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 09:41:18 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.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


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.