Dear Intel graphics folks,
since commit 17fec8a0 [1]
drm/i915: Use Graphics Base of Stolen Memory on all gen3+
Linux reads the register BSM (Base of Stolen Memory) directly to get the
base address of graphics stolen memory. With coreboot [2] and native
graphics init – note that everything works with the proprietary VGA
BIOS/Option ROM – this causes a regression [3] as this register is not
programmed at all.
From the datasheet *Mobile Intel® 945 Express Chipset Family* [4] the
register BSM is described on page 290.
Graphics Stolen Memory and TSEG are within DRAM space defined
under TOLUD. From the top of low used DRAM, (G)MCH claims 1 to
64 MBs of DRAM for internal graphics if enabled.
This register contains bits 31 to 20 of the base address of
stolen
DRAM memory. The host interface determines the base of
graphics stolen memory by subtracting the graphics stolen
memory size from TOLUD. See Device 0 TOLUD for more
explanations.
Also see Figure 12 *Main Memory Address Range* in section 9.2 on page
325.
Unfortunately I am unable to find out how the graphics stolen memory
size is determined. I’d have thought it is used for the framebuffer, but
according to page 93 (Graphics Mode select (GMS)) that the framebuffer
size can only be 1 MB or 8 MB, which contradicts that it can be up to 64
MB.
If it is determined implicitly by the value I set the BSM to, where can
I find the recommendations what size to use? I’d guess it is dependent
on the RAM size, that means dependent if the system has 512 MB or 4 GB
for example.
Thanks,
Paul
[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=17fec8a08698bcab98788e1e89f5b8e7502ababd
[2] http://www.coreboot.org/
[3] https://bugs.freedesktop.org/show_bug.cgi?id=79038
[4] http://www.intel.com/content/dam/www/public/us/en/documents/datasheets/mobile-945-express-chipset-datasheet.pdf
Document Number: 309219-006
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