linux-kernel - Re: [PATCH v4 1/4] pinctrl: rockchip: remove unnecessary locking

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Date:   Thu, 23 Mar 2017 17:51:53 +0000
From:   John Keeping <john@...anate.com>
To:     Julia Cartwright <julia@...com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Heiko Stuebner <heiko@...ech.de>, <linux-gpio@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-rockchip@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/4] pinctrl: rockchip: remove unnecessary locking

On Thu, 23 Mar 2017 11:10:20 -0500, Julia Cartwright wrote:

> One quick question below.  Apologies if this has been covered, but just
> want to be sure.
> 
> On Thu, Mar 23, 2017 at 10:59:28AM +0000, John Keeping wrote:
> > regmap_update_bits does its own locking and everything else accessed
> > here is a local variable so there is no need to lock around it.
> > 
> > Signed-off-by: John Keeping <john@...anate.com>
> > Reviewed-by: Heiko Stuebner <heiko@...ech.de>
> > Tested-by: Heiko Stuebner <heiko@...ech.de>
> > ---
> > v3: unchanged
> > v2.1:
> > - Remove RK2928 locking in rockchip_set_pull()
> > v2:
> > - Also remove locking in rockchip_set_schmitt()
> > ---
> >  drivers/pinctrl/pinctrl-rockchip.c | 33 ++-------------------------------
> >  1 file changed, 2 insertions(+), 31 deletions(-)
> > 
> > diff --git a/drivers/pinctrl/pinctrl-rockchip.c b/drivers/pinctrl/pinctrl-rockchip.c
> > index bd4b63f66220..6568c867bdcd 100644
> > --- a/drivers/pinctrl/pinctrl-rockchip.c
> > +++ b/drivers/pinctrl/pinctrl-rockchip.c  
> [..]
> > @@ -1185,17 +1177,14 @@ static int rockchip_set_drive_perpin(struct rockchip_pin_bank *bank,
> >  			rmask = BIT(15) | BIT(31);
> >  			data |= BIT(31);
> >  			ret = regmap_update_bits(regmap, reg, rmask, data);
> > -			if (ret) {
> > -				spin_unlock_irqrestore(&bank->slock, flags);
> > +			if (ret)
> >  				return ret;
> > -			}
> >  
> >  			rmask = 0x3 | (0x3 << 16);
> >  			temp |= (0x3 << 16);
> >  			reg += 0x4;
> >  			ret = regmap_update_bits(regmap, reg, rmask, temp);  
> 
> Killing the lock here means the writes to to this pair of registers (reg
> and reg + 4) can be observed non-atomically.  Have you convinced
> yourself that this isn't a problem?

I called it out in v1 [1] since this bit is new since v4.4 where I
originally wrote this patch, and didn't get any comments about it.

I've convinced myself that removing the lock doesn't cause any problems
for writing to the hardware: if the lock would prevent writes
interleaving then it means that two callers are trying to write
different drive strengths to the same pin, and even with a lock here one
of them will end up with the wrong drive strength.

But it does mean that a read via rockchip_get_drive_perpin() may see an
inconsistent state.  I think adding a new lock specifically for this
particular drive strength bit is overkill and I can't find a scenario
where this will actually matter; any driver setting a pinctrl config
must already be doing something to avoid racing two configurations
against each other, mustn't it?

[1] https://www.spinics.net/lists/arm-kernel/msg568925.html


John