Hi, Sorry for the late reply. On 4/19/22 18:45, Bjorn Helgaas wrote: > 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. Looking at the amount of exception we already now about I'm not sure if that will work well. > >> 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. So what you are suggesting is to go back to a bios-date based approach (to determine old vs new machines) combined with DMI quirks to force E820 clipping on new machines which turn out to need it despite them being new ? > >>> 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. When I first started working on this I did read the entire old email thread and IIRC this approach was reverted because the e820 based approach was deemed to be a cleaner fix. Also the single resource_alloc_from_bottom flag influenced all types of resource allocations, not just PCI host bridge window allocations. Note that the current kernel no longer has the resource_alloc_from_bottom flag. Still I think it might be worthwhile to give switching to top-down allocating for host bridge window allocs a try. Maybe we can make the desired allocation strategy a flag in the resource ? I have the feeling that if we switch to top-down allocating that we can then switch to just using _CRS and that everything will then just work, because we then match what Windows is doing... Regards, Hans > >> 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 >>> >> >
