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

[Christopher Bostic]

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>
> ---
>   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;