Re: [PATCH 2/3] serial: mxs: enable the DMA only when the rts/cts is enabled

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

Hello,

On Mon, Jul 15, 2013 at 04:41:25PM +0800, Huang Shijie wrote:
> 于 2013年07月15日 16:27, Uwe Kleine-König 写道:
> >do you want to say that the driver fails to only enable DMA when RTS/CTS
> >are available; or that today the driver can handle DMA just fine even
> >without RTS/CTS? I interpret your commit log as the latter, your patch
> >implements the former however.
> in the mxs-auart, if the RTS/CTS is not invalid, the DMA should be
> not enabled.
> 
> But current code lost the limit, a uart without the RTS/CTS may also
> enables the DMA in which case the uart
> may does not run or run in a abnormal way.
So this sounds like a fix that should go into stable and so preferably
should be the first patch in your series.

Something like:

	serial: mxs-auart: DMA unreliable without RTS/CTS

	According to [add some document name here] DMA doesn't work
	reliable without hardware handshaking. So make DMA dependant on
	a newly introdused property "fsl,uart-has-rtscts".

	Cc: stable@kernel.org # [first affected version]

The flag is only used to decide if dma should be enabled. So I think an
in-code comment would be nice, too. Is it still correct to set
AUART_CTRL2_RTSEN and AUART_CTRL2_RTS and read AUART_STAT_CTS when
fsl,uart-has-rtscts is not provided?
(BTW, mxs_auart_set_mctrl has:

	u32 ctrl = readl(u->membase + AUART_CTRL2);

	ctrl &= ~(AUART_CTRL2_RTSEN | AUART_CTRL2_RTS);
        if (mctrl & TIOCM_RTS) {
                if (tty_port_cts_enabled(&u->state->port))
                        ctrl |= AUART_CTRL2_RTSEN;
                else
                        ctrl |= AUART_CTRL2_RTS;
        }

	s->ctrl = mctrl;

A comment for the diligent reader about the difference between RTSEN and
RTS would be nice. Also I wonder if shadowing mctrl is sensible and used
correctly here.)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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