linux-kernel - Re: [PATCH v3 5/5] reset: simple: read back to make sure changes are applied

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Message-ID: <1503303660.8125.4.camel@pengutronix.de>
Date:   Mon, 21 Aug 2017 10:21:00 +0200
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Alexandru Gagniuc <alex.g@...ptrum.com>,
        linux-kernel@...r.kernel.org
Cc:     Andre Przywara <andre.przywara@....com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Baoyou Xie <baoyou.xie@...aro.org>,
        Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        Steffen Trumtrar <s.trumtrar@...gutronix.de>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Andreas Färber <afaerber@...e.de>,
        linux-arm-kernel@...ts.infradead.org, kernel@...gutronix.de
Subject: Re: [PATCH v3 5/5] reset: simple: read back to make sure changes
 are applied

Hi Alexandru,

On Wed, 2017-08-16 at 14:00 -0700, Alexandru Gagniuc wrote:
> 
> On 08/16/2017 02:47 AM, Philipp Zabel wrote:
> > Read back the register after setting or clearing a reset bit to make
> > sure that the changes are applied to the reset controller hardware.
> > Theoretically, this avoids the write to stay stuck in a store buffer
> 
> Is there hardware where this has been observed to happen, or is this 
> purely theoretical? It would be nice to have a "this is needed on 
> hardware XYZ because ABC, and doesn't affect other hardware" comment in 
> the source.

This is purely theoretical, at least in the context of reset
controllers. I'm happy to drop this patch for now.

> > during the delay of an assert-delay-deassert sequence, and makes sure
> > that the reset really is asserted for the specified duration.
> > 
> > > > Signed-off-by: Philipp Zabel <p.zabel@...gutronix.de>
> > ---
> >  drivers/reset/reset-simple.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/reset/reset-simple.c b/drivers/reset/reset-simple.c
> > index 13e7d5559acc9..d98a7e7d802d1 100644
> > --- a/drivers/reset/reset-simple.c
> > +++ b/drivers/reset/reset-simple.c
> > @@ -39,17 +39,20 @@ static int reset_simple_set(struct reset_controller_dev *rcdev,
> > > >  	int reg_width = sizeof(u32);
> > > >  	int bank = id / (reg_width * BITS_PER_BYTE);
> > > >  	int offset = id % (reg_width * BITS_PER_BYTE);
> > > > +	void __iomem *addr = data->membase + (bank * reg_width);
> > > >  	unsigned long flags;
> > > >  	u32 reg;
> > 
> > > >  	spin_lock_irqsave(&data->lock, flags);
> > 
> > > > -	reg = readl(data->membase + (bank * reg_width));
> > > > +	reg = readl(addr);
> > > >  	if (assert ^ data->active_low)
> > > >  		reg |= BIT(offset);
> > > >  	else
> > > >  		reg &= ~BIT(offset);
> > > > -	writel(reg, data->membase + (bank * reg_width));
> > > > +	writel(reg, addr);
> > > > +	/* Read back to make sure the write doesn't linger in a store buffer */
> > > > +	readl(addr);
> 
> You're not using the returned value to check that the reset was actually 
> set. This seems a very arbitrary readback workaround, which gives no 
> indication if it actually succeeded or not.

True. For those reset controllers that support status readback, this
would be a better check.

> Also the set() is now asymmetrical to clear(). In cases when releasing 
> reset on a HW block that is about to have IO performed on it, one would 
> want to make sure the reset is actually deasserted before doing any IO.

Thanks for this observation, the reset_simple_set function name is
ambiguous. I'll rename it to reset_simple_update.
The function is called both for assert and deassert operations, and
whether the bit is set or cleared depends on both the assert parameter
and the active_low flag.

regards
Philipp