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Date:   Mon, 18 Mar 2019 14:04:00 +0000
From:   Anson Huang <anson.huang@....com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
CC:     "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "festevam@...il.com" <festevam@...il.com>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "otavio@...ystems.com.br" <otavio@...ystems.com.br>,
        "stefan@...er.ch" <stefan@...er.ch>,
        Leonard Crestez <leonard.crestez@....com>,
        "schnitzeltony@...il.com" <schnitzeltony@...il.com>,
        "jan.tuerk@...rion.com" <jan.tuerk@...rion.com>,
        Robin Gong <yibin.gong@....com>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH V4 2/5] pwm: Add i.MX TPM PWM driver support

Hi, Uwe

Best Regards!
Anson Huang

> -----Original Message-----
> From: Anson Huang
> Sent: 2019年3月18日 18:08
> To: 'Uwe Kleine-König' <u.kleine-koenig@...gutronix.de>
> Cc: thierry.reding@...il.com; robh+dt@...nel.org; mark.rutland@....com;
> shawnguo@...nel.org; s.hauer@...gutronix.de; kernel@...gutronix.de;
> festevam@...il.com; linux@...linux.org.uk; otavio@...ystems.com.br;
> stefan@...er.ch; Leonard Crestez <leonard.crestez@....com>;
> schnitzeltony@...il.com; jan.tuerk@...rion.com; Robin Gong
> <yibin.gong@....com>; linux-pwm@...r.kernel.org;
> devicetree@...r.kernel.org; linux-arm-kernel@...ts.infradead.org; linux-
> kernel@...r.kernel.org; dl-linux-imx <linux-imx@....com>
> Subject: RE: [PATCH V4 2/5] pwm: Add i.MX TPM PWM driver support
> 
> Hi, Uwe
> 
> Best Regards!
> Anson Huang
> 
> > -----Original Message-----
> > From: Uwe Kleine-König [mailto:u.kleine-koenig@...gutronix.de]
> > Sent: 2019年3月18日 16:08
> > To: Anson Huang <anson.huang@....com>
> > Cc: thierry.reding@...il.com; robh+dt@...nel.org;
> > mark.rutland@....com; shawnguo@...nel.org; s.hauer@...gutronix.de;
> > kernel@...gutronix.de; festevam@...il.com; linux@...linux.org.uk;
> > otavio@...ystems.com.br; stefan@...er.ch; Leonard Crestez
> > <leonard.crestez@....com>; schnitzeltony@...il.com;
> > jan.tuerk@...rion.com; Robin Gong <yibin.gong@....com>;
> > linux-pwm@...r.kernel.org; devicetree@...r.kernel.org;
> > linux-arm-kernel@...ts.infradead.org; linux- kernel@...r.kernel.org;
> > dl-linux-imx <linux-imx@....com>
> > Subject: Re: [PATCH V4 2/5] pwm: Add i.MX TPM PWM driver support
> >
> > On Mon, Mar 18, 2019 at 07:41:02AM +0000, Anson Huang wrote:
> > > Hi,Uwe
> > > > > +	val &= ~PWM_IMX_TPM_SC_CMOD;
> > > > > +	writel(val, tpm->base + PWM_IMX_TPM_SC);
> > > >
> > > > As this interrupts the output, please only do it if necessary.
> > >
> > > OK, will do it ONLY when it is enabled previously.
> >
> > I think you only need to do that when the value actually changes.
> 
> OK, I will save the period/div count and ONLY do it when the value actually
> changes.

After further think, I added below tpm->period to save the current period settings,
ONLY when the new period to be set is different from the current period, then the 
pwm_imx_tpm_config_counter() is called, so I think no need to care about the value
changes, the value is always changed when pwm_imx_tpm_config_counter() is called.

                 if (tpm->user_count != 1 && state->period != tpm->period)
                         return -EBUSY;
                 ret = pwm_imx_tpm_config_counter(chip, state->period);
                 if (ret)
                         return ret;


