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. > 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. > 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. > Also, I assume papering over the issue like this does not fix the > kdump issue fundamentally, it just works around it, and so we might > run into this again in the future. Any ideas? The way I understand it the kdump issue is just a shortcoming of the memory zones design. Regards, Nicolas
Attachment:
signature.asc
Description: This is a digitally signed message part
