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Re: [PATCH 2/2] via-ide: Also emulate non 100% native mode

From: Mark Cave-Ayland
Subject: Re: [PATCH 2/2] via-ide: Also emulate non 100% native mode
Date: Wed, 4 Mar 2020 21:04:17 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 04/03/2020 00:22, BALATON Zoltan wrote:

>>>> So on that basis the best explanation as to what is happening is the
>>>> comment in the link you provided here:
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/arch/powerpc/platforms/chrp/pci.c?h=v4.14.172#n353
>>>> /* Pegasos2 firmware version 20040810 configures the built-in IDE 
>>>> controller
>>>> * in legacy mode, but sets the PCI registers to PCI native mode.
>>>> * The chip can only operate in legacy mode, so force the PCI class into 
>>>> legacy
>>>> * mode as well. The same fixup must be done to the class-code property in
>>>> * the IDE node /pci@80000000/ide@C,1
>>>> */
>>> I'm not sure that it makes much sense that the firmware configures the chip 
>>> one way
>>> then puts info about a different way in the device tree. There could be 
>>> bugs but this
>>> is not likely. Especially because I see in traces that the firmware does 
>>> try to
>>> configure the device in native mode. These are the first few accesses of 
>>> the firmware
>>> to via-ide:
>> But that is exactly what is happening! The comment above clearly indicates 
>> the
>> firmware incorrectly sets the IDE controller in native mode which is in exact
>> agreement with the trace you provide below. And in fact if you look at
> I may be reading the comment wrong but to me that says that "firmware 
> configures IDE
> in _legacy_ mode" whereas the trace shows it actually configures it in 
> _native_ mode
> which is complying to the CHRP doc above. But since it cannot comply to the 
> "native
> devices using OpenPIC" part it probably tries to apply the "ISA devices 
> embedded in
> PCI" part and locks IRQ to 14 and 15. Or it just wants to avoid sharing IRQs 
> as much
> as possible and the designers decided they will use IRQ14 and 15 for IDE.

Interesting. My interpretation of the comment was that the hardware can only 
in legacy mode, even though the firmware configures the PCI registers to enable
native mode (which is why the class-code and IRQ routing are wrong).

>> https://www.powerdeveloper.org/platforms/pegasos/devicetree you can see the 
>> nvramrc
>> hack that was released in order to fix the device tree to boot Linux which 
>> alters the
>> class-code and sets interrupts to 14 (which I believe is invalid, but 
>> seemingly good
>> enough here).
> Isn't this the same fixup that newer Linux kernels already include? Maybe 
> this was
> needed before Linux properly supported Pegasos2 but later kernels will do this
> anyway. This does not give us any new info we did not have before I think 
> maybe just
> easier to see all fixups in one place.
>>> pci_cfg_write via-ide 12:1 @0x9 <- 0xf
>>> pci_cfg_write via-ide 12:1 @0x40 <- 0xb
>>> pci_cfg_write via-ide 12:1 @0x41 <- 0xf2
>>> pci_cfg_write via-ide 12:1 @0x43 <- 0x35
>>> pci_cfg_write via-ide 12:1 @0x44 <- 0x18
>>> pci_cfg_write via-ide 12:1 @0x45 <- 0x1c
>>> pci_cfg_write via-ide 12:1 @0x46 <- 0xc0
>>> pci_cfg_write via-ide 12:1 @0x50 <- 0x17171717
>>> pci_cfg_write via-ide 12:1 @0x54 <- 0x14
>>> pci_cfg_read via-ide 12:1 @0x0 -> 0x5711106
>>> pci_cfg_read via-ide 12:1 @0x0 -> 0x5711106
>>> pci_cfg_read via-ide 12:1 @0x8 -> 0x1018f06
>>> pci_cfg_read via-ide 12:1 @0xc -> 0x0
>>> pci_cfg_read via-ide 12:1 @0x2c -> 0x11001af4
>>> pci_cfg_read via-ide 12:1 @0x3c -> 0x10e
>>> pci_cfg_read via-ide 12:1 @0x4 -> 0x2800080
>>> pci_cfg_read via-ide 12:1 @0x3c -> 0x10e
>>> pci_cfg_write via-ide 12:1 @0x3c <- 0x109
>>> The very first write is to turn on native mode, so I think it's not about 
>>> what the
>>> firmware does but something about how hardware is wired on Pegasos II or 
>>> the VT8231
>>> chip itself that only allows legacy interrupts instead of 100% native mode 
>>> for IDE.
>>>> Given that the DT is wrong, then we should assume that all OSs would have 
>>>> to
>>>> compensate for this in the same way as Linux, and therefore this should be 
>>>> handled
>>>> automatically.
>>>> AFAICT this then only leaves the question: why does the firmware set
>>>> PCI_INTERRUPT_LINE to 9, which is presumably why you are seeing problems 
>>>> running
>>>> MorphOS under QEMU.
>>> Linux does try to handle both true native mode and half-native mode. It 
>>> only uses
>>> half-native mode if finds IRQ14 on Pegasos, otherwise skips Pegasos 
>>> specific fixup
>>> and uses true native mode setup. I don't know what MorphOS does but I think 
>>> it justs
>>> knows that Pegasos2 has this quirk and does not look at the device tree at 
>>> all.

