From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Airlie Date: Tue, 07 May 2013 03:09:48 +0000 Subject: Re: [PATCH 3/7] drm: Update drm_addmap and drm_mmap to use PAT WC instead of MTRRs Message-Id: List-Id: References: In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Andy Lutomirski Cc: Jerome Glisse , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org >> >> From memory, even on pat system we need mtrr for VRAM is PCI BAR. We >> cover it with a write combine MTRR. The whole ioctl is use by some ddx >> or maybe even directly the XServer to do this mtrr mess in userspace. > > Egads! So we have a _DRM_WRITE_COMBINING flag, which will continue to > work fine, but almost nothing uses it. > > I'm amazed this stuff works in the current code, though. Apparently > the memory type (WC or UC) of a drm mapping is determined by the mtrr > the driver set, but if one part of the BAR is textures or the > framebuffer and another part is an outgoing command ring, won't one of > them end up with the wrong memory type? Nobody sane puts the command ring in VRAM. Dave.