On 08/07/18 16:07, Christoph Hellwig wrote:
On Fri, Jul 06, 2018 at 03:20:34PM +0100, Robin Murphy wrote:What are you trying to do? I really don't want to see more users of the hooks as they are are a horribly bad idea.I really need to fix the ongoing problem we have where, due to funky integrations, devices suffer some downstream addressing limit (described by DT dma-ranges or ACPI IORT/_DMA) which we carefully set up in dma_configure(), but then just gets lost when the driver probes and innocently calls dma_set_mask() with something wider. I think it's effectively the generalised case of the VIA 32-bit quirk, if I understand that one correctly.I'd much rather fix this in generic code. How funky are your limitations? In fact when I did the 32-bit quirk (which will also be used by a Xiling PCIe root port usable on a lot of architectures) I did initially consider adding a bus_dma_mask or similar to struct device, but opted for the most simple implementation for now. I'd be happy to chanfe this. Especially these days where busses and IP blocks are generally not tied to a specific cpu instruction set I really believe that having any more architecture code than absolutely required is a bad idea.
Oh, for sure, the generic fix would be the longer-term goal, this was just an expedient compromise because I want to get *something* landed for 4.19. Since in practice this is predominantly affecting arm64, doing the arch-specific fix to appease affected customers then working to generalise it afterwards seemed to carry the lowest risk.
That said, I think I can see a relatively safe and clean alternative approach based on converting dma_32bit_limit to a mask, so I'll spin some patches around that idea ASAP to continue the discussion.
The approach that seemed to me to be safest is largely based on the one proposed in a thread from ages ago[1]; namely to make dma_configure() better at distinguishing firmware-specified masks from bus defaults, capture the firmware mask in dev->archdata during arch_setup_dma_ops(), then use the custom set_mask routines to ensure any subsequent updates never exceed that. It doesn't seem possible to make this work robustly without storing *some* additional per-device data, and for that archdata is a lesser evil than struct device itself. Plus even though it's not actually an arch-specific issue it feels like there's such a risk of breaking other platforms that I'm reticent to even try handling it entirely in generic code.My plan for a few merge windows from now is that dma_mask and coherent_mask are 100% in device control and dma_set_mask will never fail. It will be up to the dma ops to make sure things are addressible.
It's entirely possible to plug an old PCI soundcard via a bridge adapter into a modern board where the card's 24-bit DMA mask reaches nothing but the SoC's boot flash, and no IOMMU is available (e.g. some of the smaller NXP Layercape stuff); I still think there should be an error in such rare cases when DMA is utterly impossible, but otherwise I agree it would be much nicer for drivers to just provide their preferred mask and let the ops massage it as necessary.
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