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

[Sowjanya Komatineni <skomatineni@nvidia.com>]

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).
>
> 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