Re: [PATCH] tty: serdev: serdev-ttyport: set correct tty->dev for serdev framework

[Greg KH]

On Wed, Mar 15, 2023 at 09:49:53AM +0000, Sherry Sun wrote:
> 
> 
> > -----Original Message-----
> > From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Sent: 2023年3月15日 15:40
> > To: Sherry Sun <sherry.sun@xxxxxxx>
> > Cc: jirislaby@xxxxxxxxxx; robh@xxxxxxxxxx; linux-serial@xxxxxxxxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx; dl-linux-imx <linux-imx@xxxxxxx>
> > Subject: Re: [PATCH] tty: serdev: serdev-ttyport: set correct tty->dev for
> > serdev framework
> > 
> > On Wed, Mar 15, 2023 at 03:21:43PM +0800, Sherry Sun wrote:
> > > ttyport_open() calls tty_init_dev() to initialize a tty device, but
> > > tty_get_device() cannot get the correct tty->dev for serdev tty in
> > > alloc_tty_struct(), because serdev framework does not set tty_class,
> > > so class_find_device_by_devt(tty_class, devt) may always return NULL.
> > >
> > > For serdev framework, we need to assign the correct ctrl->dev to
> > > tty->dev.
> > >
> > > Fixes: bed35c6dfa6a ("serdev: add a tty port controller driver")
> > > Signed-off-by: Sherry Sun <sherry.sun@xxxxxxx>
> > > ---
> > >  drivers/tty/serdev/serdev-ttyport.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/drivers/tty/serdev/serdev-ttyport.c
> > > b/drivers/tty/serdev/serdev-ttyport.c
> > > index d367803e2044..bba37ab90215 100644
> > > --- a/drivers/tty/serdev/serdev-ttyport.c
> > > +++ b/drivers/tty/serdev/serdev-ttyport.c
> > > @@ -112,6 +112,7 @@ static int ttyport_open(struct serdev_controller
> > *ctrl)
> > >  	tty = tty_init_dev(serport->tty_drv, serport->tty_idx);
> > >  	if (IS_ERR(tty))
> > >  		return PTR_ERR(tty);
> > > +	tty->dev = &ctrl->dev;
> > 
> > What in-kernel driver needs this change?  How has it not been a problem so
> > far?
> > 
> 
> Hi Greg, I searched the users of tty->dev under serial floder, found the following drivers need it.
> drivers/tty/serial/stm32-usart.c:780:                   pm_wakeup_event(tport->tty->dev, 0);
> drivers/tty/serial/fsl_lpuart.c:3018:           tty_dev = tty->dev;
> drivers/tty/serial/st-asc.c:266:                pm_wakeup_event(tport->tty->dev, 0);
> 
> Actually this issue was found when I tested the nxp Bluetooth driver which use serdev framework along with fsl_lpuart.c driver, when system is suspending, the following NULL pointer kernel panic is observed.
> This is because lpuart driver will check the device_may_wakeup(tty->dev) to determine if wakeup register bits need to be enabled or not before suspend, it works well the the ldisc tty, but since serdev tty doesn't set correct tty->dev, so here cause the NULL pointer panic.
> 
> root@imx8ulpevk:~# echo mem > /sys/power/state
> [   42.657779] PM: suspend entry (deep)
> [   42.664333] Filesystems sync: 0.002 seconds
> [   42.717624] Freezing user space processes ... (elapsed 0.001 seconds) done.
> [   42.727063] OOM killer disabled.
> [   42.730383] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
> [   42.753652] fec 29950000.ethernet eth0: Link is Down
> [   42.780681] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000dc
> [   42.789603] Mem abort info:
> [   42.792430]   ESR = 0x0000000096000004
> [   42.796242]   EC = 0x25: DABT (current EL), IL = 32 bits
> [   42.801661]   SET = 0, FnV = 0
> ......
> 
> > And why are you saving off a reference counted pointer without
> > incrementing the reference to the pointer?
> 
> Sorry, forgive me I am not clearly understand the requirement here, do you mean we need to add the following changes?
>     get_device(&ctrl->dev);
>     tty->dev = &ctrl->dev;
>     put_device((&ctrl->dev);

Ick, no, only do put_device() when you are finished with the pointer and
are not going to access it anymore.

> And per my understanding, the reference count needs to be increased and decreased from the user side, here we only do a initialization for the tty->dev.

Then something is not set up properly here, sorry.

greg k-h