Thanks so much Bjorn for the reply. This is very helpful. Now my understanding is more clear. I was curious whether there is any limit from the operating system(hypervisor) as to how much DMA memory can drivers map. I'll try to investigate. On Thu, Jun 12, 2014 at 5:42 AM, Bjorn Helgaas <bhelgaas@xxxxxxxxxx> wrote: > On Wed, Jun 11, 2014 at 12:23 PM, Bob Biloxi <iambobbiloxi@xxxxxxxxx> wrote: >> Hi All, >> >> I am having trouble understanding DMA window and actual amount of >> addressable DMA memory. >> >> I hope someone explains me. Let me put my understanding and doubts here: >> >> Let's say I am writing code for an ethernet device driver in the >> virtualisation(hypervisor) environment. >> >> Now, if the ethernet adapter requires certain amount of DMA memory, I >> need to allocate heap memory and dma map it and provide to the >> adapter. >> >> From the hardware perspective, we have a 64GB DMA window. >> >> I am having trouble understanding this value. Does it mean i can >> allocate 64GB of RAM(Heap memory) and dma map it? >> >> As i understand there might be a table that translates bus address to >> physical(RAM) addresses. Each entry of such table points to a 4KB >> page. If the size of each entry is 8 bytes and there are 16M such >> entries( 16M * 4K = 64GB DMA window), the size of the table comes to >> around 128M >> >> Now do I have 64GB DMA memory or 128M DMA memory? > > Documentation/DMA-API-HOWTO.txt might help answer your questions. > > The bus address to RAM address translation is done by a IOMMU > hardware. The tables you mention are I/O page tables used by the > IOMMU. The 128M occupied by the tables is kernel bookkeeping overhead > and has nothing to do with the adapter itself. > > The 64GB DMA window might be a hardware feature of the device, i.e., > maybe it can only generate 36-bit DMA addresses. That doesn't mean > you have to allocate memory for the whole window; I would guess > drivers would only allocate and map what they need. I don't know how > they figure out how much to map. > >> I want to know what is the max amount of memory that I can >> allocate(heap), dma map and provide it to the adapter. >> >> I will be really thankful in all the help that I can get!! >> >> Thanks so much >> >> >> Best Regards, >> Marc >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-pci" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
