linux-kernel - Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver

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Message-ID: <36b5627a-8639-6244-8620-4f23828e0638@nvidia.com>
Date:   Mon, 6 Apr 2020 13:05:42 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>, <thierry.reding@...il.com>,
        <jonathanh@...dia.com>, <frankc@...dia.com>, <hverkuil@...all.nl>,
        <sakari.ailus@....fi>, <helen.koike@...labora.com>
CC:     <sboyd@...nel.org>, <linux-media@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-clk@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver


On 4/6/20 12:53 PM, Dmitry Osipenko wrote:
> External email: Use caution opening links or attachments
>
>
> 06.04.2020 20:02, Sowjanya Komatineni пишет:
>> On 4/6/20 9:37 AM, Sowjanya Komatineni wrote:
>>> On 4/6/20 9:29 AM, Dmitry Osipenko wrote:
>>>> External email: Use caution opening links or attachments
>>>>
>>>>
>>>> 06.04.2020 19:12, Sowjanya Komatineni пишет:
>>>>> On 4/6/20 9:05 AM, Dmitry Osipenko wrote:
>>>>>> External email: Use caution opening links or attachments
>>>>>>
>>>>>>
>>>>>> 06.04.2020 18:35, Sowjanya Komatineni пишет:
>>>>>> ...
>>>>>>>>> +     /* wait for syncpt counter to reach frame start event
>>>>>>>>> threshold */
>>>>>>>>> +     err = host1x_syncpt_wait(chan->frame_start_sp, thresh,
>>>>>>>>> + TEGRA_VI_SYNCPT_WAIT_TIMEOUT, &value);
>>>>>>>>> +     if (err) {
>>>>>>>>> +             dev_err(&chan->video.dev,
>>>>>>>>> +                     "frame start syncpt timeout: %d\n", err);
>>>>>>>>> +             /* increment syncpoint counter for timedout events */
>>>>>>>>> + host1x_syncpt_incr(chan->frame_start_sp);
>>>>>>>> Why incrementing is done while hardware is still active?
>>>>>>>>
>>>>>>>> The sync point's state needs to be completely reset after resetting
>>>>>>>> hardware. But I don't think that the current upstream host1x driver
>>>>>>>> supports doing that, it's one of the known-long-standing problems of
>>>>>>>> the
>>>>>>>> host1x driver.
>>>>>>>>
>>>>>>>> At least the sp->max_val incrementing should be done based on the
>>>>>>>> actual
>>>>>>>> syncpoint value and this should be done after resetting hardware.
>>>>>>> upstream host1x driver don't have API to reset or to equalize max
>>>>>>> value
>>>>>>> with min/load value.
>>>>>>>
>>>>>>> So to synchronize missed event, incrementing HW syncpt counter.
>>>>>>>
>>>>>>> This should not impact as we increment this in case of missed events
>>>>>>> only.
>>>>>> It's wrong to touch sync point while hardware is active and it's
>>>>>> active
>>>>>> until being reset.
>>>>>>
>>>>>> You should re-check the timeout after hw resetting and manually put
>>>>>> the
>>>>>> syncpoint counter back into sync only if needed.
>>>>> There is possibility of timeout to happen any time even during the
>>>>> capture also and is not related to hw reset.
>>>>>
>>>>> Manual synchronization is needed when timeout of any frame events
>>>>> happen
>>>>> otherwise all subsequence frames will timeout due to mismatch in event
>>>>> counters.
>>>> My point is that hardware is stopped only after being reset, until then
>>>> you should assume that sync point could be incremented by HW at any
>>>> time.
>>>>
>>>> And if this happens that HW increments sync point after the timeout,
>>>> then the sync point counter should become out-of-sync in yours case,
>>>> IIUC. Because host1x_syncpt_incr() doesn't update the cached counter.
>>> We wait for enough time based on frame rate for syncpt increment to
>>> happen and if it doesn't happen by then definitely its missed event
>>> and we increment HW syncpoint for this timed event.
>>>
>>> cached value gets updated during syncpt wait for subsequent event.
>>>
>>> syncpt increment happens for all subsequent frame events during video
>>> capture.
>>>
>> Just to be clear, syncpt max value increment happens first and syncpt
>> trigger condition is programmed. hw syncpt increment happens based on HW
>> events.
>>
>> Wait time for HW syncpt to reach threshold is tuned to work for all
>> frame rates. So if increment doesn't happen by then, its definitely
>> missed event.
> This is questionable. Technically, speculating about whether the tuned
> value is good for all possible cases is incorrect thing to do.
>
> Although, I guess in practice it should be good enough for the starter
> and could be improved later on, once the host1x driver will be improved.

By tuned value I meant about 200ms wait timeout for frame event to 
happen is what we have been using in downstream and with BSP release 
images which works good for all sensors and bridges we supported so far.

>
>> In case of missed HW event corresponding to syncpt condition, hw syncpt
>> increment does not happen and driver increments it on timeout.
>>
>> As there is not API to equialize max with min incase of timeout/reset,
>> incrementing HW syncpt for timed out event.
>>
>> syncpt cached value gets updated during syncpt wait when it loads from
>> HW syncpt.
>>
>> As syncpt condition is already triggered, without compensating timeout
>> events or leaving syncpt max and hw syncpt in non synchronized state for
>> missed events, subsequent streamings will all timeout even on real events.
>>