Bug#1122340: I/O scheduler shows "[mq-deadline] none" on AHCI SATA drives

Package: linux
Version: 5.10.0-36-amd64
Severity: important
Tags: amd64 bullseye disk-performance ahci regression

Problem:
On AMD A320 chipset systems with AHCI SATA controller, the block I/O layer
incorrectly displays multiple schedulers as active simultaneously:
"[mq-deadline] [bfq] none". This causes severe system slowdown and desktop
unresponsiveness during disk I/O operations.

How to reproduce:
1. Boot Debian 11 (bullseye) on AMD A320 chipset motherboard (ASRock A320M-HDV)
2. Connect SATA HDD (TOSHIBA DT01ACA0 7200 RPM)
3. Check current scheduler: cat /sys/block/sda/queue/scheduler
4. Observe output: mq-deadline [bfq] none (note two active schedulers in brackets)
5. Perform large file copy or disk-intensive operation
6. Desktop environment (MATE) becomes unresponsive, system stalls

Diagnosis:
- System: Debian GNU/Linux 11 (bullseye), Kernel: 5.10.0-36-amd64, x86_64
- Motherboard: ASRock A320M-HDV (AMD Promontory A320 chipset)
- Disk: TOSHIBA DT01ACA0 (HDD, rotational=1, 7200 RPM SATA)
- Driver: ahci (multiple AHCI hosts detected: 9 instances)
- Scheduler state: mq-deadline [bfq] none (bfq active but mq-deadline also shown as active)
- Expected: Single scheduler in brackets indicating active one
- Current bug: Two schedulers shown with brackets [mq-deadline] [bfq]
- Physical system (not virtual machine)

Workaround:
Temporary fix:
# echo "bfq" > /sys/block/sda/queue/scheduler

Permanent fix via udev rule:
ACTION="" SUBSYSTEM=="block", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="bfq"

Expected behavior:
According to kernel documentation and block layer design:
1. Only ONE scheduler should be active at a time
2. The active scheduler should be shown in brackets: [bfq]
3. Available schedulers should be listed after: [bfq] mq-deadline none
4. Current display "mq-dedeadline [bfq] none" is inconsistent

Impact:
- Severe desktop usability regression (MATE environment freezes during I/O)
- System becomes unresponsive for tens of seconds during file operations
- Affects user experience significantly on HDD-based systems
- Problem appears to be in scheduler initialization/display logic

Additional notes:
- Problem persists across reboots
- Not VM-related (bare metal installation)
- Multiple AHCI hosts detected (9), may be related to initialization order
- Manual scheduler selection works correctly once set

Suggested fix locations:
1. block/elevator.c - scheduler selection/display logic
2. drivers/ata/libata-core.c - ATA device initialization
3. /lib/udev/rules.d/60-block-scheduler.rules - default scheduler assignment

System info for reference:
- Debian: 11 (bullseye)
- Kernel: 5.10.0-36-amd64
- Architecture: x86_64
- Disk: TOSHIBA DT01ACA0 (rotational=1)
- Driver: ahci (9 instances)
- Desktop: MATE
- Motherboard: ASRock A320M-HDV (AMD A320 chipset)