Messages in this thread | | | From | John Keller <> | Subject | Re: [PATCH] ia64: SN specific version of dma_get_required_mask() | Date | Tue, 25 Nov 2008 14:50:01 -0600 (CST) |
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> > Hi, > > [Sorry for the late reply and for not following the whole thread, I'm > just busy.] > > * John Keller [2008-11-18 08:08]: > > > > This patch addresses a problem on SN Altix systems with < 4GB, where > > device drivers using the dma_get_required_mask() API would be told > > to use 32 bit DMA, when 64 bit is more efficient. > > > > How exactly the use of dma_get_required_mask() relates to the crash > > kernel code you refer to is unclear to me. > > I'm not sure myself. The crashkernel reservation code on IA64 (for > other architectures I don't know any machines that have basically their > whole memory except a small amount which is used for booting mapped > above 4 GiB physical address space) needs to check if it's okay to > use memory for the crashkernel that is *all* above 4 GiB. > > This is only possible if a hardware IO/MMU is present (and working > correctly in the kdump case which isn't the case on HP IA64) and SWIOTBL > is not used because SWIOTBL needs some memory below that 4 GiB margin. > > Now I thought that there's a relationship between "memory above 4 GiB > can be used for DMA" and the return value of dma_get_required_mask(). > My assumption was: > > (dma_get_required_mask() & 0xffffffff00000000ull) > 0 > -> memory above 4 GiB can be used for DMA and so the > crashkernel memory can reside above 4 GiB > > (dma_get_required_mask() & 0xffffffff00000000ull) == 0 > -> memory above 4 GiB can not be used for DMA and so the > crashkernel memory can not all reside above 4 GiB > > Is that wrong?
In the case of SN Altix, the return value (with my patch) will always be 0xffffffffffffffffull. So, your check would work. And just as the current code's ia64_platform_is("sn2") check would always return TRUE on any SN2 system, so would your proposed use of dma_get_required_mask().
However, for SN2, memory size or location cannot be inferred from the return value, as it has no affect on the returned value. As Documentation/DMA-API.txt says: "This API returns the mask that the platform requires to operate efficiently."
And for SN2, this is always a 64 bit mask, irregardless of memory size, location, etc.
> > > If, for all platforms, the crash kernel code could use the mask returned > > from dma_get_required_mask() to do its check, then switching the code > > might be OK. But, if that's not possible for some platforms, then I'd > > wonder if dma_get_required_mask() is being used in the wrong context in > > this case. > > The crashkernel reservation code is different for every platform, so it > does not matter. However, in theory I think the check would return > correct results.
OK, I must not be understanding something. check_crashkernel_memory() appears to be coded to handle 3 or more platforms (sn2, uv, and others).
> > > Regards, > Bernhard > -- > Bernhard Walle, SUSE LINUX Products GmbH, Architecture Development >
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