Re: [PATCH] gpio: pca953x: Add GPIO settle delay

["Mendez, Judith" <jm@ti.com>]

Hi Tanmay,

On 7/13/2026 7:00 AM, Kathpalia, Tanmay wrote:
> Hi Judith,
> 
> On 7/11/2026 4:34 AM, Judith Mendez wrote:
>> Some board designs have slow RC circuits on GPIO outputs controlled by
>> PCA953X I2C expanders. On some board designs, pin voltage transitions
>> can take significant time after I2C write command completes.
>>
>> Add an optional "gpio-settle-delay-ms" device tree property for the
>> pca953x driver to add a delay after I2C write if specified, allowing to
>> compensate for slow RC circuits.
>>
>> Signed-off-by: Judith Mendez <jm@ti.com>
>> ---
>>   drivers/gpio/pca953x_gpio.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/drivers/gpio/pca953x_gpio.c b/drivers/gpio/pca953x_gpio.c
>> index 965a5fcf30b..54b957069c9 100644
>> --- a/drivers/gpio/pca953x_gpio.c
>> +++ b/drivers/gpio/pca953x_gpio.c
>> @@ -28,6 +28,7 @@
>>   #include <dm/device_compat.h>
>>   #include <dt-bindings/gpio/gpio.h>
>>   #include <linux/bitops.h>
>> +#include <linux/delay.h>
>>   #define PCA953X_INPUT           0
>>   #define PCA953X_OUTPUT          1
>> @@ -89,6 +90,7 @@ static const struct pca95xx_reg pca957x_regs = {
>>    * @reg_output: array to hold the value of output registers
>>    * @reg_direction: array to hold the value of direction registers
>>    * @regs: struct to hold the registers addresses
>> + * @settle_delay_ms: GPIO pin settle delay in milliseconds
>>    */
>>   struct pca953x_info {
>>       struct udevice *dev;
>> @@ -100,6 +102,7 @@ struct pca953x_info {
>>       u8 reg_output[MAX_BANK];
>>       u8 reg_direction[MAX_BANK];
>>       const struct pca95xx_reg *regs;
>> +    u32 settle_delay_ms;
>>   };
>>   static int pca953x_write_single(struct udevice *dev, int reg, u8 val,
>> @@ -116,6 +119,9 @@ static int pca953x_write_single(struct udevice 
>> *dev, int reg, u8 val,
>>           return ret;
>>       }
>> +    if (info->settle_delay_ms)
>> +        mdelay(info->settle_delay_ms);
>> +
>>       return 0;
>>   }
> 
> This delay is placed inside pca953x_write_single(), which is called by both
> pca953x_set_value() and pca953x_set_direction(). This results in a 
> double delay
> for every direction_output call, which I think is unintended.
> 
> Additionally, a settle delay is only meaningful after changing an output
> level — it represents the time for the pin voltage to physically settle 
> (e.g.
> through an RC circuit). Firing it on direction register writes does not 
> make
> sense. The delay should be scoped only to pca953x_set_value().

Yea did not intended.

> 
>> @@ -333,6 +339,8 @@ static int pca953x_probe(struct udevice *dev)
>>       info->bank_count = DIV_ROUND_UP(info->gpio_count, BANK_SZ);
>> +    info->settle_delay_ms = dev_read_u32_default(dev, "gpio-settle- 
>> delay-ms", 0);
>> +
> 
> As Francesco mentioned, adding u-boot specifc properties causes the U- 
> Boot DT
> to diverge from the Linux DT, which violates U-Boot's policy of 
> maintaining DT
> compatibility with Linux.
> 
>>       ret = pca953x_read_regs(dev, info->regs->output, info->reg_output);
>>       if (ret) {
>>           dev_err(dev, "Error reading output register\n");
> 
> Given that the delay is due to a slow RC circuit on the output, I would 
> suggest
> looking at the gpio-delay (gpio-delay.c, binding at gpio-delay.yaml). It is
> designed precisely for this scenario:
> "This binding describes an electrical setup where setting a GPIO output is
> delayed by some external setup, e.g. RC circuit."
Thanks, Ill check that out.

~ Judith