linux-kernel - Re: [PATCH v2] ARC: HSDK: improve reset driver

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Message-ID: <1539784482.4199.35.camel@synopsys.com>
Date:   Wed, 17 Oct 2018 13:54:43 +0000
From:   Eugeniy Paltsev <eugeniy.paltsev@...opsys.com>
To:     "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        "Eugeniy.Paltsev@...opsys.com" <Eugeniy.Paltsev@...opsys.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexey.brodkin@...opsys.com" <alexey.brodkin@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: [PATCH v2] ARC: HSDK: improve reset driver

On Fri, 2018-10-12 at 14:08 +0200, Philipp Zabel wrote:
> Hi Eugeniy,
> 
> thank you for the update.
> 
> On Fri, 2018-09-28 at 19:28 +0300, Eugeniy Paltsev wrote:
> > As for today HSDK reset driver implements only
> > .reset() callback.
> > 
> > In case of driver which implements one of standard
> > reset controller usage pattern
> > (call *_deassert() in probe(), call *_assert() in remove())
> > that leads to inoperability of this reset driver.
> > 
> > Improve HSDK reset driver by calling .reset() callback inside of
> > .deassert() callback to avoid each reset controller
> > user adaptation for work with both reset methods
> > (reset() and {.assert() & .deassert()} pair)
> > 
> > Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
> > ---
> > Changes v1->v2:
> >  * Don't call hsdk_reset_reset in .assert callback.
> > 
> >  drivers/reset/reset-hsdk.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/drivers/reset/reset-hsdk.c b/drivers/reset/reset-hsdk.c
> > index 8bce391c6943..399440f197c5 100644
> > --- a/drivers/reset/reset-hsdk.c
> > +++ b/drivers/reset/reset-hsdk.c
> > @@ -84,8 +84,21 @@ static int hsdk_reset_reset(struct reset_controller_dev *rcdev,
> >  	return ret;
> >  }
> >  
> > +static int hsdk_reset_dummy(struct reset_controller_dev *rcd, unsigned long id)
> > +{
> > +	return 0;
> > +}
> > +
> > +/*
> > + * Doing real reset from .assert isn't necessary/useful here. So we pass
> > + * 'hsdk_reset_dummy' to .assert callback to prevent -ENOTSUPP returning by
> > + * reset_control_assert() to make happy drivers which check
> > + * reset_control_{assert | deassert} return status.
> > + */
> >  static const struct reset_control_ops hsdk_reset_ops = {
> >  	.reset	= hsdk_reset_reset,
> > +	.assert = hsdk_reset_dummy,
> 
> Is this part really necessary though? reset_control_assert already
> returns 0 in shared mode, so the only thing this does is make
> reset_control_assert return 0 instead of -ENOTSUPP in exclusive mode.
> 
> The documentation states that calling reset_control_assert "on an
> exclusive reset controller guarantees that the reset will be asserted."
> Since this is clearly not the case with this driver, it is appropriate
> to keep returning an error in this case.
> 
> If there is a driver that requests an exclusive reset control, calls
> reset_control_assert, and then checks the error value to see whether
> asserting the reset succeeded, it should be made aware that
> we couldn't actually assert the reset line as requested. If the driver
> can continue operation even though the reset line was not asserted,
> it should ignore the error.
> 
> So if you need to hide this error, I'd like to know the actual case that
> is fixed by this, to see if we can't fix it in a better way.

Ok, I can drop it in my case as it will work fine with certain drivers:
(several drivers use shared reset control, other drivers use exclusive reset
control but don't check reset_control_assert() return value)

I simply want to say that this wouldn't work in all cases (without changes
in driver which use reset control).

I'll respin patch with this part dropped.

> > +	.deassert = hsdk_reset_reset,
> 
> This part is fine.
> 
> >  };
> >  
> >  static int hsdk_reset_probe(struct platform_device *pdev)
> 
> regards
> Philipp
-- 
 Eugeniy Paltsev