(Disclaimer, don't know if it's exactly the same problem).
On one of our DELL Lattitude 5400 laptops the wifi stopped working, giving:
Jul 27 22:00:04 oceanic kernel: [ 141.298350] iwlwifi 0000:00:14.3: enabling device (0000 -> 0002) Jul 27 22:00:04 oceanic kernel: [ 141.323938] iwlwifi 0000:00:14.3: firmware: direct-loading firmware iwlwifi-9000-pu-b0-jf-b0-38.ucode Jul 27 22:00:04 oceanic kernel: [ 141.324553] iwlwifi 0000:00:14.3: loaded firmware version 38.755cfdd8.0 op_mode iwlmvm Jul 27 22:00:04 oceanic kernel: [ 141.403707] iwlwifi 0000:00:14.3: Detected Intel(R) Dual Band Wireless AC 9560, REV=0x318 Jul 27 22:00:04 oceanic kernel: [ 141.646515] iwlwifi 0000:00:14.3: Microcode SW error detected. Restarting 0x0. Jul 27 22:00:04 oceanic kernel: [ 141.646524] iwlwifi 0000:00:14.3: Not valid error log pointer 0x00000000 for Init uCode Jul 27 22:00:04 oceanic kernel: [ 141.646566] iwlwifi 0000:00:14.3: SecBoot CPU1 Status: 0x3, CPU2 Status: 0x2475 Jul 27 22:00:04 oceanic kernel: [ 141.646569] iwlwifi 0000:00:14.3: Failed to start INIT ucode: -5 Jul 27 22:00:04 oceanic kernel: [ 141.658825] iwlwifi 0000:00:14.3: Failed to run INIT ucode: -5
I tried updating firmware-iwlwifi to 20190717-2~bpo10+1, no change
I upgraded to kernel 5.6.14-2~bpo10+1, and hence 9000-pu-b0-jf-b0-46.ucode which gave more output but still didn't work:
Jul 29 11:52:21 oceanic kernel: [ 12.133630] iwlwifi 0000:00:14.3: enabling device (0000 -> 0002) Jul 29 11:52:21 oceanic kernel: [ 12.150217] iwlwifi 0000:00:14.3: firmware: direct-loading firmware iwlwifi-9000-pu-b0-jf-b0-46.ucode Jul 29 11:52:21 oceanic kernel: [ 12.150233] iwlwifi 0000:00:14.3: Found debug destination: EXTERNAL_DRAM Jul 29 11:52:21 oceanic kernel: [ 12.150234] iwlwifi 0000:00:14.3: Found debug configuration: 0 Jul 29 11:52:21 oceanic kernel: [ 12.150622] iwlwifi 0000:00:14.3: loaded firmware version 46.a41adfe7.0 9000-pu-b0-jf-b0-46.ucode op_mode iwlmvm Jul 29 11:52:21 oceanic kernel: [ 12.233985] iwlwifi 0000:00:14.3: Detected Intel(R) Wireless-AC 9560 160MHz, REV=0x318 Jul 29 11:52:21 oceanic kernel: [ 12.241362] iwlwifi 0000:00:14.3: Applying debug destination EXTERNAL_DRAM Jul 29 11:52:21 oceanic kernel: [ 12.241616] iwlwifi 0000:00:14.3: Allocated 0x00400000 bytes for firmware monitor. Jul 29 11:52:21 oceanic kernel: [ 12.274019] iwlwifi 0000:00:14.3: Microcode SW error detected. Restarting 0x0. Jul 29 11:52:21 oceanic kernel: [ 12.274025] iwlwifi 0000:00:14.3: Not valid error log pointer 0x00000000 for Init uCode Jul 29 11:52:21 oceanic kernel: [ 12.274042] iwlwifi 0000:00:14.3: Fseq Registers: Jul 29 11:52:21 oceanic kernel: [ 12.274050] iwlwifi 0000:00:14.3: 0xE8680755 | FSEQ_ERROR_CODE Jul 29 11:52:21 oceanic kernel: [ 12.274059] iwlwifi 0000:00:14.3: 0x00000000 | FSEQ_TOP_INIT_VERSION Jul 29 11:52:21 oceanic kernel: [ 12.274067] iwlwifi 0000:00:14.3: 0x304500C4 | FSEQ_CNVIO_INIT_VERSION Jul 29 11:52:21 oceanic kernel: [ 12.274075] iwlwifi 0000:00:14.3: 0x0000A384 | FSEQ_OTP_VERSION Jul 29 11:52:21 oceanic kernel: [ 12.274084] iwlwifi 0000:00:14.3: 0x8F6E77E5 | FSEQ_TOP_CONTENT_VERSION Jul 29 11:52:21 oceanic kernel: [ 12.274092] iwlwifi 0000:00:14.3: 0xC5F6D9EF | FSEQ_ALIVE_TOKEN Jul 