[PATCH net-next 2/2] net: dsa: mxl862xx: use RST_DATA to skip writing zero words
From: Daniel Golle <daniel@makrotopia.org>
Date: 2026-03-21 05:20:20
Also in:
lkml
Subsystem:
maxlinear mxl862xx switch driver, networking drivers, networking [dsa], the rest · Maintainers:
Daniel Golle, Andrew Lunn, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Vladimir Oltean, Linus Torvalds
Issue the firmware's RST_DATA command before writing data payloads that contain many zero words. RST_DATA zeroes both the firmware's internal buffer and the MMD data registers in a single command, allowing the driver to skip individual MDIO writes for zero-valued words. This reduces bus traffic for the common case where API structs have many unused or default-zero fields. The optimization is applied when at least 5 zero words are found in the payload, roughly the break-even point against the cost of the extra RST_DATA command round-trip. Signed-off-by: Daniel Golle <daniel@makrotopia.org> --- drivers/net/dsa/mxl862xx/mxl862xx-host.c | 38 ++++++++++++++++++++++++ 1 file changed, 38 insertions(+)
diff --git a/drivers/net/dsa/mxl862xx/mxl862xx-host.c b/drivers/net/dsa/mxl862xx/mxl862xx-host.c
index 19ebb1dd724dd..cb129a182b0f7 100644
--- a/drivers/net/dsa/mxl862xx/mxl862xx-host.c
+++ b/drivers/net/dsa/mxl862xx/mxl862xx-host.c@@ -266,6 +266,17 @@ static int mxl862xx_get_data(struct mxl862xx_priv *priv, u16 words) MXL862XX_MMD_REG_DATA_MAX_SIZE * sizeof(u16)); } +static int mxl862xx_rst_data(struct mxl862xx_priv *priv) +{ + return mxl862xx_issue_cmd(priv, MMD_API_RST_DATA, 0); +} + +/* Minimum number of zero words in the data payload before issuing a + * RST_DATA command is worthwhile. RST_DATA costs one full command + * round-trip (~5 MDIO transactions), so the threshold must offset that. + */ +#define RST_DATA_THRESHOLD 5 + static int mxl862xx_send_cmd(struct mxl862xx_priv *priv, u16 cmd, u16 size, bool quiet) {
@@ -308,6 +319,8 @@ int mxl862xx_api_wrap(struct mxl862xx_priv *priv, u16 cmd, void *_data, u16 size, bool read, bool quiet) { __le16 *data = _data; + bool use_rst = false; + unsigned int zeros; int ret, cmd_ret; u16 max, crc, i;
@@ -321,6 +334,24 @@ int mxl862xx_api_wrap(struct mxl862xx_priv *priv, u16 cmd, void *_data, if (ret < 0) goto out; + /* If the data contains enough zero words, issue RST_DATA to zero + * both the firmware buffer and MMD registers, then skip writing + * zero words individually. + */ + for (i = 0, zeros = 0; i < size / 2 && zeros < RST_DATA_THRESHOLD; i++) + if (!data[i]) + zeros++; + + if (zeros < RST_DATA_THRESHOLD && (size & 1) && !*(u8 *)&data[i]) + zeros++; + + if (zeros >= RST_DATA_THRESHOLD) { + ret = mxl862xx_rst_data(priv); + if (ret < 0) + goto out; + use_rst = true; + } + /* Compute CRC-16 over the data payload; written as an extra word * after the data so the firmware can verify the transfer. */
@@ -354,6 +385,13 @@ int mxl862xx_api_wrap(struct mxl862xx_priv *priv, u16 cmd, void *_data, val = le16_to_cpu(data[i]); } + /* After RST_DATA, skip zero data words as the registers + * already contain zeros, but never skip the CRC at the final + * word. + */ + if (use_rst && i < max && val == 0) + continue; + ret = mxl862xx_reg_write(priv, MXL862XX_MMD_REG_DATA_FIRST + off, val);
--
2.53.0