On Tue, May 21, 2019 at 02:47:28PM +0200, Christoph Hellwig wrote: > The dma masks in struct device are always 64-bits wide. But for builds > using a 32-bit dma_addr_t we need to ensure we don't store an > unsupportable value. Before Linux 5.0 this was handled at least by > the ARM dma mapping code by never allowing to set a larger dma_mask, > but these days we allow the driver to just set the largest supported > value and never fall back to a smaller one. Ensure this always works > by truncating the value.
So how does the driver negotiation for >32bit addresses work if we don't fail for large masks? I'm thinking about all those PCI drivers that need DAC cycles for >32bit addresses, such as e1000, which negotiate via (eg): /* there is a workaround being applied below that limits * 64-bit DMA addresses to 64-bit hardware. There are some * 32-bit adapters that Tx hang when given 64-bit DMA addresses */ pci_using_dac = 0; if ((hw->bus_type == e1000_bus_type_pcix) && !dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64))) { pci_using_dac = 1; } else { err = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32)); if (err) { pr_err("No usable DMA config, aborting\n"); goto err_dma; } } and similar. If we blindly trunate the 64-bit to 32-bit, aren't we going to end up with PCI cards using DAC cycles to a host bridge that do not support DAC cycles? > > Fixes: 9eb9e96e97b3 ("Documentation/DMA-API-HOWTO: update dma_mask sections") > Signed-off-by: Christoph Hellwig <h...@lst.de> > --- > kernel/dma/mapping.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c > index f7afdadb6770..1f628e7ac709 100644 > --- a/kernel/dma/mapping.c > +++ b/kernel/dma/mapping.c > @@ -317,6 +317,12 @@ void arch_dma_set_mask(struct device *dev, u64 mask); > > int dma_set_mask(struct device *dev, u64 mask) > { > + /* > + * Truncate the mask to the actually supported dma_addr_t width to > + * avoid generating unsupportable addresses. > + */ > + mask = (dma_addr_t)mask; > + > if (!dev->dma_mask || !dma_supported(dev, mask)) > return -EIO; > > @@ -330,6 +336,12 @@ EXPORT_SYMBOL(dma_set_mask); > #ifndef CONFIG_ARCH_HAS_DMA_SET_COHERENT_MASK > int dma_set_coherent_mask(struct device *dev, u64 mask) > { > + /* > + * Truncate the mask to the actually supported dma_addr_t width to > + * avoid generating unsupportable addresses. > + */ > + mask = (dma_addr_t)mask; > + > if (!dma_supported(dev, mask)) > return -EIO; > > -- > 2.20.1 > > -- RMK's Patch system: https://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