Commit 7c20f6c1 authored by Daniel Golle's avatar Daniel Golle Committed by Jakub Kicinski
Browse files

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: default avatarDaniel Golle <daniel@makrotopia.org>
Reviewed-by: default avatarAndrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/d10bd6ad5df062d0da342c3e0d330550b3d2432b.1774185953.git.daniel@makrotopia.org


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent a319d0c8

drivers/net/dsa/mxl862xx/mxl862xx-host.c

+38 −0
Original line number Diff line number Diff line
@@ -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);