Bus Master size on X5-RX

[mrodby]

Since there is no forum for the X5-RX yet, I wasn't sure where to post this question:

What are the maximum and minimum sizes that can be chosen for the Bus Master buffer for the X5-RX? The Snap sample application has a comment saying that the size must be a multiple of 4 MB, but it does not give limits.

[jhenderson]

The size of the busmaster region is limited only by the operating system. OS calls are used to allocate page-locked, contiguous memory regions for use with bus mastering. Under 32-bit Windows, allocation of <= 32 MB regions is routinely available, whereas 32-bit Linux typically limits allocations to <= 4 MB.

This limitation is generally inconsequential except in applications attempting full-rate snapshot captures or waveform playback to/from system memory. However, even our X5 XMC cards can sustain only about 1.2 GB/s to/from system memory. Since memory subsystems on modern PCs typically support copy operations at ~5 GB/s, packets can flow between module and system memory in packets <= 1/4 of the BM size without compromising the sustained transfer rate of the module.

The new X6 cards support ~2.4 GB/s sustained rates. So in that case, the time spent copying data from the BM pool into application buffers could limit rates.

[mrodby]

Is there a way to increase the limit under 32-bit Linux, such as a kernel compile time switch?
Does 64-bit Linux have the same 4 MB limit?

[jhenderson]

I do not know of a way to increase the OS BM size limit. I believe the limit is unchanged under 64-bit Linux.