platform: generic: eswin: add EIC7700

Initial platform support for ESWIN Computing EIC7700 based on public SoC
datasheet[1] and tested on HiFive Premier P550. Vendor U-boot/Linux boots
fine, and I've tested Geekbench 6.5.0 Preview and got scores on par with
the vendor OpenSBI. System shutdown/reboot for HiFive Premier P550 and
other boards will be implemented in subsequent commits. At this point,
only SoC-level warm reset is implemented.

The files and functions are intentionally named as eic770x in many places
for future enhancements to support the 2 die version of the same SoC,
namely EIC7702, seen on DC-ROMA AI PC FML13V03 [2]. This patch set only
deals with the single die version, and the assumption is we can be either
die with id=0 or id=1, but there's only a single die in the system, or we
are only using a single die out of 2. However, the way the SoC handles 2-
die greatly affects how we configure it in a 1-die setup. EIC770X address
map has die 0/1 memory regions interleaved (see comments in eic770x.c).
If only 1 die is connected or active, it creates holes in the address map
for those regions corresponding to the remote die. When speculative-
execution or HW prefetcher touches data-cacheable regions that happen to
fall into those holes, it can trigger bus error. Specifically:

 - Remote (non-existent) die L3 zero device
 - Remote (non-existent) die cached memory region
 - Other holes in Memory Port

To make matters worse, EIC770X doesn't have cache coherent DMA, and due
to the fact that the P550 core lacks Svpbmt, the SoC maps main memory
twice as different regions, so it can bypass cache and fetch the data
directly from memory. In address space, we have two memory regions, one
as cached, the other as uncached. Thus, we also need an extra PMP entry
to protect OpenSBI blob from the uncached window. To do this, platform
code requires single_fw_region, otherwise, we'll run out of PMP entries.

EIC770X also have several feature disable/enable CSRs accessible in M
mode. By default many core features such as speculation and HW prefetch
are disabled, and M mode software is responsible of enabling. Hence,
introduce 4 new build time tunable parameters to Kconfig, which reflects
the values get updated to those CSRs:
 - ESWIN_EIC770X_FEAT0_CFG
 - ESWIN_EIC770X_FEAT1_CFG
 - ESWIN_EIC770X_L1_HWPF_CFG
 - ESWIN_EIC770X_L2_HWPF_CFG

The default values are somewhat optimal for generic workloads. They are
dumped when running SiFive's vendor OpenSBI, and in addition, with my
own tuning to address the perf regression reported by drmpeg [3]

To build the firmware+u-boot blob, Use the following, and docs [4] for
testing it with UART boot without flashing:

make FW_TEXT_START=0x80000000 \
     FW_PAYLOAD_OFFSET=0x200000 \
     FW_PAYLOAD_PATH=u-boot-nodtb.bin \
     FW_PAYLOAD_FDT_ADDR=0xf8000000 \
     FW_FDT_PATH=u-boot.dtb

[1] https://github.com/eswincomputing/EIC7700X-SoC-Technical-Reference-Manual
[2] https://github.com/geerlingguy/sbc-reviews/issues/82
[3] https://forums.sifive.com/t/low-1-core-stream-bandwidth/7274/15
[4] https://github.com/ganboing/EIC770x-Docs/blob/main/p550/bootchain/UART-Boot.md

Signed-off-by: Bo Gan <ganboing@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20251218104243.562667-6-ganboing@gmail.com
Signed-off-by: Anup Patel <anup@brainfault.org>

platform/generic/eswin/Kconfig

@@ -0,0 +1,29 @@
+# SPDX-License-Identifier: BSD-2-Clause
+
+config ESWIN_EIC770X_FEAT0_CFG
+	hex "ESWIN EIC7700X Feature Disable 0 CSR Configuration"
+	default 0x4000
+	help
+	 CSR Value to initialize EIC770X_FEAT0 (0x7c1) with.
+	 Refer to EIC770X SoC TRM for recommendations.
+
+config ESWIN_EIC770X_FEAT1_CFG
+	hex "ESWIN EIC7700X Feature Disable 1 CSR Configuration"
+	default 0x80
+	help
+	 CSR Value to initialize EIC770X_FEAT1 (0x7c2) with.
+	 Refer to EIC770X SoC TRM for recommendations.
+
+config ESWIN_EIC770X_L1_HWPF_CFG
+	hex "ESWIN EIC7700X L1 HW Prefetcher CSR Configuration"
+	default 0x1005c1be649
+	help
+	 CSR Value to initialize EIC770X_L1_HWPF (0x7c3) with.
+	 Refer to EIC770X SoC TRM for recommendations.
+
+config ESWIN_EIC770X_L2_HWPF_CFG
+	hex "ESWIN EIC7700X L2 HW Prefetcher CSR Configuration"
+	default 0x929f
+	help
+	 CSR Value to initialize EIC770X_L2_HWPF (0x7c4) with.
+	 Refer to EIC770X SoC TRM for recommendations.