On Tue, Apr 19, 2022 at 05:16:44PM +0200, Hans de Goede wrote: > Hi, > > On 4/19/22 17:03, Bjorn Helgaas wrote: > > On Tue, Apr 19, 2022 at 11:59:17AM +0200, Hans de Goede wrote: > >> On 1/1/70 01:00, Bjorn Helgaas wrote: > >>> This is still work-in-progress on the issue of PNP0A03 _CRS methods that > >>> are buggy or not interpreted correctly by Linux. > >>> > >>> The previous try at: > >>> https://lore.kernel.org/r/20220304035110.988712-1-helgaas@xxxxxxxxxx > >>> caused regressions on some Chromebooks: > >>> https://lore.kernel.org/r/Yjyv03JsetIsTJxN@xxxxxxxxxxxxx > >>> > >>> This v2 drops the commit that caused the Chromebook regression, so it also > >>> doesn't fix the issue we were *trying* to fix on Lenovo Yoga and Clevo > >>> Barebones. > >>> > >>> The point of this v2 update is to split the logging patch into (1) a pure > >>> logging addition and (2) the change to only clip PCI windows, which was > >>> previously hidden inside the logging patch and not well documented. > >>> > >>> Bjorn Helgaas (3): > >>> x86/PCI: Eliminate remove_e820_regions() common subexpressions > >>> x86: Log resource clipping for E820 regions > >>> x86/PCI: Clip only host bridge windows for E820 regions > >> > >> Thanks, the entire series looks good to me: > >> > >> Reviewed-by: Hans de Goede <hdegoede@xxxxxxxxxx> > > > > Thank you! > > > >> So what is the plan to actually fix the issue seen on some Lenovo models > >> and Clevo Barebones ? As I mentioned previously I think that since all > >> our efforts have failed so far that we should maybe reconsider just > >> using DMI quirks to ignore the E820 reservation windows for host bridges > >> on affected models ? > > > > I have been resisting DMI quirks but I'm afraid there's no other way. > > Well there is the first match adjacent windows returned by _CRS and > only then do the "covers whole region" exception check. I still > think that would work at least for the chromebook regression... Without a crystal clear strategy, I think we're going to be tweaking the algorithm forever as the _CRS/E820 mix changes. That's why I think that in the long term, a "use _CRS only, with quirks for exceptions" strategy will be simplest. > So do you want me to give that a try; or shall I write a patch > using DMI quirks. And if we go the DMI quirks, what about > matching cmdline arguments? If we add matching cmdline arguments, > which seems to be the sensible thing to do then to allow users > to test if they need the quirk, then we basically end up with my > first attempt at fixing this from 6 months ago: > > https://lore.kernel.org/linux-pci/20211005150956.303707-1-hdegoede@xxxxxxxxxx/ So I think we should go ahead with DMI quirks instead of trying to make the algorithm smarter, and yes, I think we will need commandline arguments, probably one to force E820 clipping for future machines, and one to disable it for old machines. > > I think the web we've gotten into, where vendors have used E820 to > > interact with _CRS in incompatible and undocumented ways, is not > > sustainable. > > > > I'm not aware of any spec that says the OS should use E820 to clip > > things out of _CRS, so I think the long term plan should be to > > decouple them by default. > > Right and AFAICT the reason Windows is getting away with this is > the same as with the original Dell _CRS has overlap with > physical RAM issue (1), Linux assigns address to unassigneds BAR-s > starting with the lowest available address in the bridge window, > where as Windows assigns addresses from the highest available > address in the window. Right, I agree. I'm guessing Chromebooks don't get tested with Windows at all, so we don't even have that level of testing to help. > So the real fix here might very well be > to rework the BAR assignment code to switch to fill the window > from the top rather then from the bottom. AFAICT all issues where > excluding _E820 reservations have helped are with _E820 - bridge > window overlaps at the bottom of the window. > > IOW these are really all bugs in the _CRS method for the bridge, > which Windows does not hit because it never actually uses > the lowest address(es) of the _CRS returned window. Yes. We actually did try this (https://git.kernel.org/linus/1af3c2e45e7a), but unfortunately we had to revert it. Even more unfortunately, the revert (https://git.kernel.org/linus/5e52f1c5e85f) doesn't have any details about what went wrong. > 1) At least I read in either a bugzilla, or email thread about > this that Windows allocating bridge window space from the top > was assumed to be why Windows was not impacted. > > > Straw man: > > > > - Disable E820 clipping by default. > > > > - Add a quirk to enable E820 clipping for machines older than X, > > e.g., 2023, to avoid breaking machines that currently work. > > > > - Add quirks to disable E820 clipping for individual machines like > > the Lenovo and Clevos that predate X, but E820 clipping breaks > > them. > > > > - Add quirks to enable E820 clipping for individual machines like > > the Chromebooks (and probably machines we don't know about yet) > > that have devices that consume part of _CRS but are not > > enumerable. > > > > - Communicate this to OEMs to try to prevent future machines that > > need quirks. > > > > Bjorn > > >
