[PATCH 1/2] drm/nouveau: add staging module option

[Ben Skeggs]

On 21 May 2015 at 15:49, Alexandre Courbot <gnurou at gmail.com> wrote:
> On Thu, May 21, 2015 at 1:48 PM, Ben Skeggs <skeggsb at gmail.com> wrote:
>> On 20 May 2015 at 15:56, Alexandre Courbot <acourbot at nvidia.com> wrote:
>>> Add a module option allowing to enable staging/unstable APIs. This will
>>> allow us to experiment freely with experimental APIs for a while before
>>> setting them in stone.
>>>
>>> Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
>>> ---
>>>  drm/nouveau/nouveau_drm.c          | 18 ++++++++++++++++++
>>>  drm/nouveau/uapi/drm/nouveau_drm.h |  3 +++
>>>  2 files changed, 21 insertions(+)
>>>
>>> diff --git a/drm/nouveau/nouveau_drm.c b/drm/nouveau/nouveau_drm.c
>>> index 89049335b738..e4bd6ed51e73 100644
>>> --- a/drm/nouveau/nouveau_drm.c
>>> +++ b/drm/nouveau/nouveau_drm.c
>>> @@ -75,6 +75,10 @@ MODULE_PARM_DESC(runpm, "disable (0), force enable (1), optimus only default (-1
>>>  int nouveau_runtime_pm = -1;
>>>  module_param_named(runpm, nouveau_runtime_pm, int, 0400);
>>>
>>> +MODULE_PARM_DESC(staging, "enable staging APIs");
>>> +int nouveau_staging = 0;
>>> +module_param_named(staging, nouveau_staging, int, 0400);
>>> +
>>>  static struct drm_driver driver_stub;
>>>  static struct drm_driver driver_pci;
>>>  static struct drm_driver driver_platform;
>>> @@ -895,6 +899,7 @@ nouveau_ioctls[] = {
>>>         DRM_IOCTL_DEF_DRV(NOUVEAU_GEM_CPU_PREP, nouveau_gem_ioctl_cpu_prep, DRM_UNLOCKED|DRM_AUTH|DRM_RENDER_ALLOW),
>>>         DRM_IOCTL_DEF_DRV(NOUVEAU_GEM_CPU_FINI, nouveau_gem_ioctl_cpu_fini, DRM_UNLOCKED|DRM_AUTH|DRM_RENDER_ALLOW),
>>>         DRM_IOCTL_DEF_DRV(NOUVEAU_GEM_INFO, nouveau_gem_ioctl_info, DRM_UNLOCKED|DRM_AUTH|DRM_RENDER_ALLOW),
>>> +       /* Staging ioctls */
>>>  };
>>>
>>>  long
>>> @@ -1027,6 +1032,7 @@ static void nouveau_display_options(void)
>>>         DRM_DEBUG_DRIVER("... runpm        : %d\n", nouveau_runtime_pm);
>>>         DRM_DEBUG_DRIVER("... vram_pushbuf : %d\n", nouveau_vram_pushbuf);
>>>         DRM_DEBUG_DRIVER("... pstate       : %d\n", nouveau_pstate);
>>> +       DRM_DEBUG_DRIVER("... staging      : %d\n", nouveau_staging);
>>>  }
>>>
>>>  static const struct dev_pm_ops nouveau_pm_ops = {
>>> @@ -1088,6 +1094,18 @@ err_free:
>>>  static int __init
>>>  nouveau_drm_init(void)
>>>  {
>>> +       /* Do not register staging ioctsl if option not specified */
>>> +       if (!nouveau_staging) {
>>> +               unsigned i;
>>> +
>>> +               /* This keeps us safe is no staging ioctls are defined */
>>> +               i = min(driver_stub.num_ioctls, DRM_NOUVEAU_STAGING_IOCTL);
>>> +               while (!nouveau_ioctls[i - 1].func)
>>> +                       i--;
>>> +
>>> +               driver_stub.num_ioctls = i;
>>> +       }
>> Hey Alex,
>>
>> I've got no specific objection.  But I'm curious as to why you took
>> this approach as opposed to just adding "if (!nouveau_staging) return
>> -EINVAL;" directly in the experimental ioctls?
>
> Mainly because we will likely forget to add this check (or to remove
> it) in some of the staging ioctls. The current solution doesn't
> require us to think about that - and the less things to think about,
> the better.
>
>> I think, in line with
>> what's been done in other places, having module options per-api is
>> perhaps a better choice too.
>
> Do you mean that each experimental ioctl should have its own enable
> option? I don't mind going that way if you think it is preferable. And
> in that case my comment above is void.
That would be more preferable I think, and obvious as to what exactly
you're enabling.

>
> But actually I wonder if having these experimental ioctls enabled as
> compile options (either individually or as a whole) would not be
> better. Some experimental ioctls may require code in staging (like the
> PUSHBUF_2 ioctl that I would like to submit next), and I don't think
> it is desirable to force extra code or kernel options (in this case,
> CONFIG_STAGING) to Nouveau users who will not make use of them. I
> remember that we concluded in favor or module options on IRC, but in
> the light of this, wouldn't a config option be a less intrusive
> choice?
Right, but the whole point of this is to encourage the ioctls to not
live there for too long, and progress to fully supported interfaces.

Ben.