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Tue, 28 Apr 2026 21:07:14 +0000 Message-ID: <3ef1f9c0-be0d-45fb-9420-867188241989@intel.com> Date: Tue, 28 Apr 2026 14:07:11 -0700 User-Agent: Mozilla Thunderbird Subject: Re: further issues with MGA G200 graphics chipset To: stuart hayes , Thomas Zimmermann , Jocelyn Falempe , "airlied@redhat.com" CC: , "linux-kernel@vger.kernel.org" , Pasi Vaananen References: <76aba88d-ec23-4b3c-ad91-83face0c3e94@intel.com> <6ec01703-31e0-4998-9508-a5a115ae7bc9@intel.com> <14aa4840-29c4-46df-b60d-8e1b92494ad2@redhat.com> <5155f29b-bc4d-4e60-9a26-22d568c43e46@redhat.com> <0be82b55-0f78-4ef9-8b50-f068ec36ec4f@suse.de> <3def4337-4297-45fa-8825-4147abda49f4@suse.de> Content-Language: en-US From: Jacob Keller In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MW4PR04CA0246.namprd04.prod.outlook.com (2603:10b6:303:88::11) To DS0PR11MB7579.namprd11.prod.outlook.com (2603:10b6:8:14d::5) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7579:EE_|DS0PR11MB8133:EE_ X-MS-Office365-Filtering-Correlation-Id: 91de7039-6880-4f96-3b24-08dea56a1f0d X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|366016|20046099003|22082099003|18002099003|56012099003|14066099003; 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I'd be >>>> surprised if we find more than a hand full of systems with the >>>> issue. If >>>> we see a trend or common pattern, we can generalize later on. >>>> >>> I think this is the best solution. Keep it focused for now. I believe >>> Intel has two major platforms that we care about with respect to this >>> issue. I'll see if I can dig up the data. The systems install the MGA >>> G200 for BMC use but don't seem to expose the VGA connection. >>> >>> For the specific system I have that was faulty, we have the following: >>> >>> $ for t in system-manufacturer system-product-name system-version ; \ >>>      do dmidecode -s ${t}; \ >>> done >>> Dell Inc. >>> PowerEdge XR8720t >>> Not Specified >>> >>> >>> >>> I believe there was also some concern about HP systems which similarly >>> use this chipset, but I don't have the DMI data for that one off hand. >>> I've asked some colleagues to confirm the situation and obtain that >>> data. I'll get back early next week if we think there are any other >>> systems possibly affected. >>> >>> In the mean time, I'm happy to have our team test any patch to confirm >>> that it behaves as expected and resolves the service interruptions. >> >> For now, I've modified the two places that have BMC support in the >> driver. Could you please also tell me your system's exact Matrox >> chipset or its PCI id? >> >> The patch is attached for your testing. It would work against drm-tip >> or v7.1-rc1. >> >> I've also found the page at [1], which claims that there's a Mini-DP >> port at the front. If so, I'd assume that there's also an extra >> encoder chip to replace the VGA. If we ever get specs for that, we >> could implement real support in the driver. >> >> In the meantime, the current fix should work. In the worst case, that >> Mini-DP port would give a lower default resolution. >> >> [1] https://www.dell.com/en-us/shop/ipovw/poweredge-xr8720t? >> hve=shop+now#techspecs_section >> >> Best regards >> Thomas >> >> > > So this patch disables DDC polling if the dmi_check_system() matches. If > this was to happen on systems that _do_ have a physical VGA connector, > will that port still be active, just with a resolution that may not be > compatible with the monitor that's plugged in? > That is my understanding, yes. At least as far as I can tell none of the Dell PowerEdge systems with this chipset have a true VGA port, but it is still unconfirmed if the mini DisplayPort is connected to the MGA G200 through some sort of encoder, and how it interacts with the polling disabled. What I can confirm so far is that the mini Display Port output does seem to work despite the MGA G200 driver continuously complaining about bad/faulty EDID checksums. I haven't had time again on the system to check the patch or confirm what happens with mgag200 polling disabled yet. > I don't see anyone say that the DDC polling doesn't cause too much > latency for real time kernels on other systems that do have a VGA > connector... did I miss that, or is there a chance that a lot of other > systems that use this driver might also have issues with a real time > kernel? > There were 2 problems so far identified: 1) the DDC polling was causing issues due to spinning where it could sleep. I fixed that a while ago with 0e0c8f4d16de ("drm/mgag200: fix mgag200_bmc_stop_scanout()"). This was causing 300 millisecond delays that impacted PTP functionality. This was affecting both RT and non-RT systems (though both setups are timing sensitive only one was actually using PREEMPT_RT). This issue, I believe, widely affects all systems which use the MGA G200. It has been fixed. 2) the issue I reported here. This issue appears to be possibly due to fault in the hadware, and does not happen on *every* Dell PowerEdge we have access to.. but it seems that the driver fails to read data over DDC when reading the EDID data for the connector. This results in it continuously retrying. Because i2c bit algo uses udelays, this results in enough spinning that it impacts my PTP setup. I do not know how wide the impacts from (2) are. I also do not know if the issue causes problems on systems that have a VGA port and which also have the VGA port plugged in. Given my experience, it seems entirely plausible. Here's a more detailed summary of the issue I saw: On regular (not PREEMPT_RT) kernel, the udelay on a CPU appears to block the interrupts from being fired on the CPU that is spinning. The polling doesn't use WORKQEUEUE_UNBOUND, so it schedules on the specific CPU that was executing the scheduling function. If this happens on the same CPU as the one that is assigned the IRQ for the ice driver, it won't fire. Since the polling thread ends up doing udelay for potentially a long time, it results in delaying ~20-30 milliseconds which is enough to impact the PTP functionality. I suspect this also causes issues with PREEMPT_RT but they might be different issues, due to the nature of PREEMPT_RT changing a lot of the way various critical sections work. I am fairly certain that other timing sensitive applications will have issues caused by this issue. It is plausible that such issues are below the radar for many deployments and its not ultimately causing a "real" impact for everyone.. but its definitely causing problems and hiccups for Intel as well as some of our partners and customers. The actual failure symptom we get is somewhat inconsistent. Even though it fails to read the EDID every second or 2, timestamps are only impacted every few minutes. But that missed timestamp is considered catastrophic failure for us. It results in ptp4l going to fault and losing synchronization for several seconds. It also means such setups do not pass various industry tests. We have been recently recommending that users remove the mgag200 driver. However this is a poor workaround as it results in inability to access the video over BMC, which some customers rely on. I also plan to confirm whether the mini DisplayPort is also affected by the driver removal. If it is, then removal of the driver could also result in the output being stopped. Some of our customers rely on the BMC to connect to the system, and I suspect it is useful to have local video access when debugging if something goes wrong with your remote access methods. Thus, I am trying to find another solution that resolves the issues we're having without needing to completely remove the driver. Thanks, Jake > >> >> >>> >>> Appreciate all the feedback on this thread. >>> >>> Thanks, >>> Jake >> >