Date: Tue, 14 Aug 2018 10:12:37 +0100 From: Johannes Lundberg <johalun0@gmail.com> To: freebsd-current <freebsd-current@freebsd.org> Subject: Make drm drivers use MTRR write-combine Message-ID: <CAECmPwsmL7sTYZoe%2BZHpJy6Yzce3iwabwT1THTyjBEvBRH88Jg@mail.gmail.com>
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Hi Something that we have seen for a long time on FreeBSD is the boot message Failed to add WC MTRR for [0xd0000000-0xdfffffff]: -22; performance may suffer Taking a closer look at this with memcontrol I can see that the 256 MB region that DRM wants to set as WC is already covered by this entry 0xc0000000/0x40000000 BIOS uncacheable set-by-firmware active Similar on both my Skylake and Broadwell systems. The linuxkpi wrapper can be found here: https://github.com/FreeBSDDesktop/kms-drm/blob/drm-v4.15/linuxkpi/gplv2/src/linux_mtrr.c There doesn't seem to exist a function for changing the properties of a sub region: https://github.com/FreeBSDDesktop/freebsd-base/blob/master/sys/dev/mem/memutil.c Any ideas of a good solution to this? Can this region be blacklisted or is there a safe way to split the big region into several regions with different flags when the drm driver loads? For reference, my AMD machine logs this # dmesg | grep MTRR Successfully added WC MTRR for [0xe0000000-0xefffffff]: 0; # memcontrol list --SNIP-- 0xff000/0x1000 BIOS write-protect fixed-base fixed-length set-by-firmware active 0x0/0x80000000 BIOS write-back set-by-firmware active 0x80000000/0x40000000 BIOS write-back set-by-firmware active 0xc0000000/0x20000000 BIOS write-back set-by-firmware active 0xe0000000/0x10000000 drm write-combine active Not sure if it's a BIOS or CPU vendor issue.
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