Bug 96161

I recently discovered this bug working with the drm driver, in the stolen memory fix in the i915_gem_stolen.c. The problem is general and not only related to drm driver. The fix doesn't solve anything, as the problem is Linux not calculating the end address correctly. There are no wrap into the PCI bus, as the area ends before the PCI bus, not within it.

As en example, think about a 64KB region with base zero. It occupies byte 0 - 65535 or location 0x0000 - 0xffff. It does not clash with region 0x10000.

Likewise a 64MB frame buffer or GFX stolen memory that starts 64 MB below the PCI memory region does not clash with the PCI memory region. It simply ends at the PCI memory region - 1.

Thus the first devm_request_mem_region will fail on any BIOS setting any PCI device base memory to the start of the PCI memory region. The second will also fail, as the base + 1, size - 1 will yield the same result.

Example: PCI bus memory region starts at 0xC0000000. Stolen memory size is 64 MB. Thus stolen memory starts at 0xC0000000 - 0x4000000 = 0xBC000000. The region ends at 0xBCFFFFFF and therefor do not clash with the PCI bus.

The mistake is simply that you have to substract 1 from the end of region calculation, as every region is zero based and thus starts from 0.

The devm_request_mem_region() should not return 0 in the above settings, as the 0xC0000000 is not part of the region and thus the stolen memory region are not already  claimed!!!

Best regards,

B-O Bergman
Winzent Technologies