smsc95xx 1-1.1:1.0 eth0: Failed to read reg index 0x00000114: -110

Start with the smsc95xx driver and its smsc95xx_mdio_read path, using the reported MII_ACCESS timeout and NETDEV WATCHDOG logs as the initial evidence. Review issues #60, #149, #552, and #746, then reproduce the failure on the CM3 with LAN9512; done requires identifying a reproducible cause and a verified fix.

Describe the bug
Network connectivity drops out at random on CM3 with a custom carrier board with LAN9512. Device becomes completely unreachable over any network going through that chip (e.g. Ethernet, LTE).

To reproduce
Unknown.

Expected behaviour
Network connectivity doesn't randomly drop out.

Actual behaviour
Network connectivity drops out.

System

• Which model of Raspberry Pi? CM3 with a custom carrier board with LAN9512
• Which OS and version (cat /etc/rpi-issue)? Raspberry Pi reference 2018-06-27
• Which firmware version (vcgencmd version)? VCHI initialization failed
• Which kernel version (uname -a)? 4.19.42-v7+ #1219 SMP Tue May 14 21:20:58 BST 2019 armv7l GNU/Linux

Logs

Jul 18 20:53:50 data kernel: [277401.663895] smsc95xx 1-1.1:1.0 eth0: Failed to read reg index 0x00000114: -110
Jul 18 20:53:50 data kernel: [277401.663906] smsc95xx 1-1.1:1.0 eth0: Error reading MII_ACCESS
Jul 18 20:53:50 data kernel: [277401.663916] smsc95xx 1-1.1:1.0 eth0: MII is busy in smsc95xx_mdio_read
Jul 18 20:53:50 data kernel: [277401.663926] smsc95xx 1-1.1:1.0 eth0: Failed to read MII_BMSR

Jul 18 20:54:56 data kernel: [277466.864971] WARNING: CPU: 0 PID: 0 at net/sched/sch_generic.c:461 dev_watchdog+0x294/0x298
Jul 18 20:54:56 data kernel: [277466.864977] NETDEV WATCHDOG: eth0 (smsc95xx): transmit queue 0 timed out
Jul 18 20:54:56 data kernel: [277466.864982] Modules linked in: tun cdc_ether cdc_acm rtc_ds1307 brcmfmac brcmutil sha256_generic cfg80211 rfkill raspberrypi_hwmon hwmon bcm2835_codec(C) bcm2835_v4l2(C) v4l2_mem2mem bcm2835_mmal_vchiq(C) v4l2_common videobuf2_dma_contig videobuf2_vmalloc videobuf2_memops videobuf2_v4l2 videobuf2_common videodev i2c_bcm2835 media vc_sm_cma(C) uio_pdrv_genirq uio fixed i2c_dev ip_table
s x_tables ipv6
Jul 18 20:54:56 data kernel: [277466.865151] CPU: 0 PID: 0 Comm: swapper/0 Tainted: G         C        4.19.42-v7+ #1219
Jul 18 20:54:56 data kernel: [277466.865154] Hardware name: BCM2835
Jul 18 20:54:56 data kernel: [277466.865173] [<80111eac>] (unwind_backtrace) from [<8010d430>] (show_stack+0x20/0x24)
Jul 18 20:54:56 data kernel: [277466.865184] [<8010d430>] (show_stack) from [<8080ec80>] (dump_stack+0xd4/0x118)
Jul 18 20:54:56 data kernel: [277466.865196] [<8080ec80>] (dump_stack) from [<80120904>] (__warn+0x104/0x11c)
Jul 18 20:54:56 data kernel: [277466.865204] [<80120904>] (__warn) from [<80120974>] (warn_slowpath_fmt+0x58/0x74)
Jul 18 20:54:56 data kernel: [277466.865212] [<80120974>] (warn_slowpath_fmt) from [<807355b8>] (dev_watchdog+0x294/0x298)
Jul 18 20:54:56 data kernel: [277466.865223] [<807355b8>] (dev_watchdog) from [<801965a0>] (call_timer_fn+0x3c/0x198)
Jul 18 20:54:56 data kernel: [277466.865233] [<801965a0>] (call_timer_fn) from [<801967e8>] (expire_timers+0xec/0x14c)
Jul 18 20:54:56 data kernel: [277466.865242] [<801967e8>] (expire_timers) from [<80196900>] (run_timer_softirq+0xb8/0x1ec)
Jul 18 20:54:56 data kernel: [277466.865251] [<80196900>] (run_timer_softirq) from [<80102388>] (__do_softirq+0x190/0x3f0)
Jul 18 20:54:56 data kernel: [277466.865260] [<80102388>] (__do_softirq) from [<801268dc>] (irq_exit+0xfc/0x120)
Jul 18 20:54:56 data kernel: [277466.865271] [<801268dc>] (irq_exit) from [<8017ef24>] (__handle_domain_irq+0x70/0xc4)
Jul 18 20:54:56 data kernel: [277466.865281] [<8017ef24>] (__handle_domain_irq) from [<801021b4>] (bcm2836_arm_irqchip_handle_irq+0x60/0xa4)
Jul 18 20:54:56 data kernel: [277466.865288] [<801021b4>] (bcm2836_arm_irqchip_handle_irq) from [<801019bc>] (__irq_svc+0x5c/0x7c)
Jul 18 20:54:56 data kernel: [277466.865292] Exception stack(0x80d01ea0 to 0x80d01ee8)
Jul 18 20:54:56 data kernel: [277466.865299] 1ea0: bc56838c 00000040 00000000 00000000 bc568380 bc568380 00000040 bc56838c
Jul 18 20:54:56 data kernel: [277466.865305] 1ec0: 00000001 be5ff9c0 80c63a38 80d01efc 80d01f00 80d01ef0 8069fc9c 8082bdf0
Jul 18 20:54:56 data kernel: [277466.865309] 1ee0: a0000013 ffffffff
Jul 18 20:54:56 data kernel: [277466.865318] [<801019bc>] (__irq_svc) from [<8082bdf0>] (_raw_read_lock+0x18/0x38)
Jul 18 20:54:56 data kernel: [277466.865332] [<8082bdf0>] (_raw_read_lock) from [<8069fc9c>] (led_trigger_event+0x30/0x8c)
Jul 18 20:54:56 data kernel: [277466.865341] [<8069fc9c>] (led_trigger_event) from [<806a1664>] (ledtrig_cpu+0xd4/0xf0)
Jul 18 20:54:56 data kernel: [277466.865350] [<806a1664>] (ledtrig_cpu) from [<80109a78>] (arch_cpu_idle_exit+0x1c/0x20)
Jul 18 20:54:56 data kernel: [277466.865361] [<80109a78>] (arch_cpu_idle_exit) from [<801524d0>] (do_idle+0x100/0x17c)
Jul 18 20:54:56 data kernel: [277466.865371] [<801524d0>] (do_idle) from [<8015280c>] (cpu_startup_entry+0x28/0x2c)
Jul 18 20:54:56 data kernel: [277466.865379] [<8015280c>] (cpu_startup_entry) from [<808253e4>] (rest_init+0xbc/0xc0)
Jul 18 20:54:56 data kernel: [277466.865389] [<808253e4>] (rest_init) from [<80c00fb0>] (start_kernel+0x484/0x4b4)

Additional context
I've already checked issues #60, #149, #552 and #746 but none of them really contain anything useful besides mentioning that the issue is already fixed.

nRESET pin on LAN9512 is pulled high and not connected to a GPIO pin.