Subject: a case for a common efuse API?

From: [email-masked] (Stephen Boyd)
Date: Tue, 08 Jul 2014 13:00:23 -0700

Hi,

On MSM chips we have some efuses (called qfprom) where we store things
like calibration data, speed bins, etc. We need to read out data from
the efuses in various drivers like the cpufreq, thermal, etc. This
essentially boils down to a bunch of readls on the efuse from a handful
of different drivers. In devicetree this looks a little odd because
these drivers end up having an extra reg property (or two) that points
to a register in the efuse and some length, i.e you see this:

	thermal-sensor at 34000 {
		compatible = "sensor";
		reg = <0x34000 0x1000>, <0x10018 0xc>;
		reg-names = "sensor", "efuse_calib";
	}

I imagine in DT we want something more like this:

	efuse: efuse at 10000 {
		compatible = "efuse";
		reg = <0x10000 0x1000>;
	}

	thermal-sensor at 34000 {
		compatible = "sensor";
		reg = <0x34000 0x1000>;
		efuse = <&efuse 0x18>;
	}

Where we can point to the efuse and give an address offset. From code we
could then call some efuse_read() function to read the sensor's
calibration data.

It's been suggested that we use syscon for this, but I wonder if that is
the right thing to do. With a syscon you're usually writing some
registers that got lumped together with other registers that aren't
directly related to your driver. It doesn't seem like syscon is made for
reading fuses or other things like eeproms. Maybe I'm wrong though.

Using syscon would probably work if we could add a way to point to
offsets within the syscon node (the 0x18 part in the example). In my
specific use-case the calibration data may have different offsets
depending on which SoC the hardware is instantiated in so we could also
make syscon work if the compatible field for the sensor node indicates
which SoC it is.

I added Tegra folks because I see that on Tegra this hardware is exposed
via an SoC specific API, tegra_fuse_readl(), and an associated driver in
drivers/misc/fuse/tegra/. Unfortunately I don't see any users of the API
outside of the speedo code in the same directory and the sysfs bin
attribute that may or may not be in use by some userspace code.

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