linux-kernel - Re: [PATCH 2/2 v5] tty/serial/8250: use mctrl

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Message-ID: <20190611144828.GX9224@smile.fi.intel.com>
Date:   Tue, 11 Jun 2019 17:48:28 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Stefan Roese <sr@...x.de>
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yegor Yefremov <yegorslists@...glemail.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Giulio Benetti <giulio.benetti@...ronovasrl.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 2/2 v5] tty/serial/8250: use mctrl_gpio helpers

On Tue, Jun 11, 2019 at 04:02:54PM +0200, Stefan Roese wrote:
> On 11.06.19 14:44, Andy Shevchenko wrote:
> > On Tue, Jun 11, 2019 at 12:56:03PM +0200, Stefan Roese wrote:

> > >   static inline void serial8250_out_MCR(struct uart_8250_port *up, int value)
> > >   {
> > >   	serial_out(up, UART_MCR, value);
> > > +
> > > +	if (up->gpios) {
> > > +		int mctrl_gpio = 0;
> > > +
> > > +		if (value & UART_MCR_RTS)
> > > +			mctrl_gpio |= TIOCM_RTS;
> > > +		if (value & UART_MCR_DTR)
> > > +			mctrl_gpio |= TIOCM_DTR;
> > > +
> > > +		mctrl_gpio_set(up->gpios, mctrl_gpio);
> > > +	}
> > >   }

> > >   static inline int serial8250_in_MCR(struct uart_8250_port *up)
> > >   {
> > > -	return serial_in(up, UART_MCR);
> > > +	int mctrl;
> > > +
> > > +	mctrl = serial_in(up, UART_MCR);
> > > +
> > > +	if (up->gpios) {
> > > +		int mctrl_gpio = 0;
> > > +
> > > +		/* save current MCR values */
> > > +		if (mctrl & UART_MCR_RTS)
> > > +			mctrl_gpio |= TIOCM_RTS;
> > > +		if (mctrl & UART_MCR_DTR)
> > > +			mctrl_gpio |= TIOCM_DTR;
> > > +
> > > +		mctrl_gpio = mctrl_gpio_get_outputs(up->gpios, &mctrl_gpio);
> > > +		if (mctrl_gpio & TIOCM_RTS)
> > > +			mctrl |= UART_MCR_RTS;
> > > +		else
> > > +			mctrl &= ~UART_MCR_RTS;
> > > +
> > > +		if (mctrl_gpio & TIOCM_DTR)
> > > +			mctrl |= UART_MCR_DTR;
> > > +		else
> > > +			mctrl &= ~UART_MCR_DTR;
> > > +	}
> > > +
> > > +	return mctrl;
> > >   }
> > 
> > These are using OR logic with potentially volatile data. Shouldn't we mask
> > unused bits in UART_MCR in case of up->gpios != NULL?
> 
> Sorry, I don't see, which bits you are referring to? Could you please be
> a bit more specific with the variable / macro meant (example)?

I meant that we double write values in the out() which might have some
consequences, though I hope nothing wrong with it happens.

In the in() we read the all bits in the register.

As now I look at the implementation of mctrl_gpio_get_outputs(),
I think we rather get helpers for conversion between TIOCM and UART_MCR values,
so, they can be used in get_mctrl() / set_mctrl() and above.

The logic now is understandable to me (I was confused by the conversions here
and there).

-- 
With Best Regards,
Andy Shevchenko