NXP PCF8574 and compatible ICs are simple I2C GPIO expanders.
PCF8574 incorporates quasi-bidirectional IO, and simple
communication protocol, when IO read is I2C byte read, and
IO write is I2C byte write. User can think of it as
open-drain port, when line high state is input and line low
state is output.

Signed-off-by: Dmitrii Sharikhin <d.sharikhin@yadro.com>
---
 MAINTAINERS               |   6 ++
 hw/gpio/Kconfig           |   4 +
 hw/gpio/meson.build       |   1 +
 hw/gpio/pcf8574.c         | 162 ++++++++++++++++++++++++++++++++++++++
 include/hw/gpio/pcf8574.h |  15 ++++
 5 files changed, 188 insertions(+)
 create mode 100644 hw/gpio/pcf8574.c
 create mode 100644 include/hw/gpio/pcf8574.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 65dfdc9677..bb1981d02e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2499,6 +2499,12 @@ S: Maintained
 F: hw/i2c/i2c_mux_pca954x.c
 F: include/hw/i2c/i2c_mux_pca954x.h
 
+pcf8574
+M: Dmitrii Sharikhin <d.sharikhin@yadro.com>
+S: Maintained
+F: hw/gpio/pcf8574.c
+F: include/gpio/pcf8574.h
+
 Generic Loader
 M: Alistair Francis <alistair@alistair23.me>
 S: Maintained
diff --git a/hw/gpio/Kconfig b/hw/gpio/Kconfig
index d2cf3accc8..bb731ff4ce 100644
--- a/hw/gpio/Kconfig
+++ b/hw/gpio/Kconfig
@@ -16,3 +16,7 @@ config GPIO_PWR
 
 config SIFIVE_GPIO
     bool
+
+config PCF8574
+    bool
+    depends on I2C
diff --git a/hw/gpio/meson.build b/hw/gpio/meson.build
index 8a8d03d885..c0d9a3c757 100644
--- a/hw/gpio/meson.build
+++ b/hw/gpio/meson.build
@@ -15,3 +15,4 @@ system_ss.add(when: 'CONFIG_RASPI', if_true: files(
 ))
 system_ss.add(when: 'CONFIG_ASPEED_SOC', if_true: files('aspeed_gpio.c'))
 system_ss.add(when: 'CONFIG_SIFIVE_GPIO', if_true: files('sifive_gpio.c'))
+system_ss.add(when: 'CONFIG_PCF8574', if_true: files('pcf8574.c'))
diff --git a/hw/gpio/pcf8574.c b/hw/gpio/pcf8574.c
new file mode 100644
index 0000000000..d37909e2ad
--- /dev/null
+++ b/hw/gpio/pcf8574.c
@@ -0,0 +1,162 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+/*
+ * NXP PCF8574 8-port I2C GPIO expansion chip.
+ * Copyright (c) 2024 KNS Group (YADRO).
+ * Written by Dmitrii Sharikhin <d.sharikhin@yadro.com>
+ */
+
+#include "qemu/osdep.h"
+#include "hw/i2c/i2c.h"
+#include "hw/gpio/pcf8574.h"
+#include "hw/irq.h"
+#include "migration/vmstate.h"
+#include "qemu/log.h"
+#include "qemu/module.h"
+#include "qom/object.h"
+
+/*
+ * PCF8574 and compatible chips incorporate quasi-bidirectional
+ * IO. Electrically it means that device sustain pull-up to line
+ * unless IO port is configured as output _and_ driven low.
+ *
+ * IO access is implemented as simple I2C single-byte read
+ * or write operation. So, to configure line to input user write 1
+ * to corresponding bit. To configure line to output and drive it low
+ * user write 0 to corresponding bit.
+ *
+ * In essence, user can think of quasi-bidirectional IO as
+ * open-drain line, except presence of builtin rising edge acceleration
+ * embedded in PCF8574 IC
+ *
+ * PCF8574 has interrupt request line, which is being pulled down when
+ * port line state differs from last read. Port read operation clears
+ * state and INT line returns to high state via pullup.
+ */
+
+OBJECT_DECLARE_SIMPLE_TYPE(PCF8574State, PCF8574)
+
+#define PORTS_COUNT (8)
+
+struct PCF8574State {
+    I2CSlave parent_obj;
+    uint8_t  lastrq;     /* Last requested state. If changed - assert irq */
+    uint8_t  input;      /* external electrical line state */
+    uint8_t  output;     /* Pull-up (1) or drive low (0) on bit */
+    qemu_irq handler[PORTS_COUNT];
+    qemu_irq intrq;      /* External irq request */
+};
+
+static void pcf8574_reset(DeviceState *dev)
+{
+    PCF8574State *s = PCF8574(dev);
+    s->lastrq = MAKE_64BIT_MASK(0, PORTS_COUNT);
+    s->input  = MAKE_64BIT_MASK(0, PORTS_COUNT);
+    s->output = MAKE_64BIT_MASK(0, PORTS_COUNT);
+}
+
+static inline uint8_t pcf8574_line_state(PCF8574State *s)
+{
+    /* we driving line low or external circuit does that */
+    return s->input & s->output;
+}
+
+static uint8_t pcf8574_rx(I2CSlave *i2c)
+{
+    PCF8574State *s = PCF8574(i2c);
+    uint8_t linestate = pcf8574_line_state(s);
+    if (s->lastrq != linestate) {
+        s->lastrq = linestate;
+        if (s->intrq) {
+            qemu_set_irq(s->intrq, 1);
+        }
+    }
+    return linestate;
+}
+
+static int pcf8574_tx(I2CSlave *i2c, uint8_t data)
+{
+    PCF8574State *s = PCF8574(i2c);
+    uint8_t prev;
+    uint8_t diff;
+    uint8_t actual;
+    int line = 0;
+
+    prev = pcf8574_line_state(s);
+    s->output = data;
+    actual = pcf8574_line_state(s);
+
+    for (diff = (actual ^ prev); diff; diff &= ~(1 << line)) {
+        line = ctz32(diff);
+        if (s->handler[line]) {
+            qemu_set_irq(s->handler[line], (actual >> line) & 1);
+        }
+    }
+
+    if (s->intrq) {
+        qemu_set_irq(s->intrq, actual == s->lastrq);
+    }
+
+    return 0;
+}
+
+static const VMStateDescription vmstate_pcf8574 = {
+    .name               = "pcf8574",
+    .version_id         = 0,
+    .minimum_version_id = 0,
+    .fields = (VMStateField[]) {
+        VMSTATE_I2C_SLAVE(parent_obj, PCF8574State),
+        VMSTATE_UINT8(lastrq, PCF8574State),
+        VMSTATE_UINT8(input,  PCF8574State),
+        VMSTATE_UINT8(output, PCF8574State),
+        VMSTATE_END_OF_LIST()
+    }
+};
+
+static void pcf8574_gpio_set(void *opaque, int line, int level)
+{
+    PCF8574State *s = (PCF8574State *) opaque;
+    assert(line >= 0 && line < ARRAY_SIZE(s->handler));
+
+    if (level) {
+        s->input |=  (1 << line);
+    } else {
+        s->input &= ~(1 << line);
+    }
+
+    if (pcf8574_line_state(s) != s->lastrq && s->intrq) {
+        qemu_set_irq(s->intrq, 0);
+    }
+}
+
+static void pcf8574_realize(DeviceState *dev, Error **errp)
+{
+    PCF8574State *s = PCF8574(dev);
+
+    qdev_init_gpio_in(dev, pcf8574_gpio_set, ARRAY_SIZE(s->handler));
+    qdev_init_gpio_out(dev, s->handler, ARRAY_SIZE(s->handler));
+    qdev_init_gpio_out_named(dev, &s->intrq, "nINT", 1);
+}
+
+static void pcf8574_class_init(ObjectClass *klass, void *data)
+{
+    DeviceClass   *dc = DEVICE_CLASS(klass);
+    I2CSlaveClass *k  = I2C_SLAVE_CLASS(klass);
+
+    k->recv     = pcf8574_rx;
+    k->send     = pcf8574_tx;
+    dc->realize = pcf8574_realize;
+    dc->reset   = pcf8574_reset;
+    dc->vmsd    = &vmstate_pcf8574;
+}
+
+static const TypeInfo pcf8574_infos[] = {
+    {
+        .name          = TYPE_PCF8574,
+        .parent        = TYPE_I2C_SLAVE,
+        .instance_size = sizeof(PCF8574State),
+        .class_init    = pcf8574_class_init,
+    }
+};
+
+DEFINE_TYPES(pcf8574_infos);
diff --git a/include/hw/gpio/pcf8574.h b/include/hw/gpio/pcf8574.h
new file mode 100644
index 0000000000..3291d7dbbc
--- /dev/null
+++ b/include/hw/gpio/pcf8574.h
@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+/*
+ * NXP PCF8574 8-port I2C GPIO expansion chip.
+ *
+ * Copyright (c) 2024 KNS Group (YADRO).
+ * Written by Dmitrii Sharikhin <d.sharikhin@yadro.com>
+ */
+
+#ifndef _HW_GPIO_PCF8574
+#define _HW_GPIO_PCF8574
+
+#define TYPE_PCF8574 "pcf8574"
+
+#endif /* _HW_GPIO_PCF8574 */

