Re: further issues with MGA G200 graphics chipset

[Jacob Keller]

On 4/23/2026 2:02 PM, David Airlie wrote:
> On Fri, Apr 24, 2026 at 5:42 AM Jacob Keller <jacob.e.keller@xxxxxxxxx> wrote:
>>
>> On 4/23/2026 12:22 PM, Jocelyn Falempe wrote:
>>> On 23/04/2026 18:35, Jacob Keller wrote:
>>>> On 4/23/2026 12:44 AM, Thomas Zimmermann wrote:
>>>>> Hi
>>>>>
>>>>> Am 23.04.26 um 01:55 schrieb Jacob Keller:
>>>>>> Hello,
>>>>>> <snip>>>> I'm also curious if its possible to stop polling for so
>>> long with udelay
>>>>>> in the i2c logic somehow? I am not very familiar with i2c, but it is
>>>>>> frustrating that this driver is causing yet another stall that is
>>>>>> impacting timing sensitive data. Even if in this case its due to a
>>>>>> faulty cable.. it is frustrating that such result causes the PTP
>>>>>> failures. Would switching to WQ_UNBOUND be helpful here at all?
>>>>>
>>>>> Try Dave's suggestion to avoid polling.  The driver won't be able to
>>>>> detect changes to the connector status, though.
>>>>>
>>>>
>>>> That's fine. I don't think we're even using the device. It looks like it
>>>> might only be in use for BMC, and the VGA connection isn't actually
>>>> physically available, so there are no changes to detect.
>>>>
>>>> Is this polling really only to detect when VGA is enabled? Would it make
>>>> sense to only poll on platforms which actually *have* that VGA
>>> connection?
>>>>
>>> Polling was introduced with https://patchwork.freedesktop.org/
>>> series/131977/
>>>
>>> The driver needs to know if a VGA monitor is connected or not, to
>>> provide the right available resolutions to the userspace.
>>> Otherwise you can set a high resolution that works from the BMC, but
>>> then connecting a VGA monitor will not work, as the driver won't notice
>>> that something has been connected.
>>>
>>> The mgag200 doesn't have an IRQ or a register to check if something is
>>> connected on the VGA port, so the driver uses the i2c and tries to read
>>> the EDID.
>>>
>>> Unfortunately, there is no way to know reliably if a VGA connector is
>>> present. It's possible to disable polling on some machines using DMI
>>> quirks, but I don't think this approach will scale.
>>>
>>
>> Timing sensitive setups like mine must have system admins who know to
>> manually disable mgag200 or disable polling. Many users won't be aware
>> of this. If the polling were not intrusive, this would not be an issue.
>> But....
>>
>> Faulty hardware (perhaps just a cheap pull down resistor on the VGA
>> connection as Dave Airlie suggests) means that any such affected
>> platform has a polling routine that causes significant issues on any
>> timing sensitive applications.
> 
> We could write a patch to just say if we see 10 bogus EDID polls we
> just give up and loudly say in the logs.
> 

That would certainly be a better situation for me...

> This might break some crash-cart plugins in some data centers though,
> I don't think we have contracts in Matrox or the server vendors who
> make the hw to say how they recommend finding this info.
> 

But I could see this being a problem for data centers who previously saw
"no issue" and now see "this device is causing a problem", especially if
that problem is really non-existent?

> It might be in ACPI or dmidecodes.
> 

I can try checking if anything obvious shows up in dmidecodes for the
device.

> Dave.