Re: [PATCH] drm/i915: Check for get_pages instead of shmem (filp)

[Ben Widawsky <benjamin.widawsky@intel.com>]

On Tue, Feb 09, 2016 at 11:30:34AM +0000, Dave Gordon wrote:
> On 09/02/16 00:20, Kristian Høgsberg wrote:
> >On Fri, Feb 5, 2016 at 5:48 PM, Ben Widawsky
> ><benjamin.widawsky@intel.com> wrote:
> >>This behavior of checking for a shmem backed GEM object was introduced here:
> >>commit 4c914c0c7c787b8f730128a8cdcca9c50b0784ab
> >>Author: Brad Volkin <bradley.d.volkin@intel.com>
> >>Date:   Tue Feb 18 10:15:45 2014 -0800
> >>
> >>     drm/i915: Refactor shmem pread setup
> >>
> >>It is possible for an object to not be a shmem backed GEM object (for example
> >>userptr objects). An example of how we hit this failure can be found through
> >>copy_batch() in the command parser because we allocate a userptr object for the
> >>batch which contains privileged instructions. Userptr calls
> >>drm_gem_private_object_init() which explicitly sets the filp to none.
> >>
> >>It is equally feasible to simply remove the check altogether. You'll probably
> >>oops with get_pages somewhere, but that's okay IMO because this condition
> >>should be a driver bug, and not trigger-able by userspace. On this note, the
> >>function name could probably benefit from a change, but whatever.
> >>
> >>NOTE: I manually retyped this from a test machine. So I haven't even compiled
> >>this exact patch.
> >>
> >>Cc: Chris Wilson <chris@chris-wilson.co.uk>
> >>Cc: Kristian Høgsberg <krh@bitplanet.net>
> >>Cc: Jordan Justen <jordan.l.justen@intel.com>
> >>Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
> >>---
> >>  drivers/gpu/drm/i915/i915_gem.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >>diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> >>index 66b1705..a198928 100644
> >>--- a/drivers/gpu/drm/i915/i915_gem.c
> >>+++ b/drivers/gpu/drm/i915/i915_gem.c
> >>@@ -489,7 +489,7 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
> >>
> >>         *needs_clflush = 0;
> >>
> >>-       if (!obj->base.filp)
> >>+       if (!obj->ops->get_pages)
> 
> Don't all subclasses have a get_pages() function?

Yeah, as I said in the commit message, I didn't think this was particularly
appealing, but I figured people would say something if I removed everything. I
wasn't aware of Chris' patch.

> 
> >Do we want to do what Chris did in
> >a2a4f916c2f344d4e596c875dd1e66764afec8b8 (on drm-intel-fixes):
> >
> >+       if (WARN_ON((obj->ops->flags & I915_GEM_OBJECT_HAS_STRUCT_PAGE) == 0))
> >
> >?
> 
> Yes, I think so; i915_gem_shmem_pread() is going to walk the sglist, so the
> object had better have a page array for it to iterate over.
> 
> .Dave.
> 

Okay. I'll send v2. Thanks.


-- 
Ben Widawsky, Intel Open Source Technology Center
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