Re: [PATCH v2] drm/msm: default separate_gpu_kms to auto selection

[Dmitry Baryshkov]

On Tue, Mar 17, 2026 at 04:06:30PM +0530, Mahadevan P wrote:
> 
> 
> On 3/17/2026 3:57 PM, Mahadevan P wrote:
> > On platforms with multiple display subsystems, such as SA8775P, the GPU
> > binds to the first display subsystem that probes. This implicit binding
> > prevents subsequent display subsystems from probing successfully,
> > breaking multi-display support.
> > 
> > Use the tristate separate_gpu_kms module parameter with the default
> > value set to auto (-1). In auto mode, the driver selects the binding
> > behavior based on the number of GPUs and display subsystems. This allows
> > display subsystems to probe independently when required, while
> > preserving the existing single-card behavior on simpler systems.
> > 
> > The separate_gpu_kms module parameter has the following semantics:
> > 
> >    -1 (auto, default):
> >       Select the binding mode based on hardware topology. If exactly one
> >       GPU and one display subsystem are present, bind them together to
> >       form a single DRM device. Otherwise, expose the GPU and display
> >       subsystems as separate DRM devices.
> > 
> >     0:
> >       Always bind the GPU and display together to form a single DRM
> >       device.
> > 
> >     1:
> >       Always expose the GPU and display subsystems as separate DRM
> >       devices.
> > 
> > This ensures correct probing on multi-display platforms without
> > affecting single-display, single-GPU systems.
> > 
> > Signed-off-by: Mahadevan P <mahadevan.p@xxxxxxxxxxxxxxxx>
> > ---
> > Changes in v2:
> > - EDITME: describe what is new in this series revision.
> > - EDITME: use bulletpoints and terse descriptions.
> > - Link to v1: https://lore.kernel.org/r/20260223-seperate_gpu_kms-v1-1-e8231e7f1685@xxxxxxxxxxxxxxxx
> 
> From coverletter this got added from b4 will make sure will be posted
> properly with subsequent patches.

You know, you can just edit it in the b4's coverletter and drop your
manual changelog...

> 
> > ---
> > Depends on:
> >    https://lore.kernel.org/lkml/20260124-adreno-module-table-v1-1-9c2dbb2638b4@xxxxxxxxxxxxxxxx/
> > 
> >    When separate_gpu_kms is enabled, the GPU and display drivers are
> >    probed independently. In this configuration, the Adreno GPU driver
> >    may no longer be loaded implicitly via the display subsystem.
> > 
> >    The referenced patch adds a MODULE_DEVICE_TABLE() entry for the
> >    Adreno GPU device, ensuring proper module autoloading based on
> >    device tree matching. This is required to guarantee that the GPU
> >    driver is loaded correctly when GPU and display probing are
> >    decoupled.
> > 
> > Changes in v2:
> > 
> >      - Drop dependency on Lemans dual-DPU device tree changes as this patch
> >        works independently (Dmitry)
> >      - Switch separate_gpu_kms to tristate and default to auto mode (Rob)
> >      - Rename msm_gpu_no_components() to msm_separate_gpu_kms_components() for clarity
> >      - Link to v1: https://lore.kernel.org/r/20260223-seperate_gpu_kms-v1-1-e8231e7f1685@xxxxxxxxxxxxxxxx
> > ---
> >   drivers/gpu/drm/msm/adreno/adreno_device.c |  2 +-
> >   drivers/gpu/drm/msm/msm_drv.c              | 52 ++++++++++++++++++++++++++----
> >   drivers/gpu/drm/msm/msm_drv.h              |  4 ++-
> >   drivers/gpu/drm/msm/msm_mdss.c             | 15 +++++++++
> >   4 files changed, 64 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/msm/adreno/adreno_device.c b/drivers/gpu/drm/msm/adreno/adreno_device.c
> > index 4edfe80c5be7..e40648c05797 100644
> > --- a/drivers/gpu/drm/msm/adreno/adreno_device.c
> > +++ b/drivers/gpu/drm/msm/adreno/adreno_device.c
> > @@ -272,7 +272,7 @@ static const struct component_ops a3xx_ops = {
> >   static int adreno_probe(struct platform_device *pdev)
> >   {
> >   	if (of_device_is_compatible(pdev->dev.of_node, "amd,imageon") ||
> > -	    msm_gpu_no_components())
> > +	    msm_separate_gpu_kms_components())
> >   		return msm_gpu_probe(pdev, &a3xx_ops);
> >   	return component_add(&pdev->dev, &a3xx_ops);
> > diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
> > index e5ab1e28851d..575d1aea7927 100644
> > --- a/drivers/gpu/drm/msm/msm_drv.c
> > +++ b/drivers/gpu/drm/msm/msm_drv.c
> > @@ -54,16 +54,54 @@ static bool modeset = true;
> >   MODULE_PARM_DESC(modeset, "Use kernel modesetting [KMS] (1=on (default), 0=disable)");
> >   module_param(modeset, bool, 0600);
> > -static bool separate_gpu_kms;
> > -MODULE_PARM_DESC(separate_gpu_drm, "Use separate DRM device for the GPU (0=single DRM device for both GPU and display (default), 1=two DRM devices)");
> > -module_param(separate_gpu_kms, bool, 0400);
> > +/*
> > + * separate_gpu_kms (tristate):
> > + *   -1 (default): decide automatically based on hardware topology. Split devices
> > + *                 if there is more than one GPU or more than one display master.
> > + *    0: force single DRM device (bind display + GPU)
> > + *    1: force separate DRM devices
> > + */
> > +static int separate_gpu_kms = -1;
> > +MODULE_PARM_DESC(separate_gpu_kms,
> > +		 "Use separate DRM device for the GPU (-1=auto (default), 0=single DRM device, 1=separate DRM devices)");
> > +module_param(separate_gpu_kms, int, 0400);
> >   DECLARE_FAULT_ATTR(fail_gem_alloc);
> >   DECLARE_FAULT_ATTR(fail_gem_iova);
> > -bool msm_gpu_no_components(void)
> > +static const struct of_device_id msm_gpu_match[];
> > +static int msm_count_gpus(void)
> > +{
> > +	struct device_node *np;
> > +	int count = 0;
> > +
> > +	for_each_matching_node(np, msm_gpu_match) {
> > +		if (of_device_is_available(np) && adreno_has_gpu(np))
> > +			count++;

