linux-kernel - Re: [PATCH] drm/vgm: replace idr_init() by idr_init

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Message-ID: <CAKMK7uF0buRA-FfoY2=FbZQO-cqgSwZXRWwNQ2xkBdZAS=JuRA@mail.gmail.com>
Date:   Thu, 5 Nov 2020 12:16:34 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Deepak R Varma <mh12gx2825@...il.com>
Cc:     David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/vgm: replace idr_init() by idr_init_base()

On Thu, Nov 5, 2020 at 11:31 AM Deepak R Varma <mh12gx2825@...il.com> wrote:
>
> On Thu, Nov 05, 2020 at 10:42:15AM +0100, Daniel Vetter wrote:
> > On Wed, Nov 04, 2020 at 04:53:38PM +0530, Deepak R Varma wrote:
> > > idr_init() uses base 0 which is an invalid identifier. The new function
> > > idr_init_base allows IDR to set the ID lookup from base 1. This avoids
> > > all lookups that otherwise starts from 0 since 0 is always unused.
> > >
> > > References: commit 6ce711f27500 ("idr: Make 1-based IDRs more efficient")
> > >
> > > Signed-off-by: Deepak R Varma <mh12gx2825@...il.com>
> >
> > Tiny typo in the commit message summary: s/vgm/vgem/
> >
> > Also can you pls resbumit this with intel-gfx mailing list on cc (like for
> > i915)? There's a CI bot there which runs a few vgem tests, would be good
> > to confirm nothing has been broken.
>
> Hi Daniel,
> sure. I will correct the summary typo and also feed it to the CI bot.
>
> Also, according to Felix Kuehling's comment on a similar patch for
> drm/amdkfd driver, an ID can be 0. The change I am proposing is more
> efficient for conditions that do not want to use ID as 0. Otherwise,
> id = 0 is an acceptable possibility. So, my statement that "Id 0 is an invalid
> identifier" is not true.
>
> Can you please comment if this is accurate and I should reword my log
> message as well?

You need to review the vgem code to see whether we're using id 0 as
invalid identifier or not. That's part of the work that needs to be
done here. Best would be to explain the evidence you've found in the
commit message, why id 0 is invalid for this specific code. Since yes
in general that's not true, it depends how the idr is used.
-Daniel

>
> Thank you.
> ./drv
>
> >
> > Otherwise lgtm.
> >
> > Thanks, Daniel
> >
> > > ---
> > >  drivers/gpu/drm/vgem/vgem_fence.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/vgem/vgem_fence.c b/drivers/gpu/drm/vgem/vgem_fence.c
> > > index 17f32f550dd9..2902dc6e64fa 100644
> > > --- a/drivers/gpu/drm/vgem/vgem_fence.c
> > > +++ b/drivers/gpu/drm/vgem/vgem_fence.c
> > > @@ -233,7 +233,7 @@ int vgem_fence_signal_ioctl(struct drm_device *dev,
> > >  int vgem_fence_open(struct vgem_file *vfile)
> > >  {
> > >     mutex_init(&vfile->fence_mutex);
> > > -   idr_init(&vfile->fence_idr);
> > > +   idr_init_base(&vfile->fence_idr, 1);
> > >
> > >     return 0;
> > >  }
> > > --
> > > 2.25.1
> > >
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch



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