On Mon, Mar 16, 2020 at 01:16:16PM +0000, Robin Murphy wrote: > On 2020-03-16 12:46 pm, Christoph Hellwig wrote: > > On Mon, Mar 16, 2020 at 12:12:08PM +0000, Robin Murphy wrote: > > > On 2020-03-14 12:00 am, Nicolin Chen wrote: > > > > More and more drivers set dma_masks above DMA_BIT_MAKS(32) while > > > > only a handful of drivers call dma_set_seg_boundary(). This means > > > > that most drivers have a 4GB segmention boundary because DMA API > > > > returns DMA_BIT_MAKS(32) as a default value, though they might be > > > > able to handle things above 32-bit. > > > > > > Don't assume the boundary mask and the DMA mask are related. There do > > > exist > > > devices which can DMA to a 64-bit address space in general, but due to > > > descriptor formats/hardware design/whatever still require any single > > > transfer not to cross some smaller boundary. XHCI is 64-bit yet requires > > > most things not to cross a 64KB boundary. EHCI's 64-bit mode is an example > > > of the 4GB boundary (not the best example, admittedly, but it undeniably > > > exists). > > > > Yes, which is what the boundary is for. But why would we default to > > something restrictive by default even if the driver didn't ask for it? > > I've always assumed it was for the same reason as the 64KB segment length, > i.e. it was sufficiently common as an actual restriction, but still "good > enough" for everyone else. I remember digging up all the history to > understand what these were about back when I implemented the map_sg stuff, > and from that I'd imagine the actual values are somewhat biased towards SCSI > HBAs, since they originated in the block and SCSI layers.
Yea, I did the same: commit d22a6966b8029913fac37d078ab2403898d94c63 Author: FUJITA Tomonori <[email protected]> Date: Mon Feb 4 22:28:13 2008 -0800 iommu sg merging: add accessors for segment_boundary_mask in device_dma_parameters() This adds new accessors for segment_boundary_mask in device_dma_parameters structure in the same way I did for max_segment_size. So we can easily change where to place struct device_dma_parameters in the future. dma_get_segment boundary returns 0xffffffff if dma_parms in struct device isn't set up properly. 0xffffffff is the default value used in the block layer and the scsi mid layer. Signed-off-by: FUJITA Tomonori <[email protected]> Cc: James Bottomley <[email protected]> Cc: Jens Axboe <[email protected]> Cc: Greg KH <[email protected]> Cc: Jeff Garzik <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]> _______________________________________________ iommu mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/iommu
