[PATCH net-next 4/5] net: macb: implement timestamped MDIO writes
From: James Clark
Date: Fri Oct 09 2026 - 11:28:07 EST
Implement write_sts to provide system timestamp bounds for completion
of an MDIO write. This enables tighter system timestamp bounds in PHY
implementations of gettimex64.
Take system timestamps around the command register write, then add
the time from the command write until the MDC edge that clocks the
last data bit, calculated from the peripheral clock rate and the
configured MDC divider.
On RP1 that edge comes 63.5 MDC periods after the command write. This
was measured on a Raspberry Pi CM5 with its BCM54210PE PHY. The PHY's
PHC was read with PTP_SYS_OFFSET_EXTENDED while the MDC divisor was
switched between 48 and 128 through /dev/mem. Any error in the number
of periods would make the PHC offset jump at each switch. After
compensating for drift, the jump with 63.5 periods was under 2 ns.
The MDC divider does not appear to run freely: the spread of the PHC
offsets did not change with the divisor.
Signed-off-by: James Clark <jjc@xxxxxxxxxx>
Assisted-by: LLM
---
The 63.5-period delay has been measured only on RP1, but the code
assumes it holds for all MACB and GEM variants. I am not sure whether
that is a reasonable assumption; if not, write_sts could be enabled
only for RP1.
drivers/net/ethernet/cadence/macb.h | 1 +
drivers/net/ethernet/cadence/macb_main.c | 95 ++++++++++++++++++++++--
2 files changed, 88 insertions(+), 8 deletions(-)
diff --git a/drivers/net/ethernet/cadence/macb.h b/drivers/net/ethernet/cadence/macb.h
index 1cb2778fe49..3670731690f 100644
--- a/drivers/net/ethernet/cadence/macb.h
+++ b/drivers/net/ethernet/cadence/macb.h
@@ -1334,6 +1334,7 @@ struct macb {
struct macb_or_gem_ops macbgem_ops;
struct mii_bus *mii_bus;
+ unsigned long mdio_sts_rate;
struct phylink *phylink;
struct phylink_config phylink_config;
struct phylink_pcs phylink_usx_pcs;
diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index c4223ca848d..cec6e5a48e9 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -384,11 +384,47 @@ static int macb_mdio_read_c45(struct mii_bus *bus, int mii_id, int devad,
return status;
}
-static int macb_mdio_write_c22(struct mii_bus *bus, int mii_id, int regnum,
- u16 value)
+static u64 macb_mdio_sts_delay(struct macb *bp)
+{
+ static const u16 gem_divisors[] = {
+ [GEM_CLK_DIV8] = 8,
+ [GEM_CLK_DIV16] = 16,
+ [GEM_CLK_DIV32] = 32,
+ [GEM_CLK_DIV48] = 48,
+ [GEM_CLK_DIV64] = 64,
+ [GEM_CLK_DIV96] = 96,
+ [GEM_CLK_DIV128] = 128,
+ [GEM_CLK_DIV224] = 224,
+ };
+ static const u16 macb_divisors[] = {
+ [MACB_CLK_DIV8] = 8,
+ [MACB_CLK_DIV16] = 16,
+ [MACB_CLK_DIV32] = 32,
+ [MACB_CLK_DIV64] = 64,
+ };
+ unsigned long rate = READ_ONCE(bp->mdio_sts_rate);
+ u32 config = macb_readl(bp, NCFGR);
+ u32 divisor;
+
+ if (macb_is_gem(bp))
+ divisor = gem_divisors[GEM_BFEXT(CLK, config)];
+ else
+ divisor = macb_divisors[MACB_BFEXT(CLK, config)];
+
+ /* On RP1 the MDC edge that clocks the last bit of a clause 22
+ * write comes 63.5 periods after the command write.
