Re: [PATCH for drm-misc-fixes v4 2/4] drm/hisilicon/hibmc: fix no showing when no connectors connected
From: Yongbang Shi
Date: Mon Apr 20 2026 - 11:18:43 EST
> Hi
>
> Am 20.04.26 um 10:40 schrieb Yongbang Shi:
> [...]
>>
>> From the patch you contributed,，I understand that you are using an
>> RH1288H V3 server. I have confirmed with our chief hardware engineer
>> that the board design for this series of servers does not connect the
>> BMC chip’s DDC I2C to the VGA port.
>
> Thanks a lot for double-checking. That's the info I was missing. I suggest to build a workaround into the driver to
> filter out these boards and always return 'connected' on them. IIUC even with these new callbacks, the driver would not
> work as expected.
>
Our BMC chips can distinguish between generations, but since BMC chips of the same generation are used on many different
boards, filtering by board type or board id is not feasible. We can only determine whether DDC I2C is present by
comparing BMC chips of different generations. We have mandated that the board implementation must connect to DDC I2C on
the latest BMC chip.
>>
>> [1]https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/gpu/drm/hisilicon/hibmc/
>> hibmc_drm_vdac.c?h=v6.12.82#L60
>> [2]https://lore.kernel.org/all/20250320101455.2538835-10-shiyongbang@xxxxxxxxxx/
>>
>>
>>>> Signed-off-by: Lin He <helin52@xxxxxxxxxx>
>>>> Signed-off-by: Yongbang Shi <shiyongbang@xxxxxxxxxx>
>>>> ---
>>> [...]
>>>> +static int hibmc_vdac_detect(struct drm_connector *connector,
>>>> + struct drm_modeset_acquire_ctx *ctx,
>>>> + bool force)
>>>> +{
>>>> + struct hibmc_drm_private *priv = to_hibmc_drm_private(connector-
>>>>> dev);
>>>> + struct hibmc_vdac *vdac = to_hibmc_vdac(connector);
>>>> +
>>>> + vdac->phys_state = drm_connector_helper_detect_from_ddc(connector,
>>>> + ctx, force);
>>>> +
>>>> + /* If the DP connectors are disconnected, the hibmc_vdac_detect
>>>> function
>>>> + * must return a connected state to ensure KVM display
>>>> functionality.
>>>> + * Additionally, for previous-generation products that may lack
>>>> hardware
>>>> + * link support and thus cannot detect the monitor,
>>>> hibmc_vdac_detect
>>>> + * should also return a connected state.
>>>> + */
>>>> + if (priv->dp.phys_state != connector_status_connected)
>>>> + return connector_status_connected;
>>>> +
>>>> + return vdac->phys_state;
>>> This will break user-space compositors if DP and VGA are connected at
>>> the same time. I know, because we had such logic in ast and mgag200.
>>>
>>> Today's compositors expect a single encoder-connector pair on each CRTC.
>>> That's what most contemporary hardware provides. But the server
>>> chipsets usually come with one a single CRTC and multiple connectors
>>> attached to it. Compositors fail to configure that. When we had this in
>>> ast and mgag200, Gnome would display garbage to the screen and mode-
>>> setting would fail. The bug report is at [1].
>>>
>>> In ast and mgag200, only one connector would be installed on a single
>>> system. So we could avoid the problem. It looks like that this is not an
>>> option with hibmc. VGA always seems to be present and DP seems optional.
>>> I would advice to only report one of the connectors as 'connected' if
>>> both are installed on a system.
>>>
>> According to our BMC chip specifications, both VGA and DisplayPort
>> outputs are required, but only one connection is allowed at a time,
>> which seems impractical.
>
> That's true.
>
> Yet it needs support form user space to make it work. It's not a kernel problem.
>
>>
>> Do you have any additional details regarding this issue with the Mutter
>> synthesizer? For example, steps to reproduce the issue or more specific
>> alert logs. I looked at the issue you submitted to Mutter, which
>> describes the phenomenon as "a warning that the configuration is
>> incompatible", Could you clarify what this warning specifically refers
>> to—is it a message displayed on the screen, or is it a specific entry in
>> the logs?
