[WIP] `ot_spi_device`,`ot_spi_host`: Refactoring + Passthrough mode implementation #255

ziuziakowska · 2025-10-23T14:10:21Z

This PR implements the Passthrough mode for OT SPI Device and any required changes in OT SPI Host. Passthrough mode allows for external SPI transfers to be passed through to a downstream device, and optionally intercepted or modified by OT SPI Device.

In HW, OT SPI Device and OT SPI Host are linked by their SPI bus and a wire that represents whether the Device is in Passthrough mode. If Passthrough mode is enabled, the SPI Host is entirely bypassed and SPI Device has full control over the bus, which it uses to talk to the same downstream flash as SPI Host. See Documentation reference, RTL.
This passthrough mechanism is only implemented by OT SPI Host if it has one CS line: RTL

The structure of the PR is as follows:

The initial commits consist of refactoring work in ot_spi_device for the Passthrough implementation.
The subsequent commits implement the requirements for Passthrough mode in ot_spi_host - two GPIOs are used to communicate Passthrough enabled and the Chip Select as driven by OT SPI Device, OT SPI Device and OT SPI Host are then linked together.
The final commits contain the implementation of Passthrough mode in ot_spi_device.

I have tried to handle most corner-cases I can think of, but some conflicting configuration options might not behave as expected.

The two-stage read pipeline is not yet implemented - we model all transfers as though they are over a single wire, so this would only be used to buffer all read payload bytes by two over the wire for correctness.

docs/opentitan/earlgrey.md

hw/opentitan/ot_spi_device.c

hw/riscv/ot_earlgrey.c

hw/opentitan/ot_spi_device.c

hw/riscv/ot_earlgrey.c

hw/opentitan/ot_spi_device.c

hw/opentitan/trace-events

hw/opentitan/ot_spi_device.c

rivos-eblot · 2025-10-30T07:57:22Z

hw/opentitan/ot_spi_device.c

    bool loop; /* Keep reading the buffer if end is reached */
    bool watermark; /* Read watermark hit, used as flip-flop */
    bool new_cmd; /* New command has been pushed in current SPI transaction */
+    bool cmd_addr_swap_en; /* Passthrough mode - address swap enabled */


maybe group all the passthrough related value into a structure. (cmd....?)
This would also to clear the whole thing at once in ot_spi_device_passthrough_unmatched_command_params

hw/opentitan/ot_spi_device.c

This commit simplifies command slot definitions and the associated state and control flow. The HW CFG and the HW STA commands have been combined into just HW commands, as in flash mode they are handled in hardware by some way and should therefore share the same data path. For passthrough mode, a subset of these HW commands can be intercepted (all except for WREN and WRDI). As the matched command slot number determines whether the command is a SW or HW command, the `OtSpiFlashCommand` enum has been removed in favour of using `is_sw_command` to return the boolean. Command slot matching has been brought out into its own function, `match_command_slot`, which returns whether an opcode was matched in the command info registers. The decoded slot number is only valid if this returns true. Also a bug is fixed in `ot_spi_device_exec_command` where hardcoded opcodes for read commands were used instead of the opcodes in the associated command slot. Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

... and removes `OtSpiDeviceAddrMode`. `ot_spi_device_get_command_address_size` now returns the size of the address field in the current command, using the value in `cmd_info` and the current 4B enable state. Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

See https://opentitan.org/book/hw/ip/spi_host/doc/theory_of_operation.html#pass-through-mode This commit adds the `passthrough-en` and `passthrough-cs` pins for controlling Passthrough mode behaviour. When `passthrough_en` is asserted (i.e, the upstream OT SPI Device is in Flash Passthrough mode), OT SPI Host is bypassed entirely and OT SPI Device controls the transfers on the SPI bus. This passthrough behaviour is only implemented in OT SPI Host if there is only a single CS pin. See RTL: `spi_host.sv:100` When in Passthrough mode, OT SPI Device can call into the new method `downstream_transfer` to perform SPI transfers. Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

This commit implements the Passthrough mode on SPI Device. Passthrough mode allows the device to act as a proxy for a downstream flash device, optionally intercepting commands to send back its own values, filtering commands sent downstream, or transparently translating address and payload bytes. This also "implements" the Disabled mode, which discards all bytes sent to the device. Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

ziuziakowska · 2025-10-31T16:14:32Z

I have discovered that OT SPI Device is not the only device controlling this SPI bus, the bus is actually shared by OT SPI Host 0 in Earlgrey, and OT SPI Host is bypassed entirely when Passthrough mode is enabled. This seems to only be documented here. As such this PR has been reworked a bit.

rivos-eblot

quick review.

rivos-eblot · 2025-10-31T19:58:14Z

hw/opentitan/ot_spi_host.c


    qemu_irq *cs_lines; /* CS output lines */
    SSIBus *ssi; /* SPI bus */
+    bool passthrough_en; /* Upstream SPI Device Passthrough enable line */


nit pick: move it close to on_reset to avoid creating 7 byte padding.

rivos-eblot · 2025-10-31T19:58:28Z

hw/opentitan/ot_spi_host.c

    TxFifo *tx_fifo;
    CmdFifo *cmd_fifo;

+


extra empty line added

rivos-eblot · 2025-10-31T19:59:10Z

hw/opentitan/ot_spi_host.c


-        uint8_t rx = s->fsm.output_en ? ssi_transfer(s->ssi, tx) : 0xffu;
+        uint8_t rx =
+            s->fsm.output_en ? ssi_transfer(s->ssi, tx) : SPI_DEFAULT_TX_VALUE;


