Odpověď na názor

Prave ze delame s PCIe - hw/fpga/drivery.. a alokovat souvislou pamet zvlast uspesne nejde. To co pisete je "teorie". Souvisla pamet se resila v davne dobe pres CMA ( https://lwn.net/Articles/486301/ ) a fungovalo to vicemene tak, ze se nejaka pamet rezervovala pri bootu (default v Kconfig, override pres kernel option):

There is a set of Kconfig options, which specify the default size of the reservation. All of those options are located under “Device Drivers” » “Generic Driver Options” » “Contiguous Memory Allocator” in the Kconfig menu. They allow choosing from four possibilities: the size can be an absolute value in megabytes, a percentage of total memory, the smaller of the two, or the larger of the two. The default is to allocate 16 MiBs.

Tato velikost bylo v podstate exkluzivni carve-out, a neslo tu pamet pouzit jinde.

Od soucasnych DMA zarizeni se opravdu ocekava scatter-gather podpora (tj. nespolehat na souvislou pamet), alternativne na systemu s IOMMU lze souvislou adresaci pro DMA ucely pak emulovat, takze odpada nejaka nutnost reseni problemu s runtime dynamickym CMA.