kern/57362: piixide / wd does not fall back to PIO4 after excessive DMA errors

[ltning-netbsd%anduin.net@localhost]

>Number:         57362
>Category:       kern
>Synopsis:       piixide / wd does not fall back to PIO4 after excessive DMA errors
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Apr 17 23:05:00 +0000 2023
>Originator:     Eirik Øverby
>Release:        10-BETA
>Organization:
Modirum
>Environment:
>Description:
On NetBSD 9.3, the IDE driver falls back to PIO4 after excessive DMA errors when reading/writing devices. This is the preferred behaviour, as some devices on older hardware will not operate correctly in DMA mode.

On NetBSD 10-BETA, the driver keeps trying to operate in DMA mode and if the device does not magically recover, hard power-off is the only way out.

It would be preferable if the driver downgrades to PIO4 like in previous versions. In my case, I have suffered (unimportant, backed-up) data loss as a result of this change.
>How-To-Repeat:
1. Obtain old hardware with PIO mode problems.
2. Install NetBSD 9.3, observe downgrade to PIO mode 4
3. Boot 10-BETA kernel, observe endless DMA errors
4. Boot 9.3 again; automatic boot may fail due to errors during fsck_ffs
>Fix:
The ability to disable DMA as a boot option would be useful; I am new to NetBSD so have not yet figured out if/how to set device driver flags during boot without recompiling kernel.