linux-kernel - Re: [PATCH v2 1/1] serial: 8250_dw: Allow hardware flow control to be used

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Date:   Fri, 3 Mar 2017 23:23:05 +0000
From:   James Hogan <james.hogan@...tec.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC:     Heiko Stuebner <heiko@...ech.de>, Jason Uy <jason.uy@...adcom.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Noam Camus <noamc@...hip.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Wang Hongcheng <annie.wang@....com>,
        <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        <bcm-kernel-feedback-list@...adcom.com>,
        Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
        David Daney <david.daney@...ium.com>,
        Russell King <linux@...linux.org.uk>,
        <linux-clk@...r.kernel.org>, Viresh Kumar <viresh.kumar@...com>
Subject: Re: [PATCH v2 1/1] serial: 8250_dw: Allow hardware flow control to
 be used

On Fri, Mar 03, 2017 at 03:31:06PM +0200, Andy Shevchenko wrote:
> On Fri, 2017-03-03 at 00:21 +0000, James Hogan wrote:
> > The CONFIG_HAVE_CLK=n implementation of devm_clk_get() in particular
> > seems highly questionable to me, given that commit 93abe8e4b13a ("clk:
> > add non CONFIG_HAVE_CLK routines") which added it 5 years ago says:
> > 
> > > These calls will return error for platforms that don't select
> > > HAVE_CLK
> > 
> > And NULL isn't an error in this API.
> 
> Which is okay. I dunno what should be returned from clk_round_rate() if
> clk is NULL. I would fix CLK framework, though I would like to gather
> more details.

Hmm, the common clock framwork is just one implementation of the clock
API that won't use NULL as a valid clock handle. HAVE_CLK=n is just
another implementation that does return NULL as a valid value, and
accepts that value in the other clk functions.

> Btw, I hope you also noticed this one:
> 
> http://www.spinics.net/lists/linux-serial/msg25314.html

Interesting.

Following Russel's past advise[1], the following patch on top of Heiko's
patch also fixes things for me on Octeon:

[1] https://lists.gt.net/linux/kernel/2102623

If thats an acceptable fix I'll post it properly. Thoughts?

Cheers
James

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 223ac234ddb2..e65808c482f1 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -267,6 +267,8 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
 	rate = clk_round_rate(d->clk, baud * 16);
 	if (rate < 0)
 		ret = rate;
+	else if (rate == 0)
+		ret = -ENOENT;
 	else
 		ret = clk_set_rate(d->clk, rate);
 	clk_prepare_enable(d->clk);

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