From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Luis R. Rodriguez" Subject: Re: [PATCH v1 09/47] vidoe: fbdev: atyfb: remove and fix MTRR MMIO "hole" work around Date: Sat, 28 Mar 2015 01:28:18 +0100 Message-ID: <20150328002818.GT5622__34984.6496865668$1427502613$gmane$org@wotan.suse.de> References: <1426893517-2511-1-git-send-email-mcgrof@do-not-panic.com> <1426893517-2511-10-git-send-email-mcgrof@do-not-panic.com> <20150321091514.GA22926@sci.fi> <20150327193813.GH5622@wotan.suse.de> <20150327195759.GK5622@wotan.suse.de> <20150327215655.GA29933@sci.fi> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1Ybebu-0003aY-3U for xen-devel@lists.xenproject.org; Sat, 28 Mar 2015 00:28:22 +0000 Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Andy Lutomirski Cc: Linux Fbdev development list , Daniel Vetter , Ville =?iso-8859-1?Q?Syrj=E4l=E4?= , Jan Beulich , "H. Peter Anvin" , Suresh Siddha , Tomi Valkeinen , X86 ML , Ingo Molnar , "xen-devel@lists.xenproject.org" , Ingo Molnar , Borislav Petkov , Jean-Christophe Plagniol-Villard , Antonino Daplas , Dave Airlie , Bjorn Helgaas , Thomas Gleixner , Juergen Gross , "Luis R. Rodriguez" , "linux-kernel@vger.kernel.org" , venkatesh.pallipadi@intel.com List-Id: xen-devel@lists.xenproject.org On Fri, Mar 27, 2015 at 03:02:10PM -0700, Andy Lutomirski wrote: > On Fri, Mar 27, 2015 at 2:56 PM, Ville Syrj=E4l=E4 wrote: > > On Fri, Mar 27, 2015 at 08:57:59PM +0100, Luis R. Rodriguez wrote: > >> On Fri, Mar 27, 2015 at 12:43:55PM -0700, Andy Lutomirski wrote: > >> > On Fri, Mar 27, 2015 at 12:38 PM, Luis R. Rodriguez wrote: > >> > > On Sat, Mar 21, 2015 at 11:15:14AM +0200, Ville Syrj=E4l=E4 wrote: > >> > >> On Fri, Mar 20, 2015 at 04:17:59PM -0700, Luis R. Rodriguez wrote: > >> > >> > diff --git a/drivers/video/fbdev/aty/atyfb_base.c b/drivers/vid= eo/fbdev/aty/atyfb_base.c > >> > >> > index 8025624..8875e56 100644 > >> > >> > --- a/drivers/video/fbdev/aty/atyfb_base.c > >> > >> > +++ b/drivers/video/fbdev/aty/atyfb_base.c > >> > >> > @@ -2630,21 +2630,10 @@ static int aty_init(struct fb_info *inf= o) > >> > >> > > >> > >> > #ifdef CONFIG_MTRR > >> > >> > par->mtrr_aper =3D -1; > >> > >> > - par->mtrr_reg =3D -1; > >> > >> > if (!nomtrr) { > >> > >> > - /* Cover the whole resource. */ > >> > >> > - par->mtrr_aper =3D mtrr_add(par->res_start, par->re= s_size, > >> > >> > + par->mtrr_aper =3D mtrr_add(info->fix.smem_start, > >> > >> > + info->fix.smem_len, > >> > >> > MTRR_TYPE_WRCOMB, 1); > >> > >> > >> > >> MTRRs need power of two size, so how is this supposed to work? > >> > > > >> > > As per mtrr_add_page() [0] the base and size are just supposed to = be in units > >> > > of 4 KiB, although the practice is to use powers of 2 in *some* dr= ivers this > >> > > is not standardized and by no means recorded as a requirement. Obv= iously > >> > > powers of 2 will work too and you'd end up neatly aligned as well.= mtrr_add() > >> > > will use mtrr_check() to verify the the same requirement. Furtherm= ore, > >> > > as per my commit log message: > >> > > >> > Whatever the code may or may not do, the x86 architecture uses > >> > power-of-two MTRR sizes. So I'm confused. > >> > >> There should be no confusion, I simply did not know that *was* the > >> requirement for x86, if that is the case we should add a check for that > >> and perhaps generalize a helper that does the power of two helper chan= ges, > >> the cleanest I found was the vesafb driver solution. > >> > >> Thoughts? > > > > The vesafb solution is bad since you'll only end up covering only > > the first 4MB of the framebuffer instead of the almost 8MB you want. > > Which in practice will mean throwing away half the VRAM since you really > > don't want the massive performance hit from accessing it as UC. And that > > would mean giving up decent display resolutions as well :( > > > > And the other option of trying to cover the remainder with multiple ever > > smaller MTRRs doesn't work either since you'll run out of MTRRs very > > quickly. > > > > This is precisely why I used the hole method in atyfb in the first > > place. > > > > I don't really like the idea of any new mtrr code not supporting that > > use case, especially as these things tend to be present in older machin= es > > where PAT isn't an option. > = > According to the Intel SDM, volume 3, section 11.5.2.1, table 11-6, > non-PAT CPUs that have a WC MTRR, PCD =3D 1, and PWT =3D 1 (aka UC) have > an effective memory type of UC. Hence my suggestion to add > ioremap_x86_uc and/or set_memory_x86_uc to punch a UC hole in an > otherwise WC MTRR-covered region. OK I think I get it now. And I take it this would hopefully only be used for non-PAT systems? Would there be a use case for PAT systems? I wonder if we can wrap this under some APIs to make it clean and hide this dirty thing behind the scenes, it seems a fragile and error prone and my hope would be that we won't need more specialization in this area for PAT systems. > ioremap_nocache is UC- (even on non-PAT unless I misunderstood how > this stuff works), so ioremap_nocache by itself isn't good enough. Thanks for the clarification. Luis