Re: [PATCH 2/2 v5] tty/serial/8250: use mctrl_gpio helpers

[Stefan Roese]

On 12.06.19 11:16, Andy Shevchenko wrote:
> On Wed, Jun 12, 2019 at 10:13:05AM +0200, Stefan Roese wrote:
> > On 11.06.19 16:48, Andy Shevchenko wrote:
> > > 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.
> >
> > Where is the double write to a register? Sorry, I fail to spot it.
>
> Not to the one register. From the functional point of view the same signal is
> set up twice: once per UART register, once per GPIO pins.
>
> > > 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.
> >
> > Do you something like this in mind?
>
> More likely
>
> static inline int serial8250_MCR_to_TIOCM(int mcr)

MSR_to_TIOCM (see below) ...

> {
> 	int tiocm = 0;
>
> 	if (mcr & ...)
> 		tiocm |= ...;
> 	...
>
> 	return tiocm;
> }
>
> static inline int serial8250_TIOCM_to_MCR(int tiocm)
> {
> 	... in a similar way ...
> }

While implementing such wrapper functions I noticed, that get_mctrl() /
set_mctrl() need TIOCM->MCR and MSR->TIOCM (notice MSR vs MCR here) but
serial8250_in_MCR() needs MCR->TIOCM. So there is not that much
overlay here. Additionally the wrappers would need to handle all bits
and only some of them are needed in serial8250_in/out_MCR(), so I would
need to add masking here as well.

For my taste its not really worth adding these wrappers as they won't
make things much clearer (if at all).

Thanks,
Stefan