On 11/3/24 08:09, Dmitriy Sharikhin wrote:
> NXP PCF8574 and compatible ICs are simple I2C GPIO expanders.
> PCF8574 incorporates quasi-bidirectional IO, and simple
> communication protocol, when IO read is I2C byte read, and
> IO write is I2C byte write. User can think of it as
> open-drain port, when line high state is input and line low
> state is output.
> 
> Signed-off-by: Dmitrii Sharikhin <d.sharikhin@yadro.com>
> ---
>   MAINTAINERS               |   6 ++
>   hw/gpio/Kconfig           |   4 +
>   hw/gpio/meson.build       |   1 +
>   hw/gpio/pcf8574.c         | 162 ++++++++++++++++++++++++++++++++++++++
>   include/hw/gpio/pcf8574.h |  15 ++++
>   5 files changed, 188 insertions(+)
>   create mode 100644 hw/gpio/pcf8574.c
>   create mode 100644 include/hw/gpio/pcf8574.h

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

Unfortunately your patch doesn't apply anymore:

Total patches: 1 (cherrypicked: 
<56678f4f0c1e526b7b5a04104171e4feb372e7c2.camel@yadro.com>)
Applying: hw: gpio: introduce pcf8574 driver
Patch failed at 0001 hw: gpio: introduce pcf8574 driver
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
error: patch failed: hw/gpio/Kconfig:16
error: hw/gpio/Kconfig: patch does not apply
error: patch failed: hw/gpio/meson.build:15
error: hw/gpio/meson.build: patch does not apply

Also please don't reply as v3, I almost missed v2, see
https://www.qemu.org/docs/master/devel/submitting-a-patch.html#use-git-format-patch:
"Patches are easier to find if they start a new top-level thread, rather 
than being buried in-reply-to another existing thread."

Regards,

Phil.