Hi Mihail,
On Thu, 8 Aug 2024 12:41:05 +0300
Mihail Atanassov <mihail.atanassov@xxxxxxx> wrote:
+/** + * struct drm_panthor_timestamp_info - Timestamp information +
* + * Structure grouping all queryable information relating to the
GPU timestamp. + */ +struct drm_panthor_timestamp_info { + /**
@timestamp_frequency: The frequency of the timestamp timer. */ +
__u64 timestamp_frequency; + + /** @current_timestamp: The current
timestamp. */ + __u64 current_timestamp;
As it stands, this query has nothing to do with the actual GPU so
doesn't really belong here.
It'd be more valuable, and can maybe give better calibration results
than querying the system timestamp separately in userspace, if you
reported all of:
* the system timer value
* the system timer frequency
* the GPU timer value
* the GPU timer frequency (because it _could_ be different in some
systems)
Duh, I wish this wasn't the case and all SoC vendors went for the
arch-timer which guarantees the consistency of the timestamp on the GPU
and CPU. But let's say this is a case we need to support, wouldn't it
be more useful to do the CPU/GPU calibration kernel side (basically at
init/resume time) and then expose the formula describing the
relationship between those 2 things:
CPU_time = GPU_time * GPU_to_CPU_mul / GPU_to_CPU_div +
GPU_to_CPU_offset;
* the GPU timer offset
Assuming you're talking about the TIMESTAMP_OFFSET register, my
understanding is that this offset should be set by the kernel driver to
account for any disjoint caused by suspend/resume cycles, or any
design-specific offset between the arch-timer and the timer feeding the
GPU timestamp block (hopefully the arch-timer is directly propagated to
the GPU though). The timestamp read by the GPU/CPU already has this
offset added, so I'm not sure I understand what's the value of exposing
it to userspace. As long as the CPU/GPU timestamps are consistent,
userspace probably doesn't care, but I might be missing something.
+}; + /** * struct drm_panthor_dev_query - Arguments passed to
DRM_PANTHOR_IOCTL_DEV_QUERY */
base-commit: f7f3ddb6e5c8dc7b621fd8c0903ea42190d67452