On Wed, Nov 28, 2018 at 12:48 PM Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> wrote: > > On Wed, Nov 28, 2018 at 11:27:17AM -0800, David Miller wrote: > > From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > Date: Wed, 28 Nov 2018 10:00:06 -0800 > > > > > Not all memory is accessible even to the kernel. If you have memory > > > that shows up in the last page of phys_addr_t, you just mark it > > > reserved at boot-time. > > > > It's not the physical memory at the end that needs to be reserved. > > > > It's the IOMMU mapping arena. > > True, if and only if you have an IOMMU. > > Where there isn't an IOMMU, then we'd have to reserve every page that > that translates to a bus address in the top 4K of dma_addr_t on any > bus in the system - that means knowing early in the kernel > initialisation about all buses in the system so we can detect and > reserve these pages. > The arch and platform differences/confusion as to "what is DMA error value" is the reason why dma_mapping_error is part dma ops. I understand that iy would make sense to reduce the overhead of an additional call, however, I am not sure if would be trivial change to address this. I was down this path of trying to address the missing mapping error checks a few years ago and introduced dma_mapping_error checks in the DMA DEBUG API. As you might already know that there is no common definition for the mapping error. Quick look at the defines shows: #define CALGARY_MAPPING_ERROR 0 #define S390_MAPPING_ERROR (~(dma_addr_t) 0x0) #define SPARC_MAPPING_ERROR (~(dma_addr_t)0x0) This is the reason why there is a arch or in some cases bus specific mapping_error ops is needed. We could unify this and fix all these first. I haven't looked at the patch set closely, maybe you are already doing this. thanks, -- Shuah