You need to rename the constant w/o "TX" if you want to use is as RX :)

I would have expected the SPI host output to be disabled when passthrough_en is true (?)

rivos-eblot · 2025-10-31T20:00:15Z

hw/opentitan/ot_spi_host.c

+ot_spi_host_device_passthrough_en_input(void *opaque, int irq, int level)
+{
+    OtSPIHostState *s = opaque;
+    (void)irq;


need to g_assert against expected value (0)

rivos-eblot · 2025-10-31T20:00:42Z

hw/opentitan/ot_spi_host.c

+    (void)irq;
+
+    /* Passthrough is not implemented if SPI Host has more than one CS line */
+    if (s->num_cs != 1) {


1u
need to add a trace in this case.

rivos-eblot · 2025-10-31T20:16:58Z

hw/opentitan/ot_spi_host.c

    OtSPIHostClass *sc = OT_SPI_HOST_CLASS(klass);
+    sc->downstream_transfer = &ot_spi_host_downstream_transfer;
+
+    ResettableClass *rc = RESETTABLE_CLASS(klass);


BTW passthrough_* should be reset in reset_enter

rivos-eblot · 2025-10-31T20:17:27Z

hw/opentitan/ot_spi_host.c

+static uint8_t ot_spi_host_downstream_transfer(OtSPIHostState *s, uint8_t tx)
+{
+    /* Passthrough is not implemented if SPI Host has more than one CS line */
+    if (s->num_cs != 1) {


nit pick: 1u

rivos-eblot · 2025-10-31T20:18:37Z

hw/opentitan/ot_spi_device.c

    g_assert(s->ot_id);

+    /* CS is active low, Passthrough enable is active high */
+    ibex_qdev_init_irq_default(OBJECT(s), &s->passthrough_cs, "passthrough-cs",


same note as for the other side, needs to be a public constant, and used to connect lines in the machine

rivos-eblot · 2025-10-31T20:20:53Z

hw/opentitan/ot_spi_device.c

+{
+    if (s->spihost) {
+        OtSPIHostClass *spihostc =
+            OBJECT_GET_CLASS(OtSPIHostClass, s->spihost, TYPE_OT_SPI_HOST);


more idiomatic:
OtSPIHostClass *spihostc = OT_SPI_HOST_GET_CLASS(s->sphihost);

rivos-eblot · 2025-10-31T20:22:56Z

hw/opentitan/ot_spi_device.c


+static uint8_t ot_spi_device_flash_spi_transfer(OtSPIDeviceState *s, uint8_t rx)
+{
+    if (s->spihost) {


If a SPI Device can exist w/o a SPI host, I think this should be signal as an GUEST_ERROR when passthough is enable, and may discarded before reaching this point? If not, there should be a trace here.

rivos-eblot requested changes Oct 23, 2025

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ziuziakowska mentioned this pull request Oct 28, 2025

[qemu] Earlgrey 1.0.0 spi_device passthrough suppport lowRISC/opentitan#27929

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ziuziakowska force-pushed the spi_device_passthrough branch 2 times, most recently from b7c3b04 to 2e2300b Compare October 28, 2025 13:30

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ziuziakowska force-pushed the spi_device_passthrough branch 2 times, most recently from 2264c77 to f1a68a2 Compare October 29, 2025 14:57

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ziuziakowska force-pushed the spi_device_passthrough branch from f1a68a2 to 6e8f034 Compare October 30, 2025 11:53

ziuziakowska marked this pull request as ready for review October 30, 2025 11:54

ziuziakowska requested review from AlexJones0 and jwnrt October 30, 2025 11:55

ziuziakowska force-pushed the spi_device_passthrough branch 2 times, most recently from e8ef6d7 to ad5c39c Compare October 30, 2025 14:20

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ziuziakowska marked this pull request as draft October 30, 2025 16:51

ziuziakowska changed the title ~~[WIP] ot_spi_device: Refactoring + Passthrough mode implementation~~ [WIP] ot_spi_device,ot_spi_host: Refactoring + Passthrough mode implementation Oct 31, 2025

ziuziakowska added 9 commits October 31, 2025 16:09

[ot] hw/opentitan: ot_spi_device: add trace event for byte transfer

28a37ae

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

[ot] hw/opentitan: ot_spi_device: make logging use ot_id

0dc9276

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

[ot] hw/opentitan: ot_spi_device: Create GPIOs and prop for Passthrough

13f47e6

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

[ot] hw/riscv: ot_earlgrey: link SPI Device to SPI Host #0

1ba9610

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

[ot] hw/opentitan: ot_spi_device: add trace events for Passthrough mode

28ffc3c

Signed-off-by: Alice Ziuziakowska <a.ziuziakowska@lowrisc.org>

ziuziakowska force-pushed the spi_device_passthrough branch from ca61a08 to 28ffc3c Compare October 31, 2025 16:09

rivos-eblot requested changes Oct 31, 2025

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[WIP] ot_spi_device,ot_spi_host: Refactoring + Passthrough mode implementation #255

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[WIP] ot_spi_device,ot_spi_host: Refactoring + Passthrough mode implementation #255

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ziuziakowska commented Oct 23, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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[WIP] `ot_spi_device`,`ot_spi_host`: Refactoring + Passthrough mode implementation #255

[WIP] `ot_spi_device`,`ot_spi_host`: Refactoring + Passthrough mode implementation #255

ziuziakowska commented Oct 23, 2025 •

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