[PATCH RFC 0/2] ipmi: kcs_bmc_aspeed: Support multiple LPC controller instances

[Yu-Che Hsieh <yc_hsieh@aspeedtech.com>]

Hi,

Aspeed AST2700 SoC exposes more than one physical LPC controller
(e.g. lpc0@14c31000, lpc1@14c32000, plus pcie_lpc0@12c19000 and 
pcie_lpc1@12c19800 for the PCIe-facing path), each instantiating 
its own KCS1-KCS4 with identical IDR/ODR/STR register offsets:

	lpc0: lpc@14c31000 {
		compatible = "aspeed,ast2700-lpc", "simple-mfd", "syscon";
		lpc0_kcs1: lpc-kcs@24 {
			compatible = "aspeed,ast2600-kcs-bmc";
			...
		};
		...
	};

	lpc1: lpc@14c32000 {
		compatible = "aspeed,ast2700-lpc", "simple-mfd", "syscon";
		lpc1_kcs1: kcs@24 {
			compatible = "aspeed,ast2600-kcs-bmc";
			...
		};
		...
	};

kcs_bmc_device::channel currently serves two purposes at once:

  1. Selecting which HICR0/HICR2/HICRB bit-group to touch within a single
     LPC controller's register file (aspeed_kcs_of_get_channel() derives
     this purely from the KCS node's register offset, 1..4).

  2. Naming the misc chardev (/dev/ipmi-kcsN) exposed to userspace, which
     must be unique system-wide (kcs_bmc_cdev_ipmi.c uses "ipmi-kcs%u").

Both happen to be the same value only because this driver has only ever
had to support SoCs with a single LPC controller. On AST2700, KCS1 on
every LPC controller computes to the same channel number, and whichever
instance probes second fails outright.

This series keeps a driver-private `channel` (1..4) in
struct aspeed_kcs_bmc for register access, and computes a
globally-unique kcs_bmc_device::channel as `bank * KCS_CHANNEL_MAX +
channel`, where `bank` identifies which LPC controller instance a KCS
device belongs to.

Before settling on how `bank` is derived, I considered a few options
with different tradeoffs and no clearly "correct" answer from existing
driver conventions:

  (a) Dynamic discovery: keep a driver-global list (protected by a
      mutex), keyed by the LPC controller's device_node pointer, and
      assign bank indices in first-seen-during-probe order. No DT
      changes required, but bank numbering depends on probe order,
      which is not deterministic across boots/reprobes.

  (b) Tree walk by compatible: for_each_compatible_node() over the
      whole DT matching the LPC node's own compatible string, counting
      position. Deterministic and requires no DT changes, but AST2700's
      four LPC-compatible instances (lpc0/lpc1/pcie_lpc0/pcie_lpc1)
      don't share a common parent node and don't share a consistent
      child-node naming scheme, which also rules out parent-based or
      node-name-based tree walks I looked at.

  (c) of_alias_get_id(): require board DTs to declare
      /aliases { lpc0 = &lpc0; lpc1 = &lpc1; ... }, and read the index
      back from there. This is the pattern already used by mmc/i2c/spi/
      serial for "which instance is this" numbering, including the same
      "absent alias -> fall back to previous behaviour" compatibility
      guarantee for existing DTBs.

This series implements (c), since it puts the numbering under DT-author
control rather than driver inference, generalizes across however many
LPC-compatible instances a future SoC exposes (regardless of tree
placement), and existing single-LPC-controller boards need no DT
changes at all - of_alias_get_id() returns an error when no alias is
present and the driver falls back to bank 0, which is exactly today's
behaviour.

Patches:

  Patch 1: dt-bindings: mfd: aspeed-lpc: Document lpcN alias for
           multi-instance SoCs
  Patch 2: ipmi: kcs_bmc_aspeed: Support multiple LPC controller
           instances

Before dropping the RFC tag and posting the AST2700 follow-up patches, 
I'd particularly appreciate review feedback on the bank-numbering approach. 
In particular, does option (c) seem like an acceptable use of DT aliases 
for identifying LPC controller instances, or would the community prefer a 
different approach? 

Any guidance would be very helpful.

Signed-off-by: Yu-Che Hsieh <yc_hsieh@aspeedtech.com>
---
Yu-Che Hsieh (2):
      dt-bindings: mfd: aspeed-lpc: Document lpcN alias for multi-instance SoCs
      ipmi: kcs_bmc_aspeed: Support multiple LPC controller instances

 .../devicetree/bindings/mfd/aspeed-lpc.yaml        |  7 ++++
 drivers/char/ipmi/kcs_bmc_aspeed.c                 | 37 ++++++++++++++++------
 2 files changed, 34 insertions(+), 10 deletions(-)
---
base-commit: f0e6f20cb52b14c2c441f04e21cef0c95d498cac
change-id: 20260722-upstream_kcs_multiple_lpc-735942211508

Best regards,
-- 
Yu-Che Hsieh <yc_hsieh@aspeedtech.com>