Re: [PATCH for drm-misc-fixes v4 2/4] drm/hisilicon/hibmc: fix no showing when no connectors connected

From: Thomas Zimmermann

Date: Tue Apr 21 2026 - 03:48:10 EST

Hi

Am 21.04.26 um 09:24 schrieb Yongbang Shi:
[...]

That's not really much of a problem. Just filter the older boards by some board identifier and handle it separately. We
use that all over the kernel; including graphics drives. See for example DMI_MATCH. [1]

Okay, we'll need to verify this with the BIOS and do some analysis. If necessary, we'll submit a patch as a feature
later on.

Right. It's a separate thing.

I think the design in the current patch is sound. It just needs to increment epoch_count if the physical status changes;
and it would trigger these user-space bugs. But we can only work around the latter.

Okay, I understand, but there's one more thing I need to confirm: simply adding logic to increment `epoch_count` within
`vdac_detect` isn't enough, is it? We still need to enable the `connector.polled` mechanism for periodic detection, right?

Exactly. The poll helper will periodically run the detect helper to test if the display is still attached. This is required on connectors that do not auto-detect this, such as VGA.

The VGA standard assumes that the display would be connected while the computer was running. But everyone always connected and disconnected monitors at runtime, so we have to support that.

Best regards
Thomas

like this:
hibmc_kms_init()
{
...

drm_kms_helper_poll_init(dev);

...
}

a little change like this:
vdac_detect()
{
phys_state = detect_from_hw()

if (dp.phys_state == connected) {
vdac->phys_state = phys_state;
return vdac->phys_state; /* return from here, epoch_counter will be update by drm framework */
}

if (phys_state != vdac->phys_state)
++epoch_counter;

vdac->phys_state = phys_state;

return connector_status_connected;
}

Thanks,
Yongbang.

Best regards
Thomas

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:

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)