Re: [PATCH] drm/msm: restore GEM-related IOCTLs for KMS devices

[Mahadevan P <mahadevan.p@oss.qualcomm.com>]

On 3/5/2026 6:14 AM, Dmitry Baryshkov wrote:
> On Wed, Mar 04, 2026 at 07:57:34AM -0800, Rob Clark wrote:
>> On Wed, Mar 4, 2026 at 5:34 AM Dmitry Baryshkov
>> <dmitry.baryshkov@oss.qualcomm.com> wrote:
>>> The MSM GBM backend uses MSM_GEM_NEW to allocate GEM buffers from the
>>> KMS driver, imports them into the GPU driver (msm or kgsl) and then
>>> uses them for rendering / blending. Commit 98f11fd1cf92 ("drm/msm: Take
>>> the ioctls away from the KMS-only driver") dropped all IOCTLs from the
>>> MSM KMS devices, pointing out the need to use dumb buffers, however dumb
>>> buffers should not be used by the GPU for rendering. Restore GEM-related
>>> IOCTLs for the KMS devices.
>> We shouldn't be exposing DRIVER_RENDER on KMS-only nodes.  If msm gbm
>> is allocating from a primary node, that is problematic, since there is
>> no guarantee that anything but the compositor has access to the
>> primary node.
> We are not exposing DRIVER_RENDER for KMS-only nodes. I'll check with
> the correponding colleagues, what exactly is their use case, if the
> allocation is happening in the compositor or in the app. In the latter
> case, I agree, the buffers should be allocated from the GPU.


Hi Rob,

In combination of msm-kms with kgsl, wayland clients allocate their own 
dma-bufs (typically through EGL/GLES on KGSL) and pass the fd to Weston, 
which imports them. Weston is therefore not allocating client buffers. 
GBM is mainly used for Weston-owned allocations such as render targets, 
composition buffers, and scanout/framebuffers. It can also be used by 
applications that allocate through  EGL (GBM-based path).

One additional consideration is that KGSL is not a DRM driver and does 
not expose a DRM render node. Since |gbm_create_device()| operates on 
DRM devices and relies on DRM interfaces such as |drmGetVersion()|, a 
KGSL device cannot be used as a GBM device. As a result, the MSM DRM/KMS 
node remains the natural GBM allocation point in this configuration. 
Thus we need to add render support in msm-kms driver.

Thanks,
Mahadevan


>> BR,
>> -R
>>
>>> Fixes: 98f11fd1cf92 ("drm/msm: Take the ioctls away from the KMS-only driver")
>>> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
>>> ---
>>> Note, here I assume that dumb buffers generally should not be used for
>>> rendering. That doesn't seem to be complete truth as Mesa kmsro on MSM
>>> devices uses DRM_IOCTL_MODE_CREATE_DUMB to create buffers for resources.
>>> ---
>>>   drivers/gpu/drm/msm/msm_drv.c | 10 ++++++++++
>>>   1 file changed, 10 insertions(+)
>>>
>>> diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
>>> index ed2a61c66ac9..f82c9a36a166 100644
>>> --- a/drivers/gpu/drm/msm/msm_drv.c
>>> +++ b/drivers/gpu/drm/msm/msm_drv.c
>>> @@ -799,6 +799,14 @@ static const struct drm_ioctl_desc msm_ioctls[] = {
>>>          DRM_IOCTL_DEF_DRV(MSM_VM_BIND,      msm_ioctl_vm_bind,      DRM_RENDER_ALLOW),
>>>   };
>>>
>>> +static const struct drm_ioctl_desc msm_kms_ioctls[] = {
>>> +       DRM_IOCTL_DEF_DRV(MSM_GEM_NEW,      msm_ioctl_gem_new,      DRM_RENDER_ALLOW),
>>> +       DRM_IOCTL_DEF_DRV(MSM_GEM_INFO,     msm_ioctl_gem_info,     DRM_RENDER_ALLOW),
>>> +       DRM_IOCTL_DEF_DRV(MSM_GEM_CPU_PREP, msm_ioctl_gem_cpu_prep, DRM_RENDER_ALLOW),
>>> +       DRM_IOCTL_DEF_DRV(MSM_GEM_CPU_FINI, msm_ioctl_gem_cpu_fini, DRM_RENDER_ALLOW),
>>> +       DRM_IOCTL_DEF_DRV(MSM_GEM_MADVISE,  msm_ioctl_gem_madvise,  DRM_RENDER_ALLOW),
>>> +};
>>> +
>>>   static void msm_show_fdinfo(struct drm_printer *p, struct drm_file *file)
>>>   {
>>>          struct drm_device *dev = file->minor->dev;
>>> @@ -868,6 +876,8 @@ static const struct drm_driver msm_kms_driver = {
>>>   #endif
>>>          MSM_FBDEV_DRIVER_OPS,
>>>          .show_fdinfo        = msm_show_fdinfo,
>>> +       .ioctls             = msm_kms_ioctls,
>>> +       .num_ioctls         = ARRAY_SIZE(msm_kms_ioctls),
>>>          .fops               = &fops,
>>>          .name               = "msm-kms",
>>>          .desc               = "MSM Snapdragon DRM",
>>>
>>> ---
>>> base-commit: ac47870fd795549f03d57e0879fc730c79119f4b
>>> change-id: 20260304-msm-restore-ioctls-2a1792311a09
>>>
>>> Best regards,
>>> --
>>> With best wishes
>>> Dmitry
>>>