On Thu, Nov 22, 2018 at 08:50:47AM -0800, Linus Torvalds wrote: > On Thu, Nov 22, 2018 at 6:03 AM Christoph Hellwig <hch@xxxxxx> wrote: > > > > The 0 return doesn't work for direct mappings that have ram at address > > zero and a lot of IOMMUs that start allocating bus space from address > > zero, so we can't consolidate on that, but I think we can move everyone > > to all-Fs, which the patch here does. > > Hmm. Maybe not limit it to just all ones, but actually use the > (standard, for the kernel) IS_ERR_VALUE()? > > That basically reserves the last 4095 values in an unsigned long for > error values. > > Then those functions could actually return *what* error they > encountered, using just plain > > return -ENOMEM; > > or whatever? Linus, I'm afraid that won't work very well - 32 bit platforms with 64-bit addresses (LPAE) would have dma_addr_t as a 64-bit value, which wouldn't fit into an unsigned long. IS_ERR_VALUE() would cast a 64-bit DMA address down to a 32-bit pointer (effectively masking with 0xffffffff). It would have the effect of making (eg) 0xXXXXXXXX_fffffVVV an error, where XXXXXXXX are any of the top 32-bits of a 64-bit bus address, and VVV is the error code value. That could be a problem if you hit it in several places throughout your available RAM... we'd have to mark every top page of RAM in a naturally aligned 4GB as unusable, as well as block the top page in natually aligned 4GB blocks from IOMMUs... and then what about buses that have weird offsets... So, I don't think the IS_ERR_VALUE() would work very well. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
