[Nouveau] [PATCH v2 2/2] drm/nouveau: add GEM_SET_TILING staging ioctl

[Daniel Vetter]

On Mon, Jun 15, 2015 at 06:08:21PM +0900, Alexandre Courbot wrote:
> On 06/15/2015 04:56 PM, Daniel Vetter wrote:
> >On Mon, Jun 15, 2015 at 04:09:29PM +0900, Alexandre Courbot wrote:
> >>From: Ari Hirvonen <ahirvonen at nvidia.com>
> >>
> >>Add new NOUVEAU_GEM_SET_TILING ioctl to set correct tiling
> >>mode for imported dma-bufs. This ioctl is staging for now
> >>and enabled with the "staging_tiling" module option.
> >>
> >>Signed-off-by: Ari Hirvonen <ahirvonen at nvidia.com>
> >>[acourbot at nvidia.com: carry upstream, many fixes]
> >>Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
> >>---
> >>  drm/nouveau/nouveau_bo.c           | 18 ++++++++++++
> >>  drm/nouveau/nouveau_bo.h           |  2 ++
> >>  drm/nouveau/nouveau_drm.c          |  6 ++++
> >>  drm/nouveau/nouveau_gem.c          | 58 ++++++++++++++++++++++++++++++++++++++
> >>  drm/nouveau/nouveau_gem.h          |  2 ++
> >>  drm/nouveau/nouveau_ttm.c          | 13 +--------
> >>  drm/nouveau/uapi/drm/nouveau_drm.h |  8 ++++++
> >>  7 files changed, 95 insertions(+), 12 deletions(-)
> >>
> >>diff --git a/drm/nouveau/nouveau_bo.c b/drm/nouveau/nouveau_bo.c
> >>index 6edcce1658b7..2a2ebbeb4fc0 100644
> >>--- a/drm/nouveau/nouveau_bo.c
> >>+++ b/drm/nouveau/nouveau_bo.c
> >>@@ -178,6 +178,24 @@ nouveau_bo_fixup_align(struct nouveau_bo *nvbo, u32 flags,
> >>  	*size = roundup(*size, PAGE_SIZE);
> >>  }
> >>
> >>+void
> >>+nouveau_bo_update_tiling(struct nouveau_drm *drm, struct nouveau_bo *nvbo,
> >>+			 struct nvkm_mem *mem)
> >>+{
> >>+	switch (drm->device.info.family) {
> >>+	case NV_DEVICE_INFO_V0_TESLA:
> >>+		if (drm->device.info.chipset != 0x50)
> >>+			mem->memtype = (nvbo->tile_flags & 0x7f00) >> 8;
> >>+		break;
> >>+	case NV_DEVICE_INFO_V0_FERMI:
> >>+	case NV_DEVICE_INFO_V0_KEPLER:
> >>+		mem->memtype = (nvbo->tile_flags & 0xff00) >> 8;
> >>+		break;
> >>+	default:
> >>+		break;
> >>+	}
> >>+}
> >>+
> >>  int
> >>  nouveau_bo_new(struct drm_device *dev, int size, int align,
> >>  	       uint32_t flags, uint32_t tile_mode, uint32_t tile_flags,
> >>diff --git a/drm/nouveau/nouveau_bo.h b/drm/nouveau/nouveau_bo.h
> >>index e42360983229..87d07e3533eb 100644
> >>--- a/drm/nouveau/nouveau_bo.h
> >>+++ b/drm/nouveau/nouveau_bo.h
> >>@@ -69,6 +69,8 @@ nouveau_bo_ref(struct nouveau_bo *ref, struct nouveau_bo **pnvbo)
> >>  extern struct ttm_bo_driver nouveau_bo_driver;
> >>
> >>  void nouveau_bo_move_init(struct nouveau_drm *);
> >>+void nouveau_bo_update_tiling(struct nouveau_drm *, struct nouveau_bo *,
> >>+			      struct nvkm_mem *);
> >>  int  nouveau_bo_new(struct drm_device *, int size, int align, u32 flags,
> >>  		    u32 tile_mode, u32 tile_flags, struct sg_table *sg,
> >>  		    struct reservation_object *robj,
> >>diff --git a/drm/nouveau/nouveau_drm.c b/drm/nouveau/nouveau_drm.c
> >>index 28860268cf38..45a2c88ebf8e 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_tiling, "enable staging GEM_SET_TILING ioctl");
> >>+int nouveau_staging_tiling = 0;
> >>+module_param_named(staging_tiling, nouveau_staging_tiling, int, 0600);
> >
> >Please use _unsafe here to make sure that setting this option taints the
> >kernel and gives at least a bit of a deterrent. But in the end the policy
> >is still that you can't regress anything if people complain, which means
> >you might end up with a staging ioctl locked down forever.
> 
> That would kind of kill the whole purpose of this patchset. But at the same
> time the point of having staging ioctls is to say "don't use them in
> production", so hopefully the message is clear.
> 
> >The other part I don't like with this plan is that it looks a bit like it
> >could be easily abused to evade the open source userspace requirement
> >upstream has for new interfaces. Doesn't help that your first staging
> >ioctl doesn't come with links to mesa/hwc/whatever patches attached ;-)
> 
> Well, you could abuse it - no more than 8 times though. ;)
> 
> The point is not to evade anything though, but rather to have the necessary
> kernel code land in such a way that we can experiment with Mesa and other
> user-space.
> 
> >Overall I don't think this will help - you need internal branch management
> >anyway, and upstreaming new ABI is somewhat painful for a reason: Screwing
> >things up is really expensive long-term, and the drm community has paid
> >that price a few too many times.
> 
> It seems to me that this staging feature can exactly help with that: allow
> new ioctls to mature a bit (no longer than a kernel release cycle) and avoid
> that "ah, I wish we did this differently" moment. But considering the number
> of ABIs I have driven so far (0), someone more experienced may challenge
> that belief.

Maybe some follow up from irc discussions is in order: It's really a
judgment call whether it makes sense. Imo the problem is that marking
something as staging is way too tempting and you get sloppy and end up
with a baked-in abi that you don't really want. Since in the end it
doesn't matter what you declare as staging but whether there are people
complaining about regressions when you break it (famously demonstrated
years ago with nouveau in staging and giant flamours when nouveau broke
their abi, resulting in Linus ulitmately demanding that nouveau gets
destaged asap).

Given that it's imo better to just roll with a final abi. I think in the
past we've overdone things a bit with experimental abis in the drm core,
only the atomic stuff looks big enough in retrospective to really justify
staging it. Everything else was just chickening out from committing to
things right away. In i915 the only thing we stage nowadays is new hw
enabling, simply because we start so early nowadays that with the first
cut we simply don't know yet whether the code will work perfectly on final
silicon. And we've been bitten in the past by regression reports where
people upgraded from old kernels to versions with experimental support and
couldn't boot any more at all. But that's the only exception.

You also mention the issue with having a common assembly ground for
internal patches. Staging ioctls isn't going to solve that, you need
internal trees anyway. We have lots of those for i915, you just can't see
them ;-)

Summary of my stance: In almost all cases staging ioctls is just an
illusion and doesn't give you real benefits.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch