extra large DMA buffer for PCI-E device under UIO

Jean-Francois Dagenais
2011-11-18T21:12:26Z

Hello fellow hackers.

I am maintaining a UIO based driver for a PCI-E data acquisition device.

I map BAR0 of the device to userspace. I also map two memory areas, one is used to feed instructions to the acquisition device, the other is used autonomously by the PCI device to write the acquired data.

The strategy we have been using for those two share memory areas has historically been using pci_alloc_coherent on v2.6.35 x86_64 (limited to 4MB based on my trials) and later, I made use of the VT-d (intel_iommu) to allocate as much as 128MB (an arbitrary limit) which appear contiguous to the PCI device. I use vmalloc_user to allocate 128M, then write all the physically continuous segments in a scatterlist, then use pci_map_sg which works it's way to intel_iommu. The device DMA addresses I get back are contiguous over the whole 128M. Neat! Our VT-d capable devices still use this strategy.

This large memory is mission-critical in making the acquisition device autonomous (real-time), yet keep the DMA implementation very simple. Today, we are re-using this device on a CPU architecture that has no IOMMU (intel E6XX/EG20T) and want to avoid creating a scatter-gather scheme between my driver and the FPGA (PCI device).

So I went back to the old pci_alloc_coherent method, which although limited to 4 MB, will do for early development phases. Instead of 2.6.35, we are doing preliminary development using 2.6.37 and will probably use 3.1 or more later. The cpu/device shared memory maps (1MB and 4MB) are allocated using pci_alloc_coherent and handed to UIO as physical memory using the dma_addr_t returned by the pci_alloc func.

The 1st memory map is written to by CPU and read from device.
The 2nd memory map is typically written by the device and read by the CPU, but future features may have the device also read this memory.

My initial testing on the atom E6XX show the PCI device failing when trying to read from the first memory map. I suspect PCI-E payload sizes which may be somewhat hardcoded in the FPGA firmware... we will confirm this soon.

Now from the get go I have felt lucky to have made this work because of my limited research into the intricacies of the kernel's memory management. So I ask two things:

- Is this kosher?
- Is there a better/easier/safer way to achieve this? (remember that for the second map, the more memory I have, the better. We have a gig of ram, if I take, say 256MB, that would be OK too.

I had thought about cutting out a chunk of ram from the kernel's boot args, but had always feared cache/snooping errors. Not to mention I had no idea how to "claim" or setup this memory once my driver's probe function. Maybe I would still be lucky and it would just work? mmmh...

Thanks for the help!!
/jfd

Cheers Linus!