"Don" wrote in
news:8aednfJTU6PnKr3VnZ2dnUVZ_hKdnZ2d@hiwaay1:

[snip; sections rearranged -- see below]

> RAM >= 4GB: With 4GB or more installed both max_pfn and num_physpages
> reflect 512MB more than is installed. I've tried 4GB and 8GB
> configurations.
>
> How can a driver identify at load time the amount of memory installed?

[Caveat: Most of this is i386-specific info and applies specifically to the
most common model (flatmem / 4K pages)]

The reason why more memory is indicated than is actually installed is
because the "physical address" of the memory is rearranged by the BIOS to
leave a "hole" for various devices on the motherboard as well as any add-in
PCI devices.

The true physical address layout is typically retrieved from the BIOS with
"int 15H" while still in real mode (it's referred to as "e820" information
in the kernel because e820h is the value you place into the AX register
before executing "int 15H" to request this info from the BIOS). The kernel
prints this info early in the boot process. For example, on one system
with 4GB, dmesg shows this:
BIOS-provided physical RAM map:
BIOS-e820: 0000000000000000 - 000000000009d000 (usable)
BIOS-e820: 000000000009d000 - 00000000000a0000 (reserved)
BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved)
BIOS-e820: 0000000000100000 - 00000000cffbce00 (usable)
BIOS-e820: 00000000cffbce00 - 00000000cffd0000 (ACPI data)
BIOS-e820: 00000000cffd0000 - 00000000d0000000 (reserved)
BIOS-e820: 00000000e0000000 - 00000000f0000000 (reserved)
BIOS-e820: 00000000fec00000 - 0000000100000000 (reserved)
BIOS-e820: 0000000100000000 - 0000000130000000 (usable)

Both max_pfn and num_physpages are set to encompass all of the memory
above: this also turns out to be the number of entries in the mem_map array
(an array of 'struct page' that contains an entry for every page of
physical RAM). If there is a hole, the array will also contain entries for
the pages corresponding to the hole (they're simply marked "Reserved"). So
for the system above, max_pfn would be set to (0x130000000 / 4096 [page
size]) = 1245184 and there are that many entries in the mem_map array.

> I'm working on a driver for a PCI device whose DMA engines are limited
> to 32-bit addresses. Because of the addressing limitation I'm
> implementing one procedure for +4GB installations and another for
> installations with at most 4GB of memory. If the physical addresses
> are limited to 32-bits, then the driver can use a more efficient
> method of performinh DMA. However, if an application's I/O buffers
> might reside beyond the 4BG region, then the driver must resort to a
> less efficient method for performing DMA. This criteria for which to
> use may change depending upon the response I get.

As you can see from the above, if max_pfn is above (0x100000000 / 4096) =
1048576, that indicates that there *is* memory in the system whose physical
address is not representable in 32-bits. What you need to do about it
depends on what kind of device you're driving and how the memory area is
given to the driver.

GH