linux-kernel - Re: [PATCH v8 08/10] gpu: host1x: mipi: Keep MIPI clock enabled till calibration is done

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Date:   Thu, 6 Aug 2020 20:27:15 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Sowjanya Komatineni <skomatineni@...dia.com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     jonathanh@...dia.com, frankc@...dia.com, hverkuil@...all.nl,
        sakari.ailus@....fi, robh+dt@...nel.org, helen.koike@...labora.com,
        gregkh@...uxfoundation.org, linux-media@...r.kernel.org,
        devicetree@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 08/10] gpu: host1x: mipi: Keep MIPI clock enabled till
 calibration is done

06.08.2020 20:12, Sowjanya Komatineni пишет:
> 
> On 8/6/20 9:41 AM, Sowjanya Komatineni wrote:
>>
>> On 8/6/20 9:10 AM, Dmitry Osipenko wrote:
>>> 06.08.2020 18:59, Sowjanya Komatineni пишет:
>>> ...
>>>>>> Confirmed from HW designer, calibration FSM to finish takes worst
>>>>>> case
>>>>>> 72uS so by the time it gets to sensor stream it will be done its
>>>>>> sequence and will be waiting for DONE bit.
>>>>>>
>>>>>> So disabling MIPI CAL clock on sensor stream fails is safe.
>>>>> 72us is quite a lot of time, what will happen if LP-11 happens before
>>>>> FSM finished calibration?
>>>>>
>>>>> Maybe the finish_calibration() needs to split into two parts:
>>>>>
>>>>>    1. wait for CAL_STATUS_ACTIVE before enabling sensor
>>>>>    2. wait for CAL_STATUS_DONE after enabling sensor
>>>> I don't think we need to split for active and done. Active will be 1 as
>>>> long as other pads are in calibration as well.
>>>>
>>>> We cant use active status check for specific pads under calibration.
>>>> This is common bit for all pads.
>>> Does hardware have a single FSM block shared by all pads or there is FSM
>>> per group of pads?
>>
>> MIPI CAL status register has DONE bits for individual pads status and
>> single ACTIVE bit.
>>
>> ACTIVE bit set to 1 indicates auto calibration is active which is the
>> case even when other pads (other CSI pads from other ports streaming
>> in case of parallel stream) are under calibration. Also DSI is shared
>> as well.
>>
>> We do calibration for individual pads. So, we should not rely on
>> ACTIVE bit.
>>
>>
>> MIPI driver checks for condition ACTIVE == 1 && DONE == 1 from the
>> beginning.
>>
>> But I think this also should be fixed as in case of parallel streams
>> calibration can happen in parallel waiting for ACTIVE to be cleared
>> makes all calibration callers to wait for longer than needed as ACTIVE
>> is common for all pads.
>>
>>>
>>>> Unfortunately HW don't have separate status indicating when sequence is
>>>> done to indicate its waiting for LP11.
>>>>
>>>>
>>>> To avoid all this, will remove cancel_calibration() totally and use
>>>> same
>>>> finish calibration even in case of stream failure then.
>>>>
>>> What about to add 72us delay to the end of start_calibration() in order
>>> to ensure that FSM is finished before LP-11?
>>
>> Why we should add 72uS in start_calibration() when can use same
>> finish_calibration() for both pass/fail cases?
>>
>> Only timing loose we see is in case of failure we still wait for 250ms
>> and as this is failing case I hope should be ok.
>>
> Also as we don't need cancel_calibration(), keeping tegra_mipi_wait()
> like earlier makes sense I believe as we are letting it finish going
> thru sequence.
> 
> So I think below are fixes,
> 
> 1. Existing MIPI driver, tegra_mipi_wait() to not use status ACTIVE bit
> to be 0 and use only DONE bit to be 1 for wait condition  as we are
> calibrating separately for individual pads and this ACTIVE bit is common
> for all pads where it will not be 0 in case of other parallel streams
> which may also be under calibration.

Yes, looks like it's a mistake of the current MIPI driver that it polls
the ACTIVE bit.

> 2. No need for separate cancel_calibration. So, probably earlier names
> tegra_mipi_calibrate() and tegra_mipi_wait() hols good as we are waiting
> for calibration sequence to finish irrespective of fail/pass.

The new names reflect better what those functions actually do, IMO.

What about to make finish_calibration() to take an additional argument
which corresponds to the awaited HW bits? For example if it's CSIA, then
it could be:

  tegra_mipi_finish_calibration(csi_chan->mipi, MIPI_CAL_CSIA);


Also, is it okay that DSI and CSI could change MIPI_CAL_CTRL after DSI
or CSI already started calibration?

Looking at the current start_calibration(), I think the mutex should be
kept locked and then finish_calibration() should unlock it.