Re: [PATCH] drm/panthor: Add DEV_QUERY_TIMESTAMP_INFO dev query

[Mihail Atanassov <mihail.atanassov@arm.com>]

On 28/08/2024 18:27, Boris Brezillon wrote:
> On Wed, 28 Aug 2024 18:07:03 +0200
> Boris Brezillon <boris.brezillon@collabora.com> wrote:
> 
>> On Wed, 28 Aug 2024 14:22:51 +0100
>> Mihail Atanassov <mihail.atanassov@arm.com> wrote:
>>
>>> Hi Boris,
>>>
>>> On 28/08/2024 13:09, Boris Brezillon wrote:
>>>> Hi Mihail,
>>>>
>>>> On Thu, 8 Aug 2024 12:41:05 +0300
>>>> Mihail Atanassov <mihail.atanassov@arm.com> 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;
>>>>      
>>>
>>> TIMESTAMP_OFFSET should indeed be set by the kernel (on resume). But I
>>> don't think we need to post M/D+offset to userspace. The 2 Frequencies +
>>> the scalar offset are the raw sources, and userspace can work back from
>>> there.
>>
>> Sure. No matter how you express the relationship, my point was, if the
>> calibration is supposed to happen in the kernel at resume time,
>> returning both the CPU/GPU time in DEV_QUERY_TIMESTAMP to make sure the
>> sampling is close enough that they actually represent the same
>> timestamp might not be needed, because you can easily convert from one
>> domain to the other.
> 
> I think it makes more sense after reading [1] :-). This being said, the
> maxDeviation is here to account for any latency that might exists
> between each domain sampling, so I'd be tempted to read the CPU
> monotonic time through the regular syscalls rather than add it to the
> DEV_QUERY_TIMESTAMP ioctl.
> 

Wouldn't that defeat the point of getting low-latency consecutive reads 
of both time domains? If you leave it to a separate syscall, you're at 
the mercy of a lot of things, so it's not just a scalar time delta, 
you'll get much higher measurement variance. Doing it in-kernel with no 
sleeps in the middle gets you better confidence in your samples being 
consistently correlated in time. If you have that in-kernel low latency 
correlation pairwise for all time domains you're interested in (in this 
case CPU & GPU timestamps, but you could get CPU & display IP 
timestamps, etc), you can then correlate all of the clocks in userspace.

Fundamentally, though, if you don't report CPU timestamps in v1 of the 
ioctl, you can extend later if it becomes clear that the maxDeviation is 
not low enough with the samples being across a syscall.

> [1]https://docs.vulkan.org/features/latest/features/proposals/VK_EXT_calibrated_timestamps.html

-- 
Mihail Atanassov <mihail.atanassov@arm.com>