Obsolete direct memory access (DMA) parameters

PHYSTOKVMEM

Formerly, the PHYSTOKVMEM tunable enabled mapping mainstore memory (up to 256MB) for driver compatibility, for old drivers that used phystokv on data buffers. With this tunable off, the first 1MB of memory (including device memory) was still mapped, to allow for drivers that used phystokv for device memory or BIOS data.

Typically, you did not need to change this tunable, since it was automatically set for HBA (Host Bus Adapter) drivers that may need it set. You needed to set it manually if you configured a non-HBA driver that used phystokv.

When PHYSTOKVMEM was set to ``1'', it allowed drivers using the phystokv macro to access up to 256MB of main memory. It forced HBA drivers to be loaded into memory below the threshold given by MAXDMAPAGE. It disabled the use of memory above the threshold given by MAXDMAPAGE for kernel data and bss. Typically, the kernel tried to locate itself above MAXDMAPAGE in an attempt to preserve the lower memory for more important uses.

LGDMA_RATIO

Formerly, if MAXDMAPAGE was non-zero, if memory was present at physical addresses above the threshold set by MAXDMAPAGE, and if the boot disk device had a DMA access restriction, then LGDMA_RATIO determined whether page preferences could be used in allocating memory for the filesystems. LGDMA_RATIO is in units of percent. If the ratio of the amount of memory above MAXDMAPAGE to amount of memory below MAXDMAPAGE did not exceed LGDMA_RATIO, then page preferences took effect.

When page preferences were in effect, the Virtual Memory subsystem gave disk-based filesystems (vxfs, ufs, sfs, and s5) preferential access to memory below MAXDMAPAGE. This was done in an attempt to avoid data copy up and down through DMA-accessible buffers. However, when total memory gets large, page preferences generally become counterproductive, since free page cache list residency time is negatively affected.