> 
> >
> > > > > +	/* set duty counter */
> > > > > +	tmp = readl(tpm->base + PWM_IMX_TPM_MOD) &
> > > > > +PWM_IMX_TPM_MOD_MOD_MASK;
> > > >
> > > > I recommend storing this value in driver data.
> > >
> > > NOT quite understand, as we did NOT use it in other places except
> > > the get_state, just reading the register once should be OK there.
> >
> > I had the impression it is used more than once. Will look again in the
> > next iteration. But also note that shadowing the value might already
> > be beneficial for a single call site as driver data might occupy more
> > RAM than necessary anyhow and reading from RAM is faster than from the
> hardware's register.
> > Probably this is not a fast path, so not worth the optimisation?!
> 
> OK, will save it in driver data and avoid accessing register again.
> 
> >
> > > > I wonder why MSA and MSB are two bits instead of making this a
> > > > field of width 2 with 2b10 meaning PWM mode. But maybe it's just
> > > > me not understanding the independent semantic of these two bits?
> > >
> > > I think making them a field makes more sense, but anyway we just
> > > follow the RM.
> >
> > If it makes the driver easier to understand (which I think it does)
> > feel free to derivate from the RM. Just add a comment to the definition,
> then it's fine.
> 
> OK, I will change the register definition and a comment for it.
> 
> >
> > > > Reading the reference manual I'd say in PWM mode the semantic of
> > > > ELSA and ELSB is:
> > > >
> > > > 	On counter reload set the output to ELSB
> > > > 	On counter match set the output to ELSA
> > > >
> > > > Noting that in a comment would ease understanding the code here.
> > >
> > > I added below comment for PWM modes:
> > >
> > > 137         /*
> > > 138          * set polarity (for edge-aligned PWM modes)
> > > 139          *
> > > 140          * CPWMS  MSB:MSA  ELSB:ELSA  Mode  Configuration
> > > 141          * 0      10       10         PWM   High-true pulse
> > > 142          * 0      10       00         PWM   Reserved
> > > 143          * 0      10       01         PWM   Low-true pulse
> > > 144          * 0      10       11         PWM   Reserved
> > > 145          *
> > > 146          * High-true pulse: clear output on counter match, set output on
> > > 147          * counter reload, set output when counter first enabled or
> paused.
> > > 148          *
> > > 149          * Low-true pulse: set output on counter match, clear output on
> > > 150          * counter reload, clear output when counter first enabled or
> > paused.
> > > 151          */
> >
> > I stumbled over "high-true" and "low-true" in the RM, too. In my
> > bubble this is an uncommon wording. I'd write instead:
> >
> > 	/*
> > 	 * set polarity
> > 	 *
> > 	 * ELSB:ELSA = 2b10 yields normal polarity behaviour, ELSB:ELSA
> > 	 * = 2b01 yields inversed polarity. The other values are
> > 	 * reserved.
> > 	 */
> >
> > And don't write about CPWM, MSA and MSB which are always used with
> > fixed values anyhow in the driver.
> >
> 
> OK.
> 
> > > > > +		/* disable channel */
> > > > > +		writel(PWM_IMX_TPM_CnSC_CHF,
> > > > > +			tpm->base + PWM_IMX_TPM_CnSC(pwm-
> >hwpwm));
> > > >
> > > > Clearing CHF doens't disable the channel as I read the manual.
> > >
> > > This write clears CHF as well as writing other bits 0, to disable
> > > the output. Maybe I can explicitly clear MSA/MSB/ELSA/ELSB to avoid
> > confusion.
> >
> > Ah, I misinterpreted the value written.
> >
> > > > > +static int pwm_imx_tpm_apply(struct pwm_chip *chip, struct
> > pwm_device *pwm,
> > > > > +			     struct pwm_state *state) {
> > > > > +	struct imx_tpm_pwm_chip *tpm =
> to_imx_tpm_pwm_chip(chip);
> > > > > +	struct pwm_state curstate;
> > > > > +	u32 duty_cycle = state->duty_cycle;
> > > > > +	int ret;
> > > > > +
> > > > > +	pwm_imx_tpm_get_state(chip, pwm, &curstate);
> > > > > +
> > > > > +	mutex_lock(&tpm->lock);
> > > >
> > > > What should this lock protect? Does it hurt if the state changes
> > > > between pwm_imx_tpm_get_state releasing the lock and
> > > > pwm_imx_tpm_apply taking it?
> > >
> > > The idea is to protect the share resourced by multiple channels, but
> > > I think I can make the mutex_lock includes get_state and remove the
> > > lock in
> > get_state function.
> > A
> > You might need it in .get_state to return a consistent state to the
> > caller. In this case just introduce an unlocked variant of .get_state
> > to share code between the two functions.
> >
> > And BTW the question was honest. I'm not entirely sure that you need
> > to hold the lock.
> 
> Agreed, if the different channel configuration ONLY access its own register,
> NOT any shared registers, then I think this lock is unnecessary.

Since all the functions in .apply function will need to access registers and these
registers are shared by different channels, so I think the lock is necessary. 

Anson.

> 
> >
> > > > > +		 */
> > > > > +		if (tpm->user_count != 1)
> > > > > +			return -EBUSY;
> > > >
> > > > Ideally if say period = 37 is requested but currently we have
> > > > period =
> > > > 36 and configuring 37 would result in 36 anyhow, don't return EBUSY.
> > >
> > > I think here the protection is just for making sure that is there
> > > are multiple users, period can NOT be changed, since all channels
> > > will be
> > impacted.
> >
> > I think you misunderstood what I intended to say here.
> >
> > Consider that in the case that there is only a single PWM in use
> > configuring for a period of 37 ns actually yields 36 ns because the
> > hardware cannot provide 37 ns and 36 ns is the best match.
> >
> > Then if a second user comes and requests 37 ns, the result here is,
> > that the second user gets the -EBUSY. This is ridiculous however
> > because the request is denied even though the period is already
> > configured for the length that would be configured if the second user were
> the only one.
> 
> Ah, if I understand correctly, I think I can change it to, if second user try to set
> a period different from first user, then return -EBUSY, otherwise, return
> success.
> 
> >
> > > > > +	tpm->chip.dev = &pdev->dev;
> > > > > +	tpm->chip.ops = &imx_tpm_pwm_ops;
> > > > > +	tpm->chip.base = -1;
> > > > > +	tpm->chip.npwm = PWM_IMX_TPM_MAX_CHANNEL_NUM;
> > > >
> > > > This is wrong, as some only have 2 channels?
> > >
> > > I saw we can get channel number from register, will read register to
> > > determine the channel number, but for the channel config and status
> > > saved in struct, I will still use the MAX channel number to define the array.
> >
> > Yeah, that is sensible.
> 
> I will resend V5 patch set, since there are some mis-understanding for
> previous comments.
> 
> Thanks,
> Anson.
> 
> >
> > Best regards
> > Uwe
> >
> > --
> > Pengutronix e.K.                           | Uwe Kleine-König            |
> > Industrial Linux Solutions                 |
> >
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