On Tue, Oct 13, 2020 at 03:42:07PM +0200, Ard Biesheuvel wrote: > On Tue, 13 Oct 2020 at 15:13, Lorenzo Pieralisi > <lorenzo.pieralisi@xxxxxxx> wrote: > > > > On Tue, Oct 13, 2020 at 01:22:32PM +0200, Ard Biesheuvel wrote: > > > > [...] > > > > > > > diff --git a/Documentation/arm64/arm-acpi.rst b/Documentation/arm64/arm-acpi.rst > > > > > index 47ecb9930dde..947f5b5c45ef 100644 > > > > > --- a/Documentation/arm64/arm-acpi.rst > > > > > +++ b/Documentation/arm64/arm-acpi.rst > > > > > @@ -205,6 +205,13 @@ devices available. This list of tables is not meant to be all inclusive; > > > > > in some environments other tables may be needed (e.g., any of the APEI > > > > > tables from section 18) to support specific functionality. > > > > > > > > > > +It is assumed that all DMA capable devices in the system are able to > > > > > +access the lowest 4 GB of system memory. If this is not the case, an > > > > > +IORT describing those limitations is mandatory, even if an IORT is not > > > > > +otherwise necessary to describe the I/O topology, and regardless of > > > > > +whether _DMA methods are used to describe the DMA limitations more > > > > > +precisely. Once the system has booted, _DMA methods will take precedence > > > > > +over DMA addressing limits described in the IORT. > > > > > > > > If this is a boot requirement it must be in ARM's official documentation, > > > > first, not the kernel one. > > > > > > > > I understand this is an urgent (well - no comments on why bootstrapping > > > > ACPI on Raspberry PI4 is causing all this fuss, honestly) fix but that's > > > > not a reason to rush through these guidelines. > > > > > > > > I would not add this paragraph to arm-acpi.rst, yet. > > > > > > > > > > Which documentation? ACPI compliance by itself is not sufficient for a > > > system to be able to boot Linux/arm64, which is why we documented the > > > requirements for ACPI boot on Linux/arm64 in this file. I don't think > > > we need endorsement from ARM to decide that odd platforms like this > > > need to abide by some additional rules if they want to boot in ACPI > > > mode. > > > > I think we do - if we don't we should not add this documentation either. > > > > ACPI on ARM64 software stack is based on standardized HW requirements. > > The sheer fact that we need to work around a HW deficiency shows that > > either this platform should have never been booted with ACPI or the _HW_ > > design guidelines (BSA) are not tight enough. > > > > Please note that as you may have understood I asked if we can implement > > a workaround in IORT because that's information that must be there > > regardless (and an OEM ID match in arch code - though pragmatic - > > defeats the whole purpose), I don't think we should tell Linux kernel > > developers how firmware must be written to work around blatantly > > non-compliant systems. > > > > This is not about systems being compliant or not, unless there is a > requirement somewhere that I missed that all masters in the system > must be able to access at least 32 bits of DMA. I think there is in the SBSA (4.1.3 Memory Map) but regardless, this is clearly a design bug, that's not a feature. > The problem here is that Linux/arm64 cannot deal with fully compliant > systems that communicate their [permitted] DMA limitations via a _DMA > method if this limitation happens to be that the address limit < 32 > bits. The DMA subsystem can deal with this fine, only the default DMA > zone sizing policy creates an internal issue where the DMA subsystem > is not able to allocate memory that matches the DMA constraints. > > So the 'correct' fix here would be to rework the memory allocator so > it can deal with arbitrary DMA limits at allocation time, so that any > limit returned by a _DMA method can be adhered to on the fly. > > However, we all agree that the Raspberry Pi4 is not worth that effort, > and that in the general case, SoCs with such limitations, even if they > are compliant per the spec, are not worth the trouble of complicating > this even more. So as a compromise, I think it is perfectly reasonable > to require that systems that have such limitations communicate them > via the IORT, which we can parse early, regardless of whether _DMA > methods exist as well, and whether they return the same information. > > So this is not a requirement on arm64 ACPI systems in general. It is a > requirement that expresses that we, as arm64 > contributors/[co-]maintainers, are willing to cater for such systems > if they implement their firmware in a particular way. I don't think they should implement their firmware in any particular way, that's my point, I don't want them to in the first place. To start with there is no spec I am aware of that defines when/how to use _DMA vs IORT address limits, maybe we should spell that out better somewhere and that's useful regardless. My point is: either this workaround works with firmware written with guidelines valid for all arm64 systems (not as a special case: add an IORT table because we can't parse _DMA to workaround DMA address range shenanigans) or I am not willing to merge it - I prefer to add an OEM ID quirk and show what we are forced to do to make this work. Thanks, Lorenzo
