netdev - Why is NIC driver queue depth driver dependent when it allocates system memory?

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Message-ID: <CAJx5YvHH9CoC8ZDz+MwG8RFr3eg2OtDvmU-EaqG76CiAz+W+5Q@mail.gmail.com>
Date:   Thu, 30 Jan 2020 15:02:02 +0200
From:   Martin T <m4rtntns@...il.com>
To:     netdev@...r.kernel.org
Subject: Why is NIC driver queue depth driver dependent when it allocates
 system memory?

Hi,

when I read the source code of for example tg3 driver or e1000e
driver, then looks like the driver queue is allocated from system
memory. For example, in e1000_ethtool.c kcalloc() is called to
allocate GFP_KERNEL memory.

If system memory is allocated, then why are there driver-dependent
limits? For example, in my workstation the maximum RX/TX queue for the
NIC using tg3 driver is 511 while maximum RX/TX queue for the NIC
using e1000e driver is 4096:

# Broadcom Limited NetXtreme BCM5722 Gigabit Ethernet; tg3 driver
# ethtool -g eth1
Ring parameters for eth1:
Pre-set maximums:
RX:             511
RX Mini:        0
RX Jumbo:       0
TX:             511
Current hardware settings:
RX:             200
RX Mini:        0
RX Jumbo:       0
TX:             511

# Intel Corporation Ethernet Connection (2) I219-LM; e1000e driver
# ethtool -g eth0
Ring parameters for eth0:
Pre-set maximums:
RX:             4096
RX Mini:        0
RX Jumbo:       0
TX:             4096
Current hardware settings:
RX:             256
RX Mini:        0
RX Jumbo:       0
TX:             256

#

thanks,
Martin