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

[Sowjanya Komatineni]

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)
> > > > +{
> > > > +ÂÂÂ struct device_node *node = csi->dev->of_node;
> > > > +ÂÂÂ unsigned int port_num;
> > > > +ÂÂÂ int ret;
> > > > +ÂÂÂ struct tegra_csi_channel *item;
> > > > +ÂÂÂ unsigned int tpg_channels = csi->soc->csi_max_channels;
> > > > +
> > > > +ÂÂÂ /* allocate CSI channel for each CSI x2 ports */
> > > > +ÂÂÂ for (port_num = 0; port_num < tpg_channels; port_num++) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ 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

> > 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()
> > ÂÂÂ 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,
> >
> > ÂÂÂÂÂÂÂÂ Hans