Messages in this thread | | | Subject | Re: [PATCH] dma-mapping: Relax warnings for per-device areas | From | Robin Murphy <> | Date | Fri, 6 Jul 2018 12:57:11 +0100 |
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On 05/07/18 20:36, Christoph Hellwig wrote: >> - BUG_ON(!ops); >> - WARN_ON_ONCE(dev && !dev->coherent_dma_mask); >> - >> if (dma_alloc_from_dev_coherent(dev, size, dma_handle, &cpu_addr)) >> return cpu_addr; >> >> + BUG_ON(!ops); >> + WARN_ON_ONCE(dev && !dev->coherent_dma_mask); > > I think doing dma on a device without ops is completely broken no matter > what you think of it, so I very much disagree with that part of the change. > > Also while I don't think not having a dma mask is a good idea even for > a driver purely using dma coherent pools. If the pools really are on > the device itself I can see why it might not matter, but for the case > commonly used on some ARM SOCs where we just reserve memory for certain > devices from a system pool it very much does matter. > > There really is no good excuse to not set a coherent mask in the drivers.
Right, I was rather on the fence about this - on the one hand it is objectively wrong per the API for drivers to call dma_alloc_coherent() without a prior successful dma_set_coherent_mask() call, but then I thought that in the case when they're *only* using it as a proxy for dma_alloc_from_dev_coherent() and explicitly don't want regular allocations from kernel memory to ever happen, then maybe it might be somewhat reasonable. But indeed I hadn't really given enough thought to the reserved-memory carveout case, where we definitely don't want to let a legitimate warning be hidden on a developer's machine but hit by users with different system configurations.
Fredrik, are you happy to fix up your driver to initialise a suitable mask at probe time?
Robin.
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