Re: [PATCH V2 4/4] POWERPC: Merge 32 and 64-bit dma code

[Josh Boyer <jwboyer@linux.vnet.ibm.com>]

On Mon, Oct 13, 2008 at 10:49:04AM -0400, Josh Boyer wrote:
>On Fri, Sep 12, 2008 at 03:34:46PM -0500, Becky Bruce wrote:
>>We essentially adopt the 64-bit dma code, with some changes to support
>>32-bit systems, including HIGHMEM.  dma functions on 32-bit are now
>>invoked via accessor functions which call the correct op for a device based
>>on archdata dma_ops.  If there is no archdata dma_ops, this defaults
>>to dma_direct_ops.
>>
>>In addition, the dma_map/unmap_page functions are added to dma_ops
>>because we can't just fall back on map/unmap_single when HIGHMEM is
>>enabled. In the case of dma_direct_*, we stop using map/unmap_single
>>and just use the page version - this saves a lot of ugly
>>ifdeffing.  We leave map/unmap_single in the dma_ops definition,
>>though, because they are needed by the iommu code, which does not
>>implement map/unmap_page.  Ideally, going forward, we will completely
>>eliminate map/unmap_single and just have map/unmap_page, if it's
>>workable for 64-bit.
>>
>>Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
>
>While doing a buildall this morning, I notice chrp32_defconfig fails
>to build with:
>
>drivers/built-in.o: In function `hard_dma_setup':
>floppy.c:(.text+0x6e40e): undefined reference to `isa_bridge_pcidev'
>floppy.c:(.text+0x6e412): undefined reference to `isa_bridge_pcidev'
>floppy.c:(.text+0x6e53e): undefined reference to `isa_bridge_pcidev'
>floppy.c:(.text+0x6e546): undefined reference to `isa_bridge_pcidev'
>floppy.c:(.text+0x6e54a): undefined reference to `isa_bridge_pcidev'
>make[1]: *** [.tmp_vmlinux1] Error 1
>
>(the hard_dma_setup thing is in arch/powerpc/include/asm/floppy.h).
>
>I did a git bisect and it pointed at this commit as causing the build
>to fail.  Why, I have no idea.

Ok, I was annoyed enough to look at why.

Basically, before this patch pci_map_single on 32-bit PPC seemed to
be compiled down to __dma_sync(ptr, size, direction); and the "dev"
parameter to the function was never actually used.  The compiler
seems to have optimized this out entirely, so we don't get the odd
link reference to isa_bridge_pcidev at all.  (Neither pci_map_single
or isa_bridge_pcidev are present in the vmlinux at all).

With the patch, the compiler doesn't do this code elimination
because pci_map_single boils down to dma_map_page, which calls
get_dma_direct_offset with the "dev" parameter.  So since it is
still used, the compiler can't eliminate it and hence FAIL.

I have no patch for this at the moment.  Someone should look at
it more closely, because this is causing the 5 chrp32_defconfig
users to weep.

josh