29 11:52:21 oceanic kernel: [ 12.274100] iwlwifi 0000:00:14.3: 0x08234C18 | FSEQ_CNVI_ID Jul 29 11:52:21 oceanic kernel: [ 12.274109] iwlwifi 0000:00:14.3: 0xBCDE7DBF | FSEQ_CNVR_ID Jul 29 11:52:21 oceanic kernel: [ 12.274117] iwlwifi 0000:00:14.3: 0x01000100 | CNVI_AUX_MISC_CHIP Jul 29 11:52:21 oceanic kernel: [ 12.274128] iwlwifi 0000:00:14.3: 0x01300202 | CNVR_AUX_MISC_CHIP Jul 29 11:52:21 oceanic kernel: [ 12.274138] iwlwifi 0000:00:14.3: 0x0000485B | CNVR_SCU_SD_REGS_SD_REG_DIG_DCDC_VTRIM Jul 29 11:52:21 oceanic kernel: [ 12.274179] iwlwifi 0000:00:14.3: 0xA5A5A5A2 | CNVR_SCU_SD_REGS_SD_REG_ACTIVE_VDIG_MIRROR Jul 29 11:52:21 oceanic kernel: [ 12.274224] iwlwifi 0000:00:14.3: SecBoot CPU1 Status: 0x3, CPU2 Status: 0x2386 Jul 29 11:52:21 oceanic kernel: [ 12.274227] iwlwifi 0000:00:14.3: Failed to start INIT ucode: -5 Jul 29 11:52:21 oceanic kernel: [ 12.274229] iwlwifi 0000:00:14.3: Collecting data: trigger 16 fired. Jul 29 11:52:21 oceanic kernel: [ 12.522663] iwlwifi 0000:00:14.3: Firmware not running - cannot dump error Jul 29 11:52:21 oceanic kernel: [ 12.534415] iwlwifi 0000:00:14.3: Failed to run INIT ucode: -5
In desperation I booted into windows to see if the wifi worked there, and it did.
Then I rebooted Debian and the wifi now works.
Jul 29 12:13:09 oceanic kernel: [ 15.067344] iwlwifi 0000:00:14.3: enabling device (0000 -> 0002) Jul 29 12:13:09 oceanic kernel: [ 15.072020] iwlwifi 0000:00:14.3: firmware: direct-loading firmware iwlwifi-9000-pu-b0-jf-b0-46.ucode Jul 29 12:13:09 oceanic kernel: [ 15.072039] iwlwifi 0000:00:14.3: Found debug destination: EXTERNAL_DRAM Jul 29 12:13:09 oceanic kernel: [ 15.072039] iwlwifi 0000:00:14.3: Found debug configuration: 0 Jul 29 12:13:09 oceanic kernel: [ 15.072269] iwlwifi 0000:00:14.3: loaded firmware version 46.a41adfe7.0 9000-pu-b0-jf-b0-46.ucode op_mode iwlmvm Jul 29 12:13:09 oceanic kernel: [ 15.226922] iwlwifi 0000:00:14.3: Detected Intel(R) Wireless-AC 9560 160MHz, REV=0x318 Jul 29 12:13:09 oceanic kernel: [ 15.235063] iwlwifi 0000:00:14.3: Applying debug destination EXTERNAL_DRAM Jul 29 12:13:09 oceanic kernel: [ 15.235387] iwlwifi 0000:00:14.3: Allocated 0x00400000 bytes for firmware monitor. Jul 29 12:13:09 oceanic kernel: [ 15.280681] iwlwifi 0000:00:14.3: base HW address: ac:67:5d:2d:e3:ee Jul 29 12:13:09 oceanic kernel: [ 15.518852] iwlwifi 0000:00:14.3 wlo1: renamed from wlan0 Jul 29 12:13:10 oceanic kernel: [ 15.859594] iwlwifi 0000:00:14.3: Applying debug destination EXTERNAL_DRAM Jul 29 12:13:10 oceanic kernel: [ 15.974940] iwlwifi 0000:00:14.3: Applying debug destination EXTERNAL_DRAM Jul 29 12:13:10 oceanic kernel: [ 16.039011] iwlwifi 0000:00:14.3: FW already configured (0) - re-configuring Jul 29 12:13:10 oceanic kernel: [ 16.078459] iwlwifi 0000:00:14.3: Applying debug destination EXTERNAL_DRAM Jul 29 12:13:10 oceanic kernel: [ 16.207940] iwlwifi 0000:00:14.3: Applying debug destination EXTERNAL_DRAM Jul 29 12:13:10 oceanic kernel: [ 16.275045] iwlwifi 0000:00:14.3: FW already configured (0) - re-configuring
It is possible that this problem was caused by the laptop battery running completely flat, and the Linux driver or firmware missing some initialisation step.