Re: Allocating buffers for USB transfers (again)

[Johannes Stezenbach <js@sig21.net>]

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