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

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Message-ID: <20201105174045.GA41810@localhost>
Date:   Thu, 5 Nov 2020 23:10:45 +0530
From:   Deepak R Varma <mh12gx2825@...il.com>
To:     Daniel Vetter <daniel@...ll.ch>
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 05, 2020 at 12:16:34PM +0100, Daniel Vetter wrote:
> 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
> 
You are correct. For the vgem driver, id 0 is not used. The patch
should then apply to this driver.

Thank you very much Daniel. I have just sent v2 of the patch with your
suggestions.

./drv

> >
> > 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