On Tue, Sep 20, 2016 at 02:17:44PM -0500, Bjorn Helgaas wrote:
On Tue, Sep 20, 2016 at 04:09:25PM +0100, Ard Biesheuvel wrote:
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None of these platforms can be fixed entirely in software, and given that we will not be adding quirks for new broken hardware, we should ask ourselves whether having two versions of a quirk, i.e., one for broken hardware + currently shipping firmware, and one for the same broken hardware with fixed firmware is really an improvement over what has been proposed here.
We're talking about two completely different types of quirks:
MCFG quirks to use memory-mapped config space that doesn't quite conform to the ECAM model in the PCIe spec, and
Some yet-to-be-determined method to describe address space consumed by a bridge.
The first two patches of this series are a nice implementation for 1). The third patch (ThunderX-specific) is one possibility for 2), but I don't like it because there's no way for generic software like the ACPI core to discover these resources.
Ok, so basically this means that to implement (2) we need to assign some sort of _HID to these quirky PCI bridges (so that we know what device they represent and we can retrieve their _CRS). I take from this discussion that the goal is to make sure that all non-config resources have to be declared through _CRS device objects, which is fine but that requires a FW update (unless we can fabricate ACPI devices and corresponding _CRS in the kernel whenever we match a given MCFG table signature).
We discussed this already and I think we should make a decision:
http://lists.infradead.org/pipermail/linux-arm-kernel/2016-March/414722.html
I'd like to step back and come up with some understanding of how non-broken firmware *should* deal with this issue. Then, if we *do* work around this particular broken firmware in the kernel, it would be nice to do it in a way that fits in with that understanding.
For example, if a companion ACPI device is the preferred solution, an ACPI quirk could fabricate a device with the required resources. That would address the problem closer to the source and make it more likely that the rest of the system will work correctly: /proc/iomem could make sense, things that look at _CRS generically would work (e.g, /sys/, an admittedly hypothetical "lsacpi", etc.)
Hard-coding stuff in drivers is a point solution that doesn't provide any guidance for future platforms and makes it likely that the hack will get copied into even more drivers.
OK, I see. But the guidance for future platforms should be 'do not rely on quirks', and what I am arguing here is that the more we polish up this code and make it clean and reusable, the more likely it is that will end up getting abused by new broken hardware that we set out to reject entirely in the first place.
So of course, if the quirk involves claiming resources, let's make sure that this occurs in the cleanest and most compliant way possible. But any factoring/reuse concerns other than for the current crop of broken hardware should be avoided imo.
If future hardware is completely ECAM-compliant and we don't need any more MCFG quirks, that would be great.
Yes.
But we'll still need to describe that memory-mapped config space somewhere. If that's done with PNP0C02 or similar devices (as is done on my x86 laptop), we'd be all set.
I am not sure I understand what you mean here. Are you referring to MCFG regions reported as PNP0c02 resources through its _CRS ?
IIUC PNP0C02 is a reservation mechanism, but it does not help us associate its _CRS to a specific PCI host bridge instance, right ?
If we need to work around firmware in the field that doesn't do that, one possibility is a PNP quirk along the lines of quirk_amd_mmconfig_area().
You mean matching PNP0C01/PNP0c02 and create a resource (that has to hardcoded in a static array in the kernel anyway, there is no way to retrieve it otherwise) in the corresponding PNP quirk handler ?
And it is not a given we can match against PNP0c01/PNP0c02.
So it looks like the only solution is allocating an _HID for each host bridge that is not ECAM compliant to add resources to its _CRS (unless the MCFG quirk does not need any additional data/resource, eg "use different set of PCI accessorsi 32-bit vs byte-access").
For FW that is immutable I really do not see what we can do apart from hardcoding the non-config resources (consumed by a bridge), somehow.
Thanks, Lorenzo