net: dsa: mxl862xx: use RST_DATA to skip writing zero words

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>
Link: https://patch.msgid.link/d10bd6ad5df062d0da342c3e0d330550b3d2432b.1774185953.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/dsa/mxl862xx/mxl862xx-host.c b/drivers/net/dsa/mxl862xx/mxl862xx-host.c
index 0b7f8db3182570569b6b06b122362d1cef1780a5..cadbdb590cf43275fefb0bfaddc2df14b2d2a945 100644 (file)
--- 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);