Re: New-bus unit wiring via hints..

On Oct 12, 2007, at 11:09 AM, John Baldwin wrote:

> On Thursday 11 October 2007 05:59:23 pm Marcel Moolenaar wrote:
>>
>> On Oct 11, 2007, at 2:41 PM, John Baldwin wrote:
>>
>>> 2) One of the things this fixes that is visible to users is that if
>>> your
>>>    machine gets the COM ports backwards when using ACPI it should
>>> now get
>>>    them correct (COM1 as sio0) assuming your COM ports use the
>>> default hints
>>>    and you have the default sio hints in your /boot/device.hints  
>>> file.
>>
>> I think you just pointed out the problem of using hints to wire
>> down unit numbers, because hints will stop hinting and will start
>> dictating. If I swap the serial ports in the BIOS then surely
>> my hints will be wrong and ACPI will be right. A default hints
>> file will be even more disastrous than before.
>
> hints have always dictated.  The issue is that hints are  
> enumerating devices
> that ACPI/PNPBIOS also dictate, so how does one resolve the  
> differences (do
> you ignore one or the other, or attempt to merge them, etc.).  Note  
> that
> hints can always be removed.

I think the conflict is resolved by having hints only apply to the isa
bus proper and stop tying apci to the isa bus. This also exposes other
flaws with hints, such as using flags to mark a device as (potential)
console.

> FWIW, the resources for COM1 and COM2 are largely fixed in practice on
> existing x86 hardware which is what this would affect, and hints  
> can always
> be removed.  Given the number of complaints from people where ACPI  
> enumerates
> COM2 before COM1 (and the fact that ACPI made _UID useless by not  
> giving it a
> more strict format.. it's optional and on some systems it is 0- 
> based while on
> others it is 1-based.

But aren't the complaints the immediate result of having hints in the  
first
place. Remove the hints and the complaints go away, right?

>   I've also seen pci link devices where the _UID is the
> link index from the $PIR table which == the register offset in  
> config space
> on the PCI-ISA bridge of the controlling register) and the fact  
> that COM1 and
> COM2 have de-facto fixed resources on x86, I think it's ok for x86  
> to include
> default hints for sio0 and sio1.

If the hints would be specific to the ISA bus, then it would  
definitely be
ok to have them. Even though we don't support i386 hardware, it  
should not
be a big deal to keep the support for the ISA bus. We should really stop
using acpi as an alias for isa and start treating it as a separate  
"bus".

> Do you have any comments on the idea in general of allowing busses  
> to use
> hints (or some other method) to wire devices to unit numbers?

Wiring unit numbers to particular devices is good. Tying them to hints
isn't. I would rather see us go in the direction of having the kernel
maintain a configuration file. This would allow us to extend the notion
wiring down unit numbers to keeping state across reboots. This could
be used in the future for various cases, such as:
o  video/graphics settings
o  console settings
o  fast boot or suspend from disk information
o  user specified labels or aliases for devices/disks
o  interrupt routing overrides
o  quirks
o  etc

No matter how we implement the wiring, one major issue is that the
information can be out of sync if the hardware configuration is changed.
It would be nice to have at least a boot option that tells the kernel
to ignore all the wiring information. It would allow one to boot the
kernel, correct any conflicts that would otherwise prohibit the kernel
from booting normally and restart.
Of course, if the wiring is done with hints, that would then also mean
that all the hints are ignored, which currently means that drivers that
abuse hints won't get started (i.e. syscons). That's another reason to
uncouple hints from wiring. We currently can't boot without hints even
though all hardware is found without hints -- we misuse hints as it is.

Just my $0.02,

-- 
Marcel Moolenaar
xcllnt_at_mac.com