On Sun, Oct 16, 2016 at 4:52 AM, Michał Purzyński <michalpurzyns...@gmail.com> wrote: > Could you fill me in on how the bounce buffer approach to memory management > in IXGBE and I40E works? > > Why do you allocate the same amount of memory in ixgbe_setup_rx_resources() > twice? > First time with a call to rx_ring->rx_buffer_info = vzalloc_node() - where > size represents what I set with ethtool -G (N buffers, each represented > with a struct ixgbe_rx_buffer). > Second in the rx_ring->desc = dma_alloc_coherent() of the same size as > above.
There are two different buffers being allocated here. The first is the rx_buffer_info which store data used by the driver to track memory allocations, DMA addresses, and page offsets. The allocation into desc is for descriptor rings used to signal between the device and the driver where the buffers are at, and information about the packets written back. > Second question. So we have a single ring (per queue) with a chains of > buffers "attached" to it - a chunk of memory composed from multiple pages. > Each page will have a single packet. > Does the driver replace "used" pages every time it uses one? I'm confused, > because that's kind of logical it does, there are functions doing that, yet > there are comments in the driver that "we are reusing buffers". > > My understanding is that once a page (2048 bytes here) contains a packet > data, it is only "re-attached" to SKB with minimal copying (header, > basically). That would require allocating a new page every time we used > one - attached it to SKB. A page is 4096 bytes in size. One trick we are using is to only use 2048 bytes of it at a time, then we move over to the other half by XORing the page offset value by 2048. The pages themselves have a reference count and as long as the reference count is 1 we can increase the reference count by one and always use the other half of the page. The stack itself should release the page reference reducing the reference count back to 1 hopefully before we will need to use that portion of the buffer again. If the reference count is greater than 1 we will free hand the page off to the stack and instead allocate a new page to replace it via the <driver>_alloc_mapped_page call. Hope that helps to clarify things a bit. - Alex ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot _______________________________________________ E1000-devel mailing list E1000-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/e1000-devel To learn more about Intel® Ethernet, visit http://communities.intel.com/community/wired