>
> In ast, we used to have at least one connector for the physical output and another connector for the BMC/KVM. Running
> Gnome and opening the Settings app did not work correctly. One issue is that display configuration could not be changed
> in Gnome. That's the reported error of changes not being applicable.
>
Great, regarding hibmc-drm's support for DP, my initial idea was also to create separate connectors for DP, VGA, and
KVM. However, due to some non-technical reasons, that approach wasn't implemented. Since ast has implemented it this
way, we can also consider this solution.
> IIRC I've also seen distorted output if I really pushed it. But maybe that has meanwhile been mitigated.
>
> I've been told that other compositors also don't support this scenario well.
>
> I think you'd run into this case once someone connects the DP and VGA at the same time.
>
Okay, I understand. However, so far, we have thoroughly tested the scenario where both connector0 (VGA shared with KVM)
and connector1 (DP) are displaying at the same time, and we haven’t encountered any similar issues.
I will create a dedicated connector for the KVM and conduct the necessary tests to test for this scenario.
>>
>> Over the next few days, we will focus on testing relevant scenarios
>> while setting both the VGA and DP ports to `connected` to see if we
>> encounter similar issues or warnings.
>>
>>> You also need to increment the connector's epoch_counter if the physical
>>> status changed. Doing this will trigger DRM clients to re-read the
>>> connector state and reconfigure display.
>>>
>>> Something like that
>>>
>>> dp_detect()
>>> {
>>> dp.phys_state = detect_from_hw()
>>>
>>> if (dp.phys_state changed)
>>> ++epoch_counter
>>> return connected
>>> }
>>>
>>> vdac_detect()
>>> {
>>> if (dp.phys_state == connected)
>>> return disconnected
>>>
>> Only one option is displayed here, but it does not meet our specifications.
>>
>>> vdac.phys_state = detect_from_hw()
>>>
>>> if (vdac.phys_state changed)
>>> ++epoch_counter
>>> return connected
>>> }
>>>
>>> This handles the KVM in each connector's helper. The ast driver is
>>> probably the best reference for the current logic. See [2].
>>>
>> I looked at the implementations of `detect_ctx` in ATS and MGAG200;
>> these implementations generally always return `connected`.
>>
>> If my analysis is correct, the logic should be as follows:
>> - If the `detect_ctx` function implemented by each vendor returns the
>> actual hardware state, there is no need to modify `epoch_counter`;
>> the DRM framework will execute `epoch_counter++` based on the return
>> value of the `detect_ctx` function;
>>
>> - If the `detect_ctx` function in a vendor’s code always returns
>> `connected`, then `epoch_counter++` must be performed within the
>> vendor’s `detect_ctx` function; the implementations in ATS and
>> MGAG200 appear to be like this;
>
> Exactly. In ast/mgag200, we had the case that the VGA got disconnected. We'd still return connected, as the BMC/KVM was
> present. We now had to reconfigure for the KVM. That requires modifying epoch_counter. You can see at [1] that we only
> increment if the physical status changes. [1] Same goes for re-connecting the VGA display.
>
> [1] https://elixir.bootlin.com/linux/v7.0/source/drivers/gpu/drm/ast/ast_vga.c#L56
>
>>
>>
>> Here is the pseudocode for our current logic. This logic is indeed a bit
>> hard to grasp, so I've added some more comments; it might be a bit
>> clearer than the patch:
>>
>> dp_detect()
>> {
>> dp.phys_state = detect_from_hw()
>>
>> /* epoch_counter update by drm framework */
>> return dp.phys_state;
>> }
>>
>> vdac_detect()
>> {
>> /*
>> * Scene 1: The driver runs on an older version of the BMC chip,
>> * and `dp.phys_state` is initialized to 0. To maintain
>> * consistency with the previous hibmc-drm driver
>> * logic, always return `connected`.
>> * Scene 2: The driver runs on an new version of the BMC chip,
>> * and `dp.phys_state` is real status, witch is real
>> * `disconnect`. In this scenario, we must ensure that
>> * the VGA is `connected` to guarantee that the KVM
>> * displays properly.
