Subject: Re: [PATCH net-next v4 1/2] dt-bindings: arm: mediatek: add mediatek,boottrap binding

From: Daniel Golle <[email-masked]>
Date: Thu, 18 May 2023 03:44:45 +0100

On Fri, May 12, 2023 at 08:54:36AM +0200, Krzysztof Kozlowski wrote:
> On 11/05/2023 17:53, Andrew Lunn wrote:
> > On Thu, May 11, 2023 at 04:10:20PM +0200, Daniel Golle wrote:
> >> The boottrap is used to read implementation details from the SoC, such
> >> as the polarity of LED pins. Add bindings for it as we are going to use
> >> it for the LEDs connected to MediaTek built-in 1GE PHYs.
> > 
> > What exactly is it? Fuses? Is it memory mapped, or does it need a
> > driver to access it? How is it shared between its different users?
> 
> Yes, looks like some efuse/OTP/nvmem, so it should probably use nvmem
> bindings and do not look different than other in such class.

I've asked MediaTek and they have replied with an elaborate definition.
Summary:
The boottrap is a single 32-bit wide register at 0x1001f6f0 which can
be used to read back the bias of bootstrap pins from the SoC as follows:

* bit[8]: Reference CLK source && gphy port0's LED
If bit[8] == 0:
- Reference clock source is XTRL && gphy port0's LED is pulled low on board side
If bit[8] == 1:
- Reference clock source is Oscillator && gphy port0's LED is pulled high on board side

* bit[9]: DDR type && gphy port1's LED
If bit[9] == 0:
- DDR type is DDRx16b x2 && gphy port1's LED is pulled low on board side
If bit[9] == 1:
- DDR type is DDRx16b x1 && gphy port1's LED is pulled high on board side

* bit[10]: gphy port2's LED
If bit[10] == 0:
- phy port2's LED is pulled low on board side
If bit[10] == 1:
- gphy port2's LED is pulled high on board side

* bit[11]: gphy port3's LED
If bit[11] == 0:
- phy port3's LED is pulled low on board side
If bit[11] == 1:
- gphy port3's LED is pulled high on board side

If bit[10] == 0 && bit[11] == 0:
- BROM will boot from SPIM-NOR
If bit[10] == 1 && bit[11] == 0:
- BROM will boot from SPIM-NAND
If bit[10] == 0 && bit[11] == 1:
- BROM will boot from eMMC
If bit[10] == 1 && bit[11] == 1:
- BROM will boot from SNFI-NAND

The boottrap is present in many MediaTek SoCs, however, support for
reading it is only really needed on MT7988 due to the dual-use of some
bootstrap pins as PHY LEDs.

We could say this is some kind of read-only 'syscon' node (and hence
use regmap driver to access it), that would make it easy but it's not
very accurate. Also efuse/OTP/nvmem doesn't seem accurate, though in
terms of software it could work just as well.

I will update DT bindings to contain the gained insights.

Please advise if any existing driver (syscon/regmap or efuse/OTP/nvmem)
should be used or if it's ok to just use plain mmio in the PHY driver.

Best regards

Daniel