On Thu, Jul 07, 2011 at 01:53:46PM +0200, Daniel Mack wrote: > So my approach was to force the driver using memory that is DMA > coherent, which leads to 32bit addressable memory if the device's > DMA_MASK is set accordingly. Other drivers do the same, and the patch > for doing this is found here: http://lkml.org/lkml/2010/5/7/61. Users > that see the bug all report that this patch fixes their problem with > my driver. However, it was rejected back then as other developers said > it was fixing the wrong end, and that the driver doesn't necessarily > need coherent memory. I think this is a valid approach, and since it avoids using bounce buffers it is potentially better wrt performance. BTW, usb_alloc_coherent() doc comment says you need to set the URB_NO_TRANSFER_DMA_MAP flag, which your patch fails to do. > Takashi recently posted a patch to the bugzilla entry which uses a > different approach: it introduces a function to determine suitable GFP > flags for USB devices, and passes __DMA32 to kmalloc() eventually. > However, using this flags directly with the SLUB allocator is illegal > and causes a BUG() in mm/slub.c, cache_grow(). from gfp.h: /* Do not use these with a slab allocator */ #define GFP_SLAB_BUG_MASK (__GFP_DMA32|__GFP_HIGHMEM|~__GFP_BITS_MASK) I think it means you can only use __GFP_DMA32 for get_free_pages(), not for kmalloc(). > 2. Find out exactly why these machines fail to install bounce buffers > or set up their IOMMU correctly I think this needs to be done. Johannes -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
