Re: [PATCH] sh: sh7785lcr: register the PCA9564 as I2C bus 0

[John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>]

Hello Karl,

On Sat, 2026-10-03 at 10:28 +0200, Karl Mehltretter wrote:
> The R2025S/D board information is registered on I2C bus 0. Commit
> 44454baa7ca7 ("i2c: Dynamically assign adapter id if it wasn't explictly
> specified") changed the PCA9564 platform driver to pass negative platform
> device IDs to the I2C core. The SH7785LCR device uses ID -1, so the core
> assigns it a dynamic bus number. Reserving bus 0 for the board information
> makes i2c-1 the first dynamic bus, and the RTC is never instantiated.
> 
> Set the platform device ID to 0 so the adapter is registered as i2c-0 and
> the RTC board information matches it.
> 
> Fixes: 44454baa7ca7 ("i2c: Dynamically assign adapter id if it wasn't explictly specified")
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
> ---
> 
> Tested with 29-bit and 32-bit kernels on a custom QEMU model of the
> SH7785LCR. Both registered the controller as i2c-0 and the R2025S/D as
> rtc0.
> Testing on real hardware is welcome.
> 
>  arch/sh/boards/board-sh7785lcr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/sh/boards/board-sh7785lcr.c b/arch/sh/boards/board-sh7785lcr.c
> index 25c4968f0d8b..b93423df582f 100644
> --- a/arch/sh/boards/board-sh7785lcr.c
> +++ b/arch/sh/boards/board-sh7785lcr.c
> @@ -256,7 +256,7 @@ static struct i2c_pca9564_pf_platform_data i2c_platform_data = {
>  
>  static struct platform_device i2c_device = {
>  	.name		= "i2c-pca-platform",
> -	.id		= -1,
> +	.id		= 0,
>  	.dev		= {
>  		.platform_data	= &i2c_platform_data,
>  	},

Without the patch, the rtc is not detected during boot and trying to read it out
with the hwclock utility fails:

root@tirpitz:~> hwclock
hwclock: Cannot access the Hardware Clock via any known method.
hwclock: Use the --verbose option to see the details of our search for an access method.
root@tirpitz:~> hwclock --verbose
hwclock from util-linux 2.41.3
System Time: 1791016681.183560
Trying to open: /dev/rtc0
Trying to open: /dev/rtc
Trying to open: /dev/misc/rtc
No usable clock interface found.
hwclock: Cannot access the Hardware Clock via any known method.
root@tirpitz:~>

With the patch, the rtc is detected during boot:

[    4.048000] rtc-rs5c372 0-0032: r2025sd found, 24hr
[    4.056000] rtc-rs5c372 0-0032: rtc oscillator interruption detected. Please reset the rtc clock.
[    4.072000] rtc-rs5c372 0-0032: registered as rtc0
[    4.080000] rtc-rs5c372 0-0032: rtc oscillator interruption detected. Please reset the rtc clock.
[    4.088000] rtc-rs5c372 0-0032: hctosys: unable to read the hardware clock

And can be read out with hwclock:

root@tirpitz:~> hwclock --verbose
hwclock from util-linux 2.41.3
System Time: 1791017592.121448
Trying to open: /dev/rtc0
Using the rtc interface to the clock.
Last drift adjustment done at 0 seconds after 1969
Last calibration done at 0 seconds after 1969
Hardware clock is on UTC time
Assuming hardware clock is kept in UTC time.
Waiting for clock tick...
ioctl(4, RTC_UIE_ON, 0): Invalid argument
Waiting in loop for time from /dev/rtc0 to change
...got clock tick
Time read from Hardware Clock: 2026/10/03 08:53:13
Hw clock time : 2026/10/03 08:53:13 = 1791017593 seconds since 1969
Time since last adjustment is 1791017593 seconds
Calculated Hardware Clock drift is 0.000000 seconds
2026-10-03 10:53:12.109057+02:00
root@tirpitz:~>

Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>

Adrian

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