net: stmmac: fix dwmac4 transmit performance regression

net: stmmac: fix dwmac4 transmit performance regression

dwmac4's transmit performance dropped by a factor of four due to an
incorrect assumption about which definitions are for what. This
highlights the need for sane register macros.

Commit 8409495bf6c9 ("net: stmmac: cores: remove many xxx_SHIFT
definitions") changed the way the txpbl value is merged into the
register:

        value = readl(ioaddr + DMA_CHAN_TX_CONTROL(dwmac4_addrs, chan));
-       value = value | (txpbl << DMA_BUS_MODE_PBL_SHIFT);
+       value = value | FIELD_PREP(DMA_BUS_MODE_PBL, txpbl);

With the following in the header file:

 #define DMA_BUS_MODE_PBL               BIT(16)
-#define DMA_BUS_MODE_PBL_SHIFT         16

The assumption here was that DMA_BUS_MODE_PBL was the mask for
DMA_BUS_MODE_PBL_SHIFT, but this turns out not to be the case.

The field is actually six bits wide, buts 21:16, and is called
TXPBL.

What's even more confusing is, there turns out to be a PBLX8
single bit in the DMA_CHAN_CONTROL register (0x1100 for channel 0),
and DMA_BUS_MODE_PBL seems to be used for that. However, this bit
et.al. was listed under a comment "/* DMA SYS Bus Mode bitmap */"
which is for register 0x1004.

Fix this up by adding an appropriately named field definition under
the DMA_CHAN_TX_CONTROL() register address definition.

Move the RPBL mask definition under DMA_CHAN_RX_CONTROL(), correctly
renaming it as well.

Also move the PBL bit definition under DMA_CHAN_CONTROL(), correctly
renaming it.

This removes confusion over the PBL fields.

Fixes: 8409495bf6c9 ("net: stmmac: cores: remove many xxx_SHIFT definitions")
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Bisected-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://lore.kernel.org/51859704-57fd-4913-b09d-9ac58a57f185@bootlin.com
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/E1vgY1k-00000003vOC-0Z1H@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>