linux-kernel - Re: [RFC PATCH v3 4/6] media: tegra: Add Tegra210 Video input driver

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Date:   Wed, 18 Mar 2020 09:25:12 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Hans Verkuil <hverkuil@...all.nl>, <thierry.reding@...il.com>,
        <jonathanh@...dia.com>, <frankc@...dia.com>,
        <helen.koike@...labora.com>, <sboyd@...nel.org>
CC:     <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 v3 4/6] media: tegra: Add Tegra210 Video input driver


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...

>
>>>>> 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