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Subject: Re: [RFC PATCH v3 4/6] media: tegra: Add Tegra210 Video input driver
To:     Sowjanya Komatineni <skomatineni@nvidia.com>,
        thierry.reding@gmail.com, jonathanh@nvidia.com, frankc@nvidia.com,
        helen.koike@collabora.com, sboyd@kernel.org
Cc:     linux-media@vger.kernel.org, devicetree@vger.kernel.org,
        linux-clk@vger.kernel.org, linux-tegra@vger.kernel.org,
        linux-kernel@vger.kernel.org
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From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <bc571308-93e5-e720-1cac-eb3effe1acdd@xs4all.nl>
Date:   Thu, 19 Mar 2020 15:29:52 +0100
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On 3/18/20 6:17 PM, Sowjanya Komatineni wrote:
> 
> On 3/18/20 9:25 AM, Sowjanya Komatineni wrote:
>>
>> On 3/18/20 9:14 AM, Sowjanya Komatineni wrote:
>>>
>>> On 3/18/20 4:48 AM, Hans Verkuil wrote:
>>>> External email: Use caution opening links or attachments
>>>>
>>>>
>>>> On 2/24/20 5:45 AM, Sowjanya Komatineni wrote:
>>>>> On 2/20/20 11:11 AM, Sowjanya Komatineni wrote:
>>>>>> On 2/20/20 5:33 AM, Hans Verkuil wrote:
>>>>>>> External email: Use caution opening links or attachments
>>>>>>>
>>>>>>>
>>>>>>> (Replying to myself so I can explain this a bit more)
>>>>>>>
>>>>>>> On 2/20/20 1:44 PM, Hans Verkuil wrote:
>>>>>>>>> +
>>>>>>>>> +static int tegra_csi_tpg_channels_alloc(struct tegra_csi *csi)
>>>>>>>>> +{
>>>>>>>>> +    struct device_node *node = csi->dev->of_node;
>>>>>>>>> +    unsigned int port_num;
>>>>>>>>> +    int ret;
>>>>>>>>> +    struct tegra_csi_channel *item;
>>>>>>>>> +    unsigned int tpg_channels = csi->soc->csi_max_channels;
>>>>>>>>> +
>>>>>>>>> +    /* allocate CSI channel for each CSI x2 ports */
>>>>>>>>> +    for (port_num = 0; port_num < tpg_channels; port_num++) {
>>>>>>>>> +            item = devm_kzalloc(csi->dev, sizeof(*item), 
>>>>>>>>> GFP_KERNEL);
>>>>>>>> Using devm_*alloc can be dangerous. If someone unbinds the 
>>>>>>>> driver, then
>>>>>>>> all memory allocated with devm_ is immediately freed. But if an
>>>>>>>> application
>>>>>>>> still has a filehandle open, then when it closes it it might still
>>>>>>>> reference
>>>>>>>> this already-freed memory.
>>>>>>>>
>>>>>>>> I recommend that you avoid using devm_*alloc for media drivers.
>>>>>>> A good test is to unbind & bind the driver:
>>>>>>>
>>>>>>> cd /sys/devices/platform/50000000.host1x/54080000.vi/driver
>>>>>>> echo -n 54080000.vi >unbind
>>>>>>> echo -n 54080000.vi >bind
>>>>>>>
>>>>>>> First just do this without the driver being used. That already
>>>>>>> gives me 'list_del corruption' kernel messages (list debugging
>>>>>>> is turned on in my kernel).
>>>>> Will fix in v4 to use kzalloc and also proper release v4l2 to make 
>>>>> sure
>>>>> unbind/bind works properly.
>>>>>
>>>>> BTW, tegra vi and csi are registered as clients to host1x video 
>>>>> driver.
>>>>>
>>>>> So, unbind and bind should be done with host1x video driver 
>>>>> "tegra-video"
>>>>>
>>>>> cd /sys/devices/platform/50000000.host1x/tegra-video/driver
>>>>> echo -n tegra-video > unbind
>>>>> echo -n tegra-video > bind
>>>> This still crashes with v4, at least if I am streaming with v4l2-ctl 
>>>> --stream-mmap.
>>>> Is that known?
>>>>
>>>> It's not a big deal at this moment, just want to know if this will 
>>>> be looked
>>>> at later.
>>>>
>>>> Regards,
>>>>
>>>>          Hans
>>>
>>> Weird, I tested streaming after unbind and bind as well and don't see 
>>> crash. Did below steps and tried several times unbind/bind as well.
>>>
>>> ./v4l2-ctl --stream-mmap --stream-count=1 -d /dev/video3
>>> cd /sys/devices/platform/50000000.host1x/tegra-video/driver
>>> echo -n tegra-video > unbind
>>> sleep 1
>>> echo -n tegra-video > bind
>>> cd /home/ubuntu
>>> ./v4l2-ctl --stream-mmap --stream-count=1 -d /dev/video3
>>>
>>> Can you post call trace when you saw crash?
>>
>> Tried unbind when  node is open with v4l2-ctl --sleep 10 as well and 
>> bind back.
>>
>> I don't see crash. Will confirm on doing unbind/bind with stream-mmap...
>>
> Able to repro when unbind/bind happens while stream-mmap.

That's indeed what I did. I don't want to try it again since I'm working from home
and the Jetson is in the office. And once it crashes I need someone in the office
to press the reset button. I hope I can pick it up next week to keep it at home as
that will make testing a lot easier.

> 
> Will look and have fix in v5.

Nice!

Thank you,

	Hans

> 
> Thanks Hans.
> 
>>>
>>>>>>> Note that this first test is basically identical to a rmmod/modprobe
>>>>>>> of the driver. But when I compiled the driver as a module it didn't
>>>>>>> create any video device nodes! Nor did I see any errors in the 
>>>>>>> kernel
>>>>>>> log. I didn't pursue this, and perhaps I did something wrong, but 
>>>>>>> it's
>>>>>>> worth taking a look at.
>>>>>>>
>>>>>>> The next step would be to have a video node open with:
>>>>>>>
>>>>>>> v4l2-ctl --sleep 10
>>>>>>>
>>>>>>> then while it is sleeping unbind the driver and see what happens
>>>>>>> when v4l2-ctl exits.
>>>>>>>
>>>>>>> Worst case is when you are streaming:
>>>>>>>
>>>>>>> v4l2-ctl --stream-mmap
>>>>>>>
>>>>>>> and then unbind.
>>>>>>>
>>>>>>> In general, the best way to get this to work correctly is:
>>>>>>>
>>>>>>> 1) don't use devm_*alloc
>>>>>>> 2) set the release callback of struct v4l2_device and do all freeing
>>>>>>> there.
>>>>>>> 3) in the platform remove() callback you call 
>>>>>>> media_device_unregister()
>>>>>>>      and video_unregister_device().
>>>>>> Reg 3, in current patch, media_device_unregister is called in
>>>>>> host1x_video_remove
>>>>>> video_unregister_device happens during host1x_video_remove ->
>>>>>> host1x_device_exit -> tegra_vi_exit -> tegra_vi_channels_cleanup
>>>>>>
>>>>>>> It's worth getting this right in this early stage, rather than 
>>>>>>> fixing it
>>>>>>> in the future.
>>>>>>>
>>>>>>> Regards,
>>>>>>>
>>>>>>>           Hans