+ */
+ return div64_ul(127ULL * divisor * NSEC_PER_SEC, 2 * rate);
+}
+
+static int macb_mdio_write_c22_sts(struct mii_bus *bus, int mii_id, int regnum,
+ u16 value, struct ptp_system_timestamp *sts)
{
struct macb *bp = bus->priv;
+ unsigned long flags;
+ u64 delay_ns;
int status;
+ u32 cmd;
status = pm_runtime_resume_and_get(&bp->pdev->dev);
if (status < 0)
@@ -398,12 +434,32 @@ static int macb_mdio_write_c22(struct mii_bus *bus, int mii_id, int regnum,
if (status < 0)
goto mdio_write_exit;
- macb_writel(bp, MAN, (MACB_BF(SOF, MACB_MAN_C22_SOF)
- | MACB_BF(RW, MACB_MAN_C22_WRITE)
- | MACB_BF(PHYA, mii_id)
- | MACB_BF(REGA, regnum)
- | MACB_BF(CODE, MACB_MAN_C22_CODE)
- | MACB_BF(DATA, value)));
+ cmd = MACB_BF(SOF, MACB_MAN_C22_SOF)
+ | MACB_BF(RW, MACB_MAN_C22_WRITE)
+ | MACB_BF(PHYA, mii_id)
+ | MACB_BF(REGA, regnum)
+ | MACB_BF(CODE, MACB_MAN_C22_CODE)
+ | MACB_BF(DATA, value);
+
+ if (sts) {
+ delay_ns = macb_mdio_sts_delay(bp);
+ local_irq_save(flags);
+ ptp_read_system_prets(sts);
+ /* macb_writel() is relaxed; order it after the timestamp. */
+ mb();
+ }
+ macb_writel(bp, MAN, cmd);
+ if (sts) {
+ /* Flush the posted write before taking the upper bound. */
+ macb_readl(bp, NSR);
+ /* Order the read-back before the system timestamp. */
+ rmb();
+ ptp_read_system_postts(sts);
+ local_irq_restore(flags);
+
+ ptp_adjust_system_prets(sts, delay_ns);
+ ptp_adjust_system_postts(sts, delay_ns);
+ }
status = macb_mdio_wait_for_idle(bp);
if (status < 0)
@@ -415,6 +471,12 @@ static int macb_mdio_write_c22(struct mii_bus *bus, int mii_id, int regnum,
return status;
}
+static int macb_mdio_write_c22(struct mii_bus *bus, int mii_id, int regnum,
+ u16 value)
+{
+ return macb_mdio_write_c22_sts(bus, mii_id, regnum, value, NULL);
+}
+
static int macb_mdio_write_c45(struct mii_bus *bus, int mii_id,
int devad, int regnum,
u16 value)
@@ -1141,6 +1203,13 @@ static int macb_mdiobus_register(struct macb *bp, struct device_node *mdio_np)
return mdiobus_register(bp->mii_bus);
}
+static bool macb_mdio_init_sts(struct macb *bp)
+{
+ bp->mdio_sts_rate = clk_get_rate(bp->pclk);
+
+ return bp->mdio_sts_rate != 0;
+}
+
static int macb_mii_init(struct macb *bp)
{
struct device_node *mdio_np, *np = bp->pdev->dev.of_node;
@@ -1166,6 +1235,8 @@ static int macb_mii_init(struct macb *bp)
bp->mii_bus->name = "MACB_mii_bus";
bp->mii_bus->read = &macb_mdio_read_c22;
bp->mii_bus->write = &macb_mdio_write_c22;
+ if (macb_mdio_init_sts(bp))
+ bp->mii_bus->write_sts = &macb_mdio_write_c22_sts;
bp->mii_bus->read_c45 = &macb_mdio_read_c45;
bp->mii_bus->write_c45 = &macb_mdio_write_c45;
snprintf(bp->mii_bus->id, MII_BUS_ID_SIZE, "%s-%x",
@@ -3095,12 +3166,20 @@ static void macb_configure_dma(struct macb *bp)
static void macb_init_hw(struct macb *bp)
{
+ unsigned long rate;
u32 config;
macb_reset_hw(bp);
macb_set_hwaddr(bp);
config = macb_mdc_clk_div(bp);
+ /* Record the pclk rate the MDC divider is chosen from, for
+ * write_sts, which can't call clk_get_rate() under the MDIO bus
+ * lock.
+ */
+ rate = clk_get_rate(bp->pclk);
+ if (rate)
+ WRITE_ONCE(bp->mdio_sts_rate, rate);
/* Make eth data aligned.
* If RSC capable, that offset is ignored by HW.
*/
--
2.56.0