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    SubjectRe: [Xen-devel] [PATCH 1/1] XEN: Use correct masking in xen_swiotlb_alloc_coherent.
    On Tue, Sep 04, 2012 at 05:44:46PM +0100, David Vrabel wrote:
    > On 04/09/12 16:12, Stefano Panella wrote:
    > > On 09/04/2012 03:55 PM, David Vrabel wrote:
    > >> On 04/09/12 15:37, Konrad Rzeszutek Wilk wrote:
    > >>> On Tue, Sep 04, 2012 at 03:07:42PM +0100, Stefano Panella wrote:
    > >>>> So if hwdev->coherent_dma_mask is set to 0xffffffffffffffff our
    > >>>> dma_mask will
    > >>>> be u64 set to 0xffffffffffffffff even if we set it to
    > >>>> DMA_BIT_MASK(32) previously.
    > >>> That is what I was missing. Let me include that in the git commit and
    > >>> also
    > >>> put this patch on the stable tree.
    > >> Note that this appears to be a work around for a bug in the sound system
    > >> or Intel HDA device driver which is incorrectly truncating a dma_addr_t
    > >> to a u32. So by ensuring a DMA_BIT_MASK(32) when the dma_addr_t is
    > >> truncated it still works.
    > >>
    > >> David
    > > Sorry David, I am not completely sure I understand your argument in
    > > favour of a bug in the
    > > sound system or Intel HDA device driver. Could you please elaborate more
    > > in detail about this?
    >
    > The HDA driver claims to support 64-bit DMA addresses, so it should be
    > perfectly fine for the Xen DMA mapping/allocation functions to return
    > DMA addresses > 4 GiB. For most devices this seems to work just fine.
    >
    > I think that somewhere between the buffer being mapped or allocated and
    > the address being programmed into the hardware, the 64-bit dma_addr_t
    > value is being truncated to 32 bits giving an invalid DMA address.
    >
    > The patch would avoid this by setting the DMA mask to 32-bit and only
    > returning DMA address < 4 GiB which can quite happily be stuffed into a
    > u32 without losing bits.
    >
    > I think it would be useful (without the patch applied) to print the DMA
    > addresses returned by the allocate/mapping functions and the address
    > programmed into the hardware. It will be easily to spot if it got
    > truncated or not.

    Just enable DMA debug API (CONFIG_DEBUG_DMA_API) and use this fancy module:


    /*
    *
    * This program is free software; you can redistribute it and/or modify
    * it under the terms of the GNU General Public License v2.0 as published by
    * the Free Software Foundation
    *
    * This program is distributed in the hope that it will be useful,
    * but WITHOUT ANY WARRANTY; without even the implied warranty of
    * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    * GNU General Public License for more details.
    */

    #include <linux/module.h>
    #include <linux/string.h>
    #include <linux/types.h>
    #include <linux/init.h>
    #include <linux/stat.h>
    #include <linux/err.h>
    #include <linux/ctype.h>
    #include <linux/slab.h>
    #include <linux/limits.h>
    #include <linux/device.h>
    #include <linux/pci.h>
    #include <linux/blkdev.h>
    #include <linux/device.h>

    #include <linux/init.h>
    #include <linux/mm.h>
    #include <linux/fcntl.h>
    #include <linux/slab.h>
    #include <linux/kmod.h>
    #include <linux/major.h>
    #include <linux/highmem.h>
    #include <linux/blkdev.h>
    #include <linux/module.h>
    #include <linux/blkpg.h>
    #include <linux/buffer_head.h>
    #include <linux/mpage.h>
    #include <linux/mount.h>
    #include <linux/uio.h>
    #include <linux/namei.h>
    #include <asm/uaccess.h>

    #include <linux/pagemap.h>
    #include <linux/pagevec.h>

    #include <linux/dma-debug.h>

    #define DUMP_DMA_FUN "0.1"

    MODULE_AUTHOR("Konrad Rzeszutek Wilk <konrad@virtualiron>");
    MODULE_DESCRIPTION("dump dma");
    MODULE_LICENSE("GPL");
    MODULE_VERSION(DUMP_DMA_FUN);

    static int __init dump_dma_init(void)
    {
    debug_dma_dump_mappings(NULL);
    return 0;
    }

    static void __exit dump_dma_exit(void)
    {
    }

    module_init(dump_dma_init);
    module_exit(dump_dma_exit);
    >
    > David


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