Re: [PATCH v2] clk-si5341: Support NVM programming through sysfs

[Stephen Boyd <sboyd@kernel.org>]

Quoting Mike Looijmans (2021-01-05 23:04:36)
> Export an attribute program_nvm_bank that when read reports the current
> bank value. To program the chip's current state into NVM, write the
> magic value 0xC7 into this attribute.
> 
> This allows the clock chip to be programmed "in system" to reduce boot
> time by 300ms and allows the clock to be up and running before the
> kernel boots (e.g. for bootloader usage). Some vendors initialize PLLs
> only in their bootloader and thus need the clock running at boot.
> 
> Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
> ---
> v2: Add description in Documentation/ABI/
>     Use regmap_read_poll_timeout()
>     Abort on sysfs_create_group error
> 
>  .../ABI/testing/sysfs-devices-clk-si5341      | 24 +++++++
>  drivers/clk/clk-si5341.c                      | 68 +++++++++++++++++++
>  2 files changed, 92 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sysfs-devices-clk-si5341
> 
> diff --git a/Documentation/ABI/testing/sysfs-devices-clk-si5341 b/Documentation/ABI/testing/sysfs-devices-clk-si5341
> new file mode 100644
> index 000000000000..7243b82a3729
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-devices-clk-si5341
> @@ -0,0 +1,24 @@
> +What:          /sys/bus/i2c/devices/.../clk-si534*/program_nvm_bank

I'm still curious why this is exposed here instead of through nvmem
framework. I think we talked about it before, but it would be great if
the commit text could quiet any concerns about that so we have a record
of why using the existing nvmem framework wasn't suitable.

> +Date:          Jan 2021
> +KernelVersion: 5.12
> +Contact:       Mike Looijmans <mike.looijmans@topic.nl>
> +Description:   Allows programming the NVM memory of the clock chip, so it boots
> +               up in the current configuration. This saves boot time (300ms
> +               clock initialization) and allows the clock to be available
> +               before the kernel boots (e.g. u-boot ethernet clock).
> +
> +               Write the magic value 0xc7 to this attribute to program the
> +               chip's current settings into its NVM storage. This magic value
> +               is taken from the datasheet, it's the same value that must be
> +               written to the hardware to program it. Programming can only be
> +               done twice in the lifetime of the chip.
> +
> +               Read the value to check the state of the chip. This returns the
> +               raw value as read from the hardware. Possible values:
> +               0x03: Not programmed
> +               0x0f: Programmed once
> +               0x3f: Programmed twice, can no longer be programmed
> +
> +Users:         User space applications for embedded boards equipped with one
> +               or more Si534x programmable clock devices. Would typically be
> +               used at the end of production stages.