On Fri, 9 Oct 2020 at 10:36, Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx> wrote: > > On Fri, 2020-10-09 at 09:37 +0200, Ard Biesheuvel wrote: > > On Fri, 9 Oct 2020 at 09:11, Christoph Hellwig <hch@xxxxxx> wrote: > > > On Thu, Oct 08, 2020 at 12:05:25PM +0200, Nicolas Saenz Julienne wrote: > > > > Sadly I just realised that the series is incomplete, we have RPi4 users that > > > > want to boot unsing ACPI, and this series would break things for them. I'll > > > > have a word with them to see what we can do for their use-case. > > > > > > Stupid question: why do these users insist on a totally unsuitable > > > interface? And why would we as Linux developers care to support such > > > a aims? > > > > The point is really whether we want to revert changes in Linux that > > made both DT and ACPI boot work without quirks on RPi4. > > Well, and broke a big amount of devices that were otherwise fine. > Yeah that was unfortunate. > > Having to check the RPi4 compatible string or OEM id in core init code is > > awful, regardless of whether you boot via ACPI or via DT. > > > > The problem with this hardware is that it uses a DMA mask which is > > narrower than 32, and the arm64 kernel is simply not set up to deal > > with that at all. On DT, we have DMA ranges properties and the likes > > to describe such limitations, on ACPI we have _DMA methods as well as > > DMA range attributes in the IORT, both of which are now handled > > correctly. So all the information is there, we just have to figure out > > how to consume it early on. > > Is it worth the effort just for a single board? I don't know about ACPI but > parsing dma-ranges that early at boot time is not trivial. My intuition tells > me that it'd be even harder for ACPI, being a more complex data structure. > Yes, it will be harder, especially for the _DMA methods. > > Interestingly, this limitation always existed in the SoC, but it > > wasn't until they started shipping it with more than 1 GB of DRAM that > > it became a problem. This means issues like this could resurface in > > the future with existing SoCs when they get shipped with more memory, > > and so I would prefer fixing this in a generic way. > > Actually what I proposed here is pretty generic. Specially from arm64's > perspective. We call early_init_dt_scan(), which sets up zone_dma_bits based on > whatever it finds in DT. Both those operations are architecture independent. > arm64 arch code doesn't care about the logic involved in ascertaining > zone_dma_bits. I get that the last step isn't generic. But it's all setup so as > to make it as such whenever it's worth the effort. > The problem is that, while we are providing a full description of the SoC's capabilities, we short circuit this by inserting knowledge into the code (that is shared between all DT architectures) that "brcm,bcm2711" is special, and needs a DMA zone override. I think for ACPI boot, we might be able to work around this by cold plugging the memory above 1 GB, but I have to double check whether it won't get pulled into ZONE_DMA32 anyway (unless anyone can answer that for me here from the top of their head)