If !adreno_has_gpu() we definitely should be using separate binding.

> > +	}
> > +
> > +	return count;
> > +}
> > +
> > +static bool msm_separate_gpu_kms_auto(void)
> > +{
> > +	int gpus = msm_count_gpus();
> > +	int mdss = msm_mdss_count_masters();

This breaks MDP4 case.

> > +
> > +	if (gpus <= 0 || mdss <= 0)
> > +		return false;
> > +
> > +	/* If exactly one GPU and one display subsystem single card */
> > +	return (gpus > 1) || (mdss > 1);
> > +}
> > +
> > +bool msm_separate_gpu_kms_components(void)
> >   {
> > -	return separate_gpu_kms;
> > +	if (separate_gpu_kms == 1)
> > +		return true;
> > +	if (separate_gpu_kms == 0)
> > +		return false;
> > +	return msm_separate_gpu_kms_auto();
> >   }
> >   static int msm_drm_uninit(struct device *dev, const struct component_ops *gpu_ops)
> > @@ -1030,7 +1068,7 @@ static int add_gpu_components(struct device *dev,
> >   static int msm_drm_bind(struct device *dev)
> >   {
> >   	return msm_drm_init(dev,
> > -			    msm_gpu_no_components() ?
> > +			    msm_separate_gpu_kms_components() ?
> >   				    &msm_kms_driver :
> >   				    &msm_driver,
> >   			    NULL);
> > @@ -1069,7 +1107,7 @@ int msm_drv_probe(struct device *master_dev,
> >   			return ret;
> >   	}
> > -	if (!msm_gpu_no_components()) {
> > +	if (!msm_separate_gpu_kms_components()) {
> >   		ret = add_gpu_components(master_dev, &match);
> >   		if (ret)
> >   			return ret;
> > diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
> > index 6d847d593f1a..64a5ad35f7a2 100644
> > --- a/drivers/gpu/drm/msm/msm_drv.h
> > +++ b/drivers/gpu/drm/msm/msm_drv.h
> > @@ -555,6 +555,8 @@ void msm_kms_shutdown(struct platform_device *pdev);
> >   bool msm_disp_drv_should_bind(struct device *dev, bool dpu_driver);
> > -bool msm_gpu_no_components(void);
> > +bool msm_separate_gpu_kms_components(void);
> > +
> > +int msm_mdss_count_masters(void);
> >   #endif /* __MSM_DRV_H__ */
> > diff --git a/drivers/gpu/drm/msm/msm_mdss.c b/drivers/gpu/drm/msm/msm_mdss.c
> > index 9047e8d9ee89..00e3ac7dab3a 100644
> > --- a/drivers/gpu/drm/msm/msm_mdss.c
> > +++ b/drivers/gpu/drm/msm/msm_mdss.c
> > @@ -73,6 +73,21 @@ static int msm_mdss_parse_data_bus_icc_path(struct device *dev,
> >   	return 0;
> >   }
> > +static const struct of_device_id mdss_dt_match[];
> > +
> > +int msm_mdss_count_masters(void)

No masters please.

> > +{
> > +	struct device_node *np;
> > +	int count = 0;
> > +
> > +	for_each_matching_node(np, mdss_dt_match) {
> > +		if (of_device_is_available(np))
> > +			count++;
> > +	}
> > +
> > +	return count;
> > +}
> > +
> >   static void msm_mdss_irq(struct irq_desc *desc)
> >   {
> >   	struct msm_mdss *msm_mdss = irq_desc_get_handler_data(desc);
> > 
> > ---
> > base-commit: b84a0ebe421ca56995ff78b66307667b62b3a900
> > change-id: 20260316-separate_gpu_kms-04d2cf4d91e2
> > 
> > Best regards,

-- 
With best wishes
Dmitry