[PATCH linux dev-4.13 2/6] gpio/aspeed: Use a cache of output data registers

[Andrew Jeffery]

On Wed, 9 May 2018, at 01:47, Christopher Bostic wrote:
> Reviewed-by: Christopher Bostic <cbostic at linux.vnet.ibm.com>
> 
> 
> On 5/7/18 8:06 PM, Benjamin Herrenschmidt wrote:
> > The current driver does a read/modify/write of the output
> > registers when changing a bit in __aspeed_gpio_set().
> >
> > This is sub-optimal for a couple of reasons:
> >
> >    - If any of the neighbouring GPIOs (sharing the shared
> > register) isn't (yet) configured as an output, it will
> > read the current input value, and then apply it to the
> > output latch, which may not be what the user expects. There
> > should be no bug in practice as aspeed_gpio_dir_out() will
> > establish a new value but it's not great either.
> >
> >    - The GPIO block in the aspeed chip is clocked rather
> > slowly (typically 25Mhz). That extra MMIO read halves the maximum
> > speed at which we can toggle the GPIO.
> >
> > This provides a significant performance improvement to the GPIO
> > based FSI master.
> >
> > Signed-off-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>

Reviewed-by: Andrew Jeffery <andrew at aj.id.au>

> > ---
> >   drivers/gpio/gpio-aspeed.c | 21 +++++++++++++++++++--
> >   1 file changed, 19 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpio/gpio-aspeed.c b/drivers/gpio/gpio-aspeed.c
> > index ac54b9b25f74..a62cbf3ed4a8 100644
> > --- a/drivers/gpio/gpio-aspeed.c
> > +++ b/drivers/gpio/gpio-aspeed.c
> > @@ -54,6 +54,8 @@ struct aspeed_gpio {
> >   	u8 *offset_timer;
> >   	unsigned int timer_users[4];
> >   	struct clk *clk;
> > +
> > +	u32 *dcache;
> >   };
> >
> >   struct aspeed_gpio_bank {
> > @@ -231,12 +233,13 @@ static void __aspeed_gpio_set(struct gpio_chip *gc, unsigned int offset,
> >   	u32 reg;
> >
> >   	addr = bank_val_reg(gpio, bank, GPIO_DATA);
> > -	reg = ioread32(addr);
> > +	reg = gpio->dcache[GPIO_BANK(offset)];
> >
> >   	if (val)
> >   		reg |= GPIO_BIT(offset);
> >   	else
> >   		reg &= ~GPIO_BIT(offset);
> > +	gpio->dcache[GPIO_BANK(offset)] = reg;
> >
> >   	iowrite32(reg, addr);
> >   }
> > @@ -848,7 +851,7 @@ static int __init aspeed_gpio_probe(struct platform_device *pdev)
> >   	const struct of_device_id *gpio_id;
> >   	struct aspeed_gpio *gpio;
> >   	struct resource *res;
> > -	int rc;
> > +	int rc, i, banks;
> >
> >   	gpio = devm_kzalloc(&pdev->dev, sizeof(*gpio), GFP_KERNEL);
> >   	if (!gpio)
> > @@ -889,6 +892,20 @@ static int __init aspeed_gpio_probe(struct platform_device *pdev)
> >   	gpio->chip.base = -1;
> >   	gpio->chip.irq_need_valid_mask = true;
> >
> > +	/* Allocate a cache of the output registers */
> > +	banks = gpio->config->nr_gpios >> 5;
> > +	gpio->dcache = devm_kzalloc(&pdev->dev,
> > +				    sizeof(u32) * banks, GFP_KERNEL);
> > +	if (!gpio->dcache)
> > +		return -ENOMEM;
> > +
> > +	/* Populate it with initial values read from the HW */
> > +	for (i = 0; i < banks; i++) {
> > +		const struct aspeed_gpio_bank *bank = &aspeed_gpio_banks[i];
> > +		gpio->dcache[i] = ioread32(gpio->base + bank->val_regs +
> > +					   GPIO_DATA);
> > +	}
> > +
> >   	rc = devm_gpiochip_add_data(&pdev->dev, &gpio->chip, gpio);
> >   	if (rc < 0)
> >   		return rc;
>