Re: [alsa-devel] [RFC PATCH] ASoC: wm8731: let codec to manage clock by itself

[Lars-Peter Clausen]

On 02/03/2015 06:26 PM, Lars-Peter Clausen wrote:
> On 02/03/2015 06:17 PM, Russell King - ARM Linux wrote:
> > On Tue, Feb 03, 2015 at 05:53:48PM +0100, Lars-Peter Clausen wrote:
> > > On 02/03/2015 01:44 PM, Mark Brown wrote:
> > > > On Tue, Feb 03, 2015 at 08:54:57AM +0100, Manuel Lauss wrote:
> > > >
> > > > > +    wm8731->mclk = devm_clk_get(&spi->dev, "mclk");
> > > > > +    if (IS_ERR(wm8731->mclk)) {
> > > > > +        wm8731->mclk = NULL;
> > > > > +        dev_warn(&spi->dev, "assuming static MCLK\n");
> > > > > +    }
> > > >
> > > > This is broken for both deferred probe and in the case where the clock
> > > > API genuinely returns a NULL clock.  Other than that it's the kind of
> > > > thing that we've done for some other drivers, though it's not good to
> > > > have to do this.  Check them for correct behaviour.
> > >
> > > Ideally we'd introduce a {devm_}clk_get_optional(), with the same semantics
> > > as gpiod_get_optional(), which handles the finer details of differentiating
> > > between clock specified, but not yet probed, clock specified, but
> > > incorrectly and no clock specified, so this doesn't have to be done over and
> > > over by each driver.
> >
> > No, we don't need to.  It clk_get() already knows this distinction, and
> > it appropriately returns -ENOENT vs -EPROBE_DEFER according to whether
> > there's a clock specified in DT or not.
>
> I know, but it returns a error when no clock is specified (-ENOENT), whereas
> gpiod_get_optional()-like semantics mean, it would return no error.

What I wanted to say is that pretty much every user of clk_get() that wants 
a optional clock gets the handling wrong. E.g. they check for PTR_ERR(clk) 
== -EPROBE_DEFER rather than checking for PTR_ERR(clk) != -ENOENT. Which 
causes errors when the clock is specified, but incorrectly specified (e.g. 
invalid phandle or specifier) to be silently ignored.

My hope is that having a explicit API for requesting a optional clock might 
make it easier for users to gets things right.

If you have coccinelle you can use the following script to find good and bad 
users:

@@
expression clk;
@@
clk =
(
devm_clk_get
|
clk_get
)
 (...);
<+...
(
*PTR_ERR(clk) == -EPROBE_DEFER
|
*PTR_ERR(clk) != -ENOENT
)
...+>


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/