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Subject: Re: [PATCH V1] i2c: busses: tegra: Add suspend-resume support
To:     Bitan Biswas <bbiswas@nvidia.com>,
        Laxman Dewangan <ldewangan@nvidia.com>,
        Thierry Reding <treding@nvidia.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        linux-i2c@vger.kernel.org, linux-tegra@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Shardar Mohammed <smohammed@nvidia.com>,
        Sowjanya Komatineni <skomatineni@nvidia.com>,
        Mantravadi Karthik <mkarthik@nvidia.com>
References: <1559195718-6693-1-git-send-email-bbiswas@nvidia.com>
 <c8bad04b-67ef-bcdf-04df-4aa61271e81c@gmail.com>
 <9142282b-ab76-53a0-13ce-c43b8adc575f@nvidia.com>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <4f14a218-332c-0263-c6c5-73a13b2446f0@gmail.com>
Date:   Fri, 31 May 2019 15:43:56 +0300
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31.05.2019 11:50, Bitan Biswas пишет:
> 
> 
> On 5/30/19 4:27 AM, Dmitry Osipenko wrote:
>> 30.05.2019 8:55, Bitan Biswas пишет:
>>> Post suspend I2C registers have power on reset values. Before any
>>> transfer initialize I2C registers to prevent I2C transfer timeout
>>> and implement suspend and resume callbacks needed. Fix below errors
>>> post suspend:
>>>
>>> 1) Tegra I2C transfer timeout during jetson tx2 resume:
>>>
>>> [   27.520613] pca953x 1-0074: calling pca953x_resume+0x0/0x1b0 @
>>> 2939, parent: i2c-1
>>> [   27.633623] tegra-i2c 3160000.i2c: i2c transfer timed out
>>> [   27.639162] pca953x 1-0074: Unable to sync registers 0x3-0x5. -110
>>> [   27.645336] pca953x 1-0074: Failed to sync GPIO dir registers: -110
>>> [   27.651596] PM: dpm_run_callback(): pca953x_resume+0x0/0x1b0
>>> returns -110
>>> [   27.658375] pca953x 1-0074: pca953x_resume+0x0/0x1b0 returned -110
>>> after 127152 usecs
>>> [   27.666194] PM: Device 1-0074 failed to resume: error -110
>>>
>>> 2) Tegra I2C transfer timeout error on jetson Xavier post resume.
>>>
>>> Signed-off-by: Bitan Biswas <bbiswas@nvidia.com>
>>> ---
>>>   drivers/i2c/busses/i2c-tegra.c | 24 ++++++++++++++++++++++++
>>>   1 file changed, 24 insertions(+)
>>>
>>> diff --git a/drivers/i2c/busses/i2c-tegra.c
>>> b/drivers/i2c/busses/i2c-tegra.c
>>> index ebaa78d..f6a377f 100644
>>> --- a/drivers/i2c/busses/i2c-tegra.c
>>> +++ b/drivers/i2c/busses/i2c-tegra.c
>>> @@ -1687,9 +1687,33 @@ static int tegra_i2c_remove(struct
>>> platform_device *pdev)
>>>   }
>>>     #ifdef CONFIG_PM_SLEEP
>>> +static int tegra_i2c_suspend(struct device *dev)
>>> +{
>>> +    struct tegra_i2c_dev *i2c_dev = dev_get_drvdata(dev);
>>> +
>>> +    i2c_mark_adapter_suspended(&i2c_dev->adapter);
>>> +
>>> +    return 0;
>>> +}
>>> +
>>> +static int tegra_i2c_resume(struct device *dev)
>>> +{
>>> +    struct tegra_i2c_dev *i2c_dev = dev_get_drvdata(dev);
>>> +    int ret;
>>> +
>>> +    i2c_lock_bus(&i2c_dev->adapter, I2C_LOCK_ROOT_ADAPTER);
>>> +    ret = tegra_i2c_init(i2c_dev, false);
>>> +    i2c_unlock_bus(&i2c_dev->adapter, I2C_LOCK_ROOT_ADAPTER);
>>
>> Why the locking is needed here?
> 
> async resume could result in stress test issues if some client accesses
> the i2c instance. This ensures the i2c instance is locked till the
> initialization is complete.

1) This doesn't make much sense.. if client could access I2C during of
tegra_i2c_init execution, then what stops it to perform the access
before the lock is taken?

2) The whole point of the i2c_mark_adapter_* API is to catch those
faulty clients that have a broken suspend-resume sequence. Client will
get a -ESHUTDOWN on trying to issue I2C transfer while controller is
marked as suspended.

3) Please don't use async suspend-resume where it doesn't make sense.

Corollary: you should drop the locking because it doesn't do anything
useful.