Writing a Windows 64-bit device driver for a 32-bit PCI device

Question

I'm evaluating to port a device driver I wrote several years ago from 32 to 64 bits. The physical device is a 32-bit PCI card. That is, the device is 32 bits but I need to access it from Win7x64. The device presents some registers to the Windows world and then performs heavy bus master data transferring into a chunk of driver-allocated memory.

I've read in the Microsoft documentation that you can signal whether the driver supports 64-bit DMA or not. If it doesn't, then the DMA is double buffered. However, I'm not sure if this is the case. My driver would/could be a full 64-bit one, so it could support 64-bit addresses in the processor address space, but the actual physical device WON'T support it. In fact, the device BARs must be mapped under 4 GB and the device must get a PC RAM address to perform bus master below 4 GB. Does this mean that my driver will go through double buffering always? This is a very performance-sensitive process and the double buffering could prevent the whole system from working.

Of course, designing a new 64-bit PCI (or PCI-E) board is out of question.

Anybody could give me some resources for this process (apart from MS pages)?

Thanks a lot!

Answer 1

This is an old post, I hope the answer is still relevant...

There are two parts here, PCI target and PCI master access.

PCI target access: The driver maps PCI BARs to 64bit virtual address space and the driver just reads/writes through a pointer.

PCI master access: You need to create a DmaAdapter object by calling IoGetDmaAdapter(). When creating, you also describe your device is a 32bit (see DEVICE_DESCRIPTION parameter). Then you call DmaAdapter::AllocateCommonBuffer() method to allocate a contiguous DMA buffer in PC RAM.

I am not sure about double-buffering though. From my experience, double-buffering is not used, instead, DmaAdapter::AllocateCommonBuffer() simply fails if cannot allocate a buffer that satisfies the DEVICE_DESCRIPTION (in your case - 32bit dma addressing).

Answer 2

There's no problem writing a 64-bit driver for a device only capable of 32-bit PCI addressing. As Alexey pointed out, the DMA adapter object you create specifies the HW addressing capabilities of your device. As you allocate DMA buffers, the OS takes this into account and will make sure to allocate these within your HW's accessible region. Linux drivers behave similar, where your driver must supply a DMA address mask to associate with your device that DMA functions used later will refer to.

The performance hit you could run into is if your application allocates a buffer that you need to DMA to/from. This buffer could be scattered all throughout memory, with pages in memory above 4G. If your driver plans to DMA to these, it will need to lock the buffer pages in RAM during the DMA and build an SGL for your DMA engine based on the page locations. The problem is, for those pages above 4G, the OS would then have to copy/move them to pages under 4G so that your DMA engine is able to access them. That is where the potential performance hit is.