features. This allows
>> other parts of the driver to adjust their behaviour based on the feature
>> bits enabled for a given architecture.
> 
> I'm not convinced we need this just yet. AFAICT, the only feature flag
> being added in this patchset is PANTHOR_HW_FEATURE_PWR_CONTROL, and
> most of this is abstracted away with function pointers already. The
> only part that tests this FEATURE_PWR_CONTROL flag is the
> initialization, which could very much be abstracted away with a
> function pointer (NULL meaning no PWR block present). There might be
> other use cases you're planning to use this for, so I'd like to hear
> about them to make my final opinion on that.
> 

I see your point — the intent here is mainly to have the feature flag
reflect hardware-level changes. In this series, for example, it
corresponds to the addition of the new PWR_CONTROL block.

Another use case would be arch v11, where a new PRFCNT_FEATURES register
was introduced. In that case, we might want to adjust the
counters_per_block [1] value depending on that register’s value.

I would also expect this mechanism to remain useful for future hardware
revisions, as it provides a clean way to describe architectural
differences without scattering version-specific checks throughout the
code, while still being lighter-weight than function pointers.

[1] https://lore.kernel.org/dri-devel/cover.1753449448.git.lukas.zapolskas@arm.com/

Kind regards,
Karunika Choo

>>
>> Signed-off-by: Karunika Choo <karunika.choo@arm.com>
>> ---
>>  drivers/gpu/drm/panthor/panthor_hw.c |  5 +++++
>>  drivers/gpu/drm/panthor/panthor_hw.h | 18 ++++++++++++++++++
>>  2 files changed, 23 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/panthor/panthor_hw.c b/drivers/gpu/drm/panthor/panthor_hw.c
>> index b6e7401327c3..34536526384d 100644
>> --- a/drivers/gpu/drm/panthor/panthor_hw.c
>> +++ b/drivers/gpu/drm/panthor/panthor_hw.c
>> @@ -186,3 +186,8 @@ int panthor_hw_init(struct panthor_device *ptdev)
>>  
>>  	return 0;
>>  }
>> +
>> +bool panthor_hw_has_feature(struct panthor_device *ptdev, enum panthor_hw_feature feature)
>> +{
>> +	return test_bit(feature, ptdev->hw->features);
>> +}
>> diff --git a/drivers/gpu/drm/panthor/panthor_hw.h b/drivers/gpu/drm/panthor/panthor_hw.h
>> index 39752de3e7ad..7a191e76aeec 100644
>> --- a/drivers/gpu/drm/panthor/panthor_hw.h
>> +++ b/drivers/gpu/drm/panthor/panthor_hw.h
>> @@ -4,14 +4,32 @@
>>  #ifndef __PANTHOR_HW_H__
>>  #define __PANTHOR_HW_H__
>>  
>> +#include <linux/types.h>
>> +
>>  struct panthor_device;
>>  
>> +/**
>> + * enum panthor_hw_feature - Bit position of each HW feature
>> + *
>> + * Used to define GPU specific features based on the GPU architecture ID.
>> + * New feature flags will be added with support for newer GPU architectures.
>> + */
>> +enum panthor_hw_feature {
>> +	/** @PANTHOR_HW_FEATURES_END: Must be last. */
>> +	PANTHOR_HW_FEATURES_END
>> +};
>> +
>> +
>>  /**
>>   * struct panthor_hw - GPU specific register mapping and functions
>>   */
>>  struct panthor_hw {
>> +	/** @features: Bitmap containing panthor_hw_feature */
>> +	DECLARE_BITMAP(features, PANTHOR_HW_FEATURES_END);
>>  };
>>  
>>  int panthor_hw_init(struct panthor_device *ptdev);
>>  
>> +bool panthor_hw_has_feature(struct panthor_device *ptdev, enum panthor_hw_feature feature);
>> +
>>  #endif /* __PANTHOR_HW_H__ */
>