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Date: Tue, 21 Jun 2016 23:22:58 +0200
From: Boris Brezillon <boris.brezillon@...e-electrons.com>
To: Brian Norris <briannorris@...omium.org>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>,
Thierry Reding <thierry.reding@...il.com>,
Linux PWM List <linux-pwm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Brian Norris <computersforpeace@...il.com>,
Doug Anderson <dianders@...omium.org>,
linux-renesas-soc@...r.kernel.org,
Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v2] pwm: improve args checking in pwm_apply_state()
On Tue, 21 Jun 2016 11:37:31 -0700
Brian Norris <briannorris@...omium.org> wrote:
> Hi Geert,
>
> On Tue, Jun 21, 2016 at 04:42:04PM +0200, Geert Uytterhoeven wrote:
> > On Fri, May 27, 2016 at 6:45 PM, Brian Norris <briannorris@...omium.org> wrote:
> > > It seems like in the process of refactoring pwm_config() to utilize the
> > > newly-introduced pwm_apply_state() API, some args/bounds checking was
> > > dropped.
> > >
> > > In particular, I noted that we are now allowing invalid period
> > > selections. e.g.:
> > >
> > > # echo 1 > /sys/class/pwm/pwmchip0/export
> > > # cat /sys/class/pwm/pwmchip0/pwm1/period
> > > 100
> > > # echo 101 > /sys/class/pwm/pwmchip0/pwm1/duty_cycle
> > > [... driver may or may not reject the value, or trigger some logic bug ...]
> > >
> > > It's better to see:
> > >
> > > # echo 1 > /sys/class/pwm/pwmchip0/export
> > > # cat /sys/class/pwm/pwmchip0/pwm1/period
> > > 100
> > > # echo 101 > /sys/class/pwm/pwmchip0/pwm1/duty_cycle
> > > -bash: echo: write error: Invalid argument
> > >
> > > This patch reintroduces some bounds checks in both pwm_config() (for its
> > > signed parameters; we don't want to convert negative values into large
> > > unsigned values) and in pwm_apply_state() (which fix the above described
> > > behavior, as well as other potential API misuses).
> > >
> > > Fixes: 5ec803edcb70 ("pwm: Add core infrastructure to allow atomic updates")
> > > Signed-off-by: Brian Norris <briannorris@...omium.org>
> > > ---
> > > v2:
> > > * changed subject, as this covers more scope now
> > > * add Fixes tag, as this is a v4.7-rc regression
> > > * add more bounds/args checks in pwm_apply_state() and pwm_config()
> > >
> > > drivers/pwm/core.c | 3 ++-
> > > include/linux/pwm.h | 3 +++
> > > 2 files changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
> > > index dba3843c53b8..ed337a8c34ab 100644
> > > --- a/drivers/pwm/core.c
> > > +++ b/drivers/pwm/core.c
> > > @@ -457,7 +457,8 @@ int pwm_apply_state(struct pwm_device *pwm, struct pwm_state *state)
> > > {
> > > int err;
> > >
> > > - if (!pwm)
> > > + if (!pwm || !state || !state->period ||
> > > + state->duty_cycle > state->period)
> > > return -EINVAL;
> >
> > This check breaks the LCD backlight on r8a7740/armadillo.
> > Apparently both period and duty_cycle are zero during the first invocation.
> > Later, these are initialized from DT, cfr.
> >
> > pwms = <&tpu 2 33333 PWM_POLARITY_INVERTED>;
> >
> > in arch/arm/boot/dts/r8a7740-armadillo800eva.dts.
>
> Hmm, this isn't super obvious how to best fix. On one hand, the
> pwm_config() API used to reject period<=0, but on the other hand, I
> think its replacement (pwm_apply_state()) is getting used in more places
> than it used to be, and not all of them are really handling the "atomic
> update" concept yet. Seems like a product of Boris's multi-phase attempt
> to convert the PWM APIs to support atomic updates -- and many users
> haven't really converted yet.
>
> > With added debug printing, the difference between failure and success is:
> >
> > renesas-tpu-pwm e6600000.pwm: TPU PWM -1 registered
> > tpu_pwm_request:223
> > pwm_apply_state:460: pwm backlight/2: period 0, duty_cycle 0
> > +Ignoring failure
> > +pwm_apply_state:479: polarity 0 -> 1
> > +tpu_pwm_set_polarity:343
> > +pwm_apply_state:502: period 0 -> 0
> > +pwm_apply_state:503: duty_cycle 0 -> 0
> > +pwm_apply_state:516: enabled 0 -> 0
> > pwm_config:238: pwm backlight/2: duty_ns 33333, period_ns 33333
> > pwm_apply_state:460: pwm backlight/2: period 33333, duty_cycle 33333
> > -pwm_apply_state:479: polarity 0 -> 0
> > +pwm_apply_state:479: polarity 1 -> 1
> > pwm_apply_state:502: period 0 -> 33333
> > pwm_apply_state:503: duty_cycle 0 -> 33333
> > tpu_pwm_config:267
> > pwm_apply_state:516: enabled 0 -> 0
> > pwm_apply_state:460: pwm backlight/2: period 33333, duty_cycle 33333
> > -pwm_apply_state:479: polarity 0 -> 0
> > +pwm_apply_state:479: polarity 1 -> 1
> > pwm_apply_state:502: period 33333 -> 33333
> > pwm_apply_state:503: duty_cycle 33333 -> 33333
> > pwm_apply_state:516: enabled 0 -> 1
> > tpu_pwm_enable:354
>
> I'm not sure I 100% understand this debug log, but I think maybe the
> problem is in pwm_apply_args(), which calls pwm_disable() and
> pwm_set_polarity() sequentially, without ever configuring a period? What
> if pwm_apply_args() were to configure the period for us?
>
> Boris, any thoughts?
I think this is partly caused by this commit:
a8c3862551e0 ("pwm: Keep PWM state in sync with hardware state")
But again, we need that one if we don't want to override the current
PWM period with the pwm_args one when the driver support hardware
readout.
This leaves 2 choices:
1/ assign the period to pargs->period if ->get_state() is not
implemented
2/ remove the !period check in pwm_apply_state() when state->enabled is
false (do we really care if period = 0 when all we want to do is
disable the PWM?)
Thierry, what's you're opinion.
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