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>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
---
v2: no changes
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 0b7f8db318257..cadbdb590cf43 100644
--- a/drivers/net/dsa/mxl862xx/mxl862xx-host.c
+++ b/drivers/net/dsa/mxl862xx/mxl862xx-host.c
@@ -283,6 +283,17 @@ 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)
{
@@ -318,6 +329,8 @@ 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;
@@ -331,6 +344,24 @@ 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.
*/
@@ -367,6 +398,13 @@ 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 word 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