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

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Date:   Wed, 15 Mar 2017 17:25:33 +0100
From:   Heiko Stuebner <heiko@...ech.de>
To:     John Keeping <john@...anate.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/4] pinctrl: rockchip: remove unnecessary locking

Am Montag, 13. März 2017, 18:38:10 CET schrieb John Keeping:
> 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.
> 
> I originally wrote this on to of v4.4 which doesn't have the split
> registers for drive strength, where some additional locking might be
> necessary.  But I don't think it can be "slock" given that the following
> patches will convert that to a raw spinlock and regmap uses a normal
> spinlock internally.
> 
> Signed-off-by: John Keeping <john@...anate.com>

Reviewed-by: Heiko Stuebner <heiko@...ech.de>

I have a hard time remembering why these locks are in there in the first place. 
Not only has regmap its own locking, all the grf-registers handling iomux (and 
also the new schmitt triggers) are hiword-mask registers (meaning you enable 
bit x+16 to get write access to bit x), so there isn't locking needed to 
prevent concurrent register access in the first place.

I can only guess  this was meant as a safeguard for earlier socs (
- rk2928 - single-core Cortex-A9
- rk2918 Cortex-A8
- earlier ARM9) that hadn't these hiword-mask mechanisms yet.
I don't think we'll ever have support for those (they're old and nobody cares) 
but we can of course revisit this if at some point somebody is interested in 
affected socs.

Of course that is only true for the grf-based parts, not for the gpio 
registers.

> ---
>  drivers/pinctrl/pinctrl-rockchip.c | 20 +-------------------
>  1 file changed, 1 insertion(+), 19 deletions(-)
> 
> diff --git a/drivers/pinctrl/pinctrl-rockchip.c
> b/drivers/pinctrl/pinctrl-rockchip.c index a838c8bb3129..1defe83a5c4d
> 100644
> --- a/drivers/pinctrl/pinctrl-rockchip.c
> +++ b/drivers/pinctrl/pinctrl-rockchip.c
> @@ -649,7 +649,6 @@ static int rockchip_set_mux(struct rockchip_pin_bank
> *bank, int pin, int mux) int iomux_num = (pin / 8);
>  	struct regmap *regmap;
>  	int reg, ret, mask, mux_type;
> -	unsigned long flags;
>  	u8 bit;
>  	u32 data, rmask;
> 
> @@ -698,15 +697,11 @@ static int rockchip_set_mux(struct rockchip_pin_bank
> *bank, int pin, int mux) if (ctrl->iomux_recalc && (mux_type &
> IOMUX_RECALCED))
>  		ctrl->iomux_recalc(bank->bank_num, pin, &reg, &bit, &mask);
> 
> -	spin_lock_irqsave(&bank->slock, flags);
> -
>  	data = (mask << (bit + 16));
>  	rmask = data | (data >> 16);
>  	data |= (mux & mask) << bit;
>  	ret = regmap_update_bits(regmap, reg, rmask, data);
> 
> -	spin_unlock_irqrestore(&bank->slock, flags);
> -
>  	return ret;
>  }
> 
> @@ -1132,7 +1127,6 @@ static int rockchip_set_drive_perpin(struct
> rockchip_pin_bank *bank, struct rockchip_pinctrl *info = bank->drvdata;
>  	struct rockchip_pin_ctrl *ctrl = info->ctrl;
>  	struct regmap *regmap;
> -	unsigned long flags;
>  	int reg, ret, i;
>  	u32 data, rmask, rmask_bits, temp;
>  	u8 bit;
> @@ -1160,8 +1154,6 @@ static int rockchip_set_drive_perpin(struct
> rockchip_pin_bank *bank, return ret;
>  	}
> 
> -	spin_lock_irqsave(&bank->slock, flags);
> -
>  	switch (drv_type) {
>  	case DRV_TYPE_IO_1V8_3V0_AUTO:
>  	case DRV_TYPE_IO_3V3_ONLY:
> @@ -1182,17 +1174,14 @@ 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);
> 
> -			spin_unlock_irqrestore(&bank->slock, flags);
>  			return ret;
>  		case 18 ... 21:
>  			/* setting fully enclosed in the second register */
> @@ -1200,7 +1189,6 @@ static int rockchip_set_drive_perpin(struct
> rockchip_pin_bank *bank, bit -= 16;
>  			break;
>  		default:
> -			spin_unlock_irqrestore(&bank->slock, flags);
>  			dev_err(info->dev, "unsupported bit: %d for pinctrl drive type: %d
\n",
>  				bit, drv_type);
>  			return -EINVAL;
> @@ -1212,7 +1200,6 @@ static int rockchip_set_drive_perpin(struct
> rockchip_pin_bank *bank, rmask_bits = RK3288_DRV_BITS_PER_PIN;
>  		break;
>  	default:
> -		spin_unlock_irqrestore(&bank->slock, flags);
>  		dev_err(info->dev, "unsupported pinctrl drive type: %d\n",
>  			drv_type);
>  		return -EINVAL;
> @@ -1224,7 +1211,6 @@ static int rockchip_set_drive_perpin(struct
> rockchip_pin_bank *bank, data |= (ret << bit);
> 
>  	ret = regmap_update_bits(regmap, reg, rmask, data);
> -	spin_unlock_irqrestore(&bank->slock, flags);
> 
>  	return ret;
>  }
> @@ -1336,16 +1322,12 @@ static int rockchip_set_pull(struct
> rockchip_pin_bank *bank, return ret;
>  		}
> 
> -		spin_lock_irqsave(&bank->slock, flags);
> -
>  		/* enable the write to the equivalent lower bits */
>  		data = ((1 << RK3188_PULL_BITS_PER_PIN) - 1) << (bit + 16);
>  		rmask = data | (data >> 16);
>  		data |= (ret << bit);
> 
>  		ret = regmap_update_bits(regmap, reg, rmask, data);
> -
> -		spin_unlock_irqrestore(&bank->slock, flags);
>  		break;
>  	default:
>  		dev_err(info->dev, "unsupported pinctrl type\n");