I think this is the other way around? From the code above:

        if (viaide->irq != 14)

Doesn't this suggest that chrp_pci_fixup_vt8231_ata() exits without applying 
the fix
if it finds PCI_INTERRUPT_LINE set to 9 from the firmware above?

Do you have a copy of the full DT and the firmware revision number that was 
used to
generate your Linux boot output on real hardware?

>> Again to summarise: this is a known bug in Pegasos2 firmware and the VIA is a
>> standard chip, so let's try and figure out exactly what is happening using a 
>> real
>> firmware and emulate that behaviour in QEMU. This should then make all 
>> guests happy,
>> regardless of architecture, without requiring the introduction of feature 
>> bits or
>> risk of introducing other incompatibilities.
> I think I've already done that with this patch (within the limits possible in 
> QEMU).
> The Pegasos2 seems to always use IRQ14 and 15 even when IDE is set to native 
> mode
> (which the firmware does immediately, I'm using a real firmware to test) and 
> all
> guests are happy with this. This behaviour is confirmed by excpectations of 
> AmigaOS
> and MorphOS drivers and also Linux comments (although those comments may get 
> the
> reason wrong, they confirm the behaviour). I'm not sure how real hardware 
> implements
> this behaviour and also not sure if it's a bug in the firmware or rather a 
> peculiar
> design choice for Pegasos2. But that probably does not matter for the fact 
> that it's
> how it works which is all we need to emulate it.
> Also consider that QEMU via-ide is only emulating native mode of the chip 
> because we
> can't switch between the two modes so it's either legacy only or native only 
> because
> all other implemented controllers are either ISA or native PCI so QEMU does 
> not have
> way to deregister ISA IDE and PCI code does not have way to use io BARs 
> despite not
> being enabled via PCI config which could be needed to use them when device is 
> in
> legacy mode. Previously via-ide was emulating legacy only and that worked 
> with Linux
> but not with anything else. I'm not planning to rewrite large parts of the 
> IDE and
> PCI code to allow switching back and forth which is not even needed. Unless 
> you know
> a better way to implement this I think the proposed patch is achieving this 
> with
> minimal changes. I don't see a need to more exactly emulate some kind of 
> hardware bug
> or peculiar design choices of a board than what's needed to make clients 
> happy and boot.
> Is this series good enough now to be merged or are any changes needed? I'd 
> like to
> not miss the deadline for the freeze and get this delayed for months for not 
> good
> reason because I'm not sure when will I have time to work on it again.

I think there's still time to get something done before freeze, however I'm not
convinced that we understand the actual problem correctly (and also the use of
feature bits feels somewhat odd to me).

One more thing I don't understand: I had a glance over the logs you posted over 
https://osdn.net/projects/qmiga/ticket/38949 and you mention that everything 
works up
to the point where BMDMA is enabled.

>From memory working with cmd646 both the BMDMA and non-BMDMA interrupts end up
calling into the same *_set_irq() function in the emulated controller. So what 
is the
difference between the initial state where IRQs function enough to start to 
load an
OS, and at the point where BMDMA is enabled by the OS driver and things stop 
How does the IRQ routing compare?



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