On Wed, 2008-11-19 at 11:15 +0200, Tomas Winkler wrote: > >> You are off here. The constrain is just that address will fit in 32 > >> bit register. I don't know why this constrain, but there is always a > >> reason. > > > > There's also the PCI-E constraint of having to split up DMA at 4k > > boundaries, which is what I was talking about above. > > Have to check this but I'm not sure if this is anyhow related. It's not. I misunderstood how RX DMA works, but given that you actually have to allocate the buffers that large the HW has to support splitting. > Not just that, it was never confirmed this is the issue it was the > first place we started to look. > This code is there from time zero of the driver something like kernel > 2.6.18 and the problem popped only in kernel 2.6.27 so what else has > changed. Yeah, it's weird, and it doesn't even seem to solve it for everyone. So there must be another thing too. > Yes I have CONFIG_SWIOTLB=y in my config spec for X86_64, nevertheless > you've mentioned that the allocation problem is due to headroom > allocation in skb?! Well, the skb "head" is _all_ of the skb data that is not part of an extra buffer. So skb->data is the "head" while the skb fraglist is referred to as the data. That can be a bit confusing. Anyway skb->data is just allocated with kmalloc and on my machine that has 128 byte alignment. The IOMMU maps pages, so doesn't change the alignment. If you are using swiotlb (just having it in the config isn't enough, check the kernel messages) then much DMA will be done with bounce buffer. > > Incidentally, where does the 36 bit limit come from? The RX has a 40 bit > > limit if you see that it uses 32 bit pointer that's shifted by 8 bits. > > I'm checking with the HW guys Seems that isn't necessary, there are some things like the rbb stats that are only shifted down by 4 bits, so they need a 36 bit mask. Bit unfortunate that this single allocations forces all into the mask, but I don't right now see how to just get that single one into 36 and the rest into 40. johannes
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