>> */
>> if (dp.phys_state != connected)
>> return connected;
>>
>> /*
>> * DP is connected: This driver is definitely running on the
>> * latest BMC chip, and the DDC I2C is definitely
>> * connected to the VGA port, so we can confirm whether
>> * the VGA is present by reading the EDID directly via
>> * I2C.
>> */
>> vdac.phys_state = detect_from_hw()
>>
>> /* epoch_counter update by drm framework */
>> return vdac.phys_state;
>
> Let's assume the DP is disconnected. If VGA's physical status goes from connected to disconnected, user space will not
> know that it needs to reconfigure for the BMC. (or vice versa)
>
> To give you some context from earlier discussions around the other drivers: We had the scenario that the user boots up
> without a connected VGA display. This would enable the BMC, which sometimes picks a large resolution; say >2k per
> dimension. That is problematic for KVM over networks. And if the user now plugs in a physical VGA display, they would be
> stuck with the large resolution on the framebuffer console. Maybe the VGA display wouldn't even support such a high-res
> mode. So we set the default for the BMC to 1024x768. That is well manageable for all of today's VGA displays and does
> not impose too much overhead for KVM over networks. As a drawback, it's a bit small on monitors and not nice to look
> at. So we modify epoch_counter whenever the physical status changes. This triggers compositors to reconfigure their
> screens for the new display.
>
Yes, this issue does exist. If we uses the `connector.polled` mechanism to monitor the VGA's presence in real time and
update the `epoch_counter`, which might resolve the issue. However, upon further consideration, I don't think this is
the optimal solution. If we create a separate connector for KVM and distinguish between older-generation BMC chips and
newer-generation BMC chips, the problem will be easily solved. In addition, DP and VGA, as well as VGA and KVM, will be
decoupled.
A temporary solution:
/* called by poll work */
vdac_detect()
{
/* Let’s examine this logic on its own to gain a better understanding. */
if (older-generation)
return connected;
vdac.phys_state = detect_from_hw();
if (vdac.phys_state changed)
++epoch_counter;
if (dp.phys_state != connected) {
return connected;
}
return vdac.phys_state;
}
A more elegant solution:
|--connector0(KVM): always connected, do not update epoch_counter.
|
crtc--|--connector1(VGA): detect real time by `connector.polled`, update epoch_counter.
|
|--connector1(DP): detect real time by hpd interrupt, update epoch_counter.
kvm_detect()
{
return connected;
}
/* called by poll work */
vdac_detect()
{
/* for older-generation */
if (older-generation)
return connected;
/* for newer-generation */
vdac.phys_state = detect_from_hw();
/* epoch_counter update by drm framework */
return vdac.phys_state;
}
dp_detect()
{
dp.phys_state = detect_from_hw();
/* epoch_counter update by drm framework */
return dp.phys_state;
}
Thanks,
Yongbang.
> Best regards
> Thomas
>
>> }
>>
>> Sorry for the delay; it took me a little while to figure out how best to
>> explain this bug and our current implementation.
>>
>> Thanks,
>> Yongbang.
>>
>>> [1] https://gitlab.gnome.org/GNOME/mutter/-/issues/3858
>>> [2] https://elixir.bootlin.com/linux/v7.0/source/drivers/gpu/drm/ast/
>>> ast_vga.c#L47
>>>
>>> Best regards
>>> Thomas
>>>
>>>> +}
>>>> +
>>>> static const struct drm_connector_helper_funcs
>>>> hibmc_connector_helper_funcs = {
>>>> .get_modes = hibmc_connector_get_modes,
>>>> - .detect_ctx = drm_connector_helper_detect_from_ddc,
>>>> + .detect_ctx = hibmc_vdac_detect,
>>>> };
>>>> static const struct drm_connector_funcs hibmc_connector_funcs = {
>>>> @@ -130,6 +145,8 @@ int hibmc_vdac_init(struct hibmc_drm_private *priv)
>>>> connector->polled = DRM_CONNECTOR_POLL_CONNECT |
>>>> DRM_CONNECTOR_POLL_DISCONNECT;
>>>> + vdac->phys_state = connector_status_connected;
>>>> +
>>>> return 0;
>>>> err:
>