Add get/set_module_eeprom_by_page ethtool ops to netdevsim, enabling
testing of kernel features that depend on module EEPROM access (e.g.
CMIS loopback) without real hardware.

The EEPROM is backed by a 256-page x 128-byte array exposed as binary
debugfs files under ports/<N>/ethtool/module/pages/{0..255}. Offsets
0-127 map to page 0 (lower memory), 128-255 to the requested page's
upper memory, following the CMIS layout. Error injection via get_err
and set_err follows the existing netdevsim pattern.

Signed-off-by: Björn Töpel <bjorn@kernel.org>
---
 drivers/net/netdevsim/ethtool.c   | 79 +++++++++++++++++++++++++++++++
 drivers/net/netdevsim/netdevsim.h | 11 +++++
 2 files changed, 90 insertions(+)

diff --git a/drivers/net/netdevsim/ethtool.c b/drivers/net/netdevsim/ethtool.c
index 36a201533aae..2145ccc8a9bd 100644
--- a/drivers/net/netdevsim/ethtool.c
+++ b/drivers/net/netdevsim/ethtool.c
@@ -195,6 +195,67 @@ nsim_get_fec_stats(struct net_device *dev, struct ethtool_fec_stats *fec_stats,
 	values[2].per_lane[3] = 0;
 }
 
+static u8 *nsim_module_eeprom_ptr(struct netdevsim *ns,
+				  const struct ethtool_module_eeprom *page_data, u32 *len)
+{
+	u32 offset;
+	u8 page;
+
+	if (page_data->offset < NSIM_MODULE_EEPROM_PAGE_LEN) {
+		page = 0;
+		offset = page_data->offset;
+	} else {
+		page = page_data->page;
+		offset = page_data->offset - NSIM_MODULE_EEPROM_PAGE_LEN;
+	}
+
+	if (page >= NSIM_MODULE_EEPROM_PAGES)
+		return NULL;
+
+	*len = min_t(u32, page_data->length, NSIM_MODULE_EEPROM_PAGE_LEN - offset);
+	return ns->ethtool.module.pages[page] + offset;
+}
+
+static int nsim_get_module_eeprom_by_page(struct net_device *dev,
+					  const struct ethtool_module_eeprom *page_data,
+					  struct netlink_ext_ack *extack)
+{
+	struct netdevsim *ns = netdev_priv(dev);
+	u32 len;
+	u8 *ptr;
+
+	if (ns->ethtool.module.get_err)
+		return -ns->ethtool.module.get_err;
+
+	ptr = nsim_module_eeprom_ptr(ns, page_data, &len);
+	if (!ptr)
+		return -EINVAL;
+
+	memcpy(page_data->data, ptr, len);
+
+	return len;
+}
+
+static int nsim_set_module_eeprom_by_page(struct net_device *dev,
+					  const struct ethtool_module_eeprom *page_data,
+					  struct netlink_ext_ack *extack)
+{
+	struct netdevsim *ns = netdev_priv(dev);
+	u32 len;
+	u8 *ptr;
+
+	if (ns->ethtool.module.set_err)
+		return -ns->ethtool.module.set_err;
+
+	ptr = nsim_module_eeprom_ptr(ns, page_data, &len);
+	if (!ptr)
+		return -EINVAL;
+
+	memcpy(ptr, page_data->data, len);
+
+	return 0;
+}
+
 static int nsim_get_ts_info(struct net_device *dev,
 			    struct kernel_ethtool_ts_info *info)
 {
@@ -222,6 +283,8 @@ static const struct ethtool_ops nsim_ethtool_ops = {
 	.set_fecparam			= nsim_set_fecparam,
 	.get_fec_stats			= nsim_get_fec_stats,
 	.get_ts_info			= nsim_get_ts_info,
+	.get_module_eeprom_by_page	= nsim_get_module_eeprom_by_page,
+	.set_module_eeprom_by_page	= nsim_set_module_eeprom_by_page,
 };
 
 static void nsim_ethtool_ring_init(struct netdevsim *ns)
@@ -237,6 +300,7 @@ static void nsim_ethtool_ring_init(struct netdevsim *ns)
 void nsim_ethtool_init(struct netdevsim *ns)
 {
 	struct dentry *ethtool, *dir;
+	int i;
 
 	ns->netdev->ethtool_ops = &nsim_ethtool_ops;
 
@@ -270,4 +334,19 @@ void nsim_ethtool_init(struct netdevsim *ns)
 			   &ns->ethtool.ring.rx_mini_max_pending);
 	debugfs_create_u32("tx_max_pending", 0600, dir,
 			   &ns->ethtool.ring.tx_max_pending);
+
+	dir = debugfs_create_dir("module", ethtool);
+	debugfs_create_u32("get_err", 0600, dir, &ns->ethtool.module.get_err);
+	debugfs_create_u32("set_err", 0600, dir, &ns->ethtool.module.set_err);
+
+	dir = debugfs_create_dir("pages", dir);
+	for (i = 0; i < NSIM_MODULE_EEPROM_PAGES; i++) {
+		char name[8];
+
+		ns->ethtool.module.page_blobs[i].data = ns->ethtool.module.pages[i];
+		ns->ethtool.module.page_blobs[i].size = NSIM_MODULE_EEPROM_PAGE_LEN;
+
+		snprintf(name, sizeof(name), "%u", i);
+		debugfs_create_blob(name, 0600, dir, &ns->ethtool.module.page_blobs[i]);
+	}
 }
diff --git a/drivers/net/netdevsim/netdevsim.h b/drivers/net/netdevsim/netdevsim.h
index f767fc8a7505..965d0aa0940b 100644
--- a/drivers/net/netdevsim/netdevsim.h
+++ b/drivers/net/netdevsim/netdevsim.h
@@ -82,6 +82,16 @@ struct nsim_ethtool_pauseparam {
 	bool report_stats_tx;
 };
 
+#define NSIM_MODULE_EEPROM_PAGES	256
+#define NSIM_MODULE_EEPROM_PAGE_LEN	128
+
+struct nsim_ethtool_module {
+	u32 get_err;
+	u32 set_err;
+	u8 pages[NSIM_MODULE_EEPROM_PAGES][NSIM_MODULE_EEPROM_PAGE_LEN];
+	struct debugfs_blob_wrapper page_blobs[NSIM_MODULE_EEPROM_PAGES];
+};
+
 struct nsim_ethtool {
 	u32 get_err;
 	u32 set_err;
@@ -90,6 +100,7 @@ struct nsim_ethtool {
 	struct ethtool_coalesce coalesce;
 	struct ethtool_ringparam ring;
 	struct ethtool_fecparam fec;
+	struct nsim_ethtool_module module;
 };
 
 struct nsim_rq {
-- 
2.53.0