net: stmmac: Track the TX DMA threshold per queue

Commit 3a6c12a0c6c3 ("net: stmmac: bump tc when get underflow error from
DMA descriptor") made dwmac4 raise the DMA threshold of the queues that
report TX underflows. This is done by incrementing a module global
parameter 'tc' by 64 on each underflow.

This leads to inconsistent TX DMA thresholds between queues and causes
issues on platforms with multiple stmmac controllers, as 'tc' is shared
by all of them. Once 'tc' goes over 256 the bumping stops, and queues
that did not underflow before are left at the initial threshold.

On the Renesas RZ/V2H platform, which has two dwmac4 controllers, we
observed sporadic iperf3 TCP TX failures:

  iperf3 -c <server> -t5 --bind-dev end1
  ...
  [  5]   0.00-5.00   sec  1.00 MBytes  1.68 Mbits/sec   87       sender
  [  5]   0.00-5.00   sec   768 KBytes  1.26 Mbits/sec            receiver

This happens because the TX queues of end1 were underflowing on a large
fraction of the packets, but the DMA TX threshold was never raised from
the default of 64 bytes. The threshold was not raised because end0
had underflowed first and had already pushed 'tc' past 256. Which
controller is affected depends on which one underflows first.

To fix this, make the TX DMA threshold per queue instead of global.
Each time an underflow happens on a queue, its TX DMA threshold gets
incremented, without affecting other queues or controllers. With this
change, 'tc' is only used as the initial value, and the RX threshold
is no longer bumped together with the TX one, as TX underflows have
nothing to do with the RX side.

Fixes: 3a6c12a0c6c3 ("net: stmmac: bump tc when get underflow error from DMA descriptor")
Signed-off-by: Ovidiu Panait <ovidiu.panait.rb@renesas.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20261013114503.57487-2-ovidiu.panait.rb@renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions