linux-kernel - Re: [PATCH 6/7] drm/amdgpu: Ensure kunmap is called on error

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Message-ID: <20211215210949.GW3538886@iweiny-DESK2.sc.intel.com>
Date:   Wed, 15 Dec 2021 13:09:49 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Christian König <christian.koenig@....com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Patrik Jakobsson <patrik.r.jakobsson@...il.com>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        linux-arm-msm@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org
Subject: Re: [PATCH 6/7] drm/amdgpu: Ensure kunmap is called on error

On Tue, Dec 14, 2021 at 08:09:29AM +0100, Christian König wrote:
> Am 14.12.21 um 04:37 schrieb Ira Weiny:
> > On Mon, Dec 13, 2021 at 09:37:32PM +0100, Christian König wrote:
> > > Am 11.12.21 um 00:24 schrieb ira.weiny@...el.com:
> > > > From: Ira Weiny <ira.weiny@...el.com>
> > > > 
> > > > The default case leaves the buffer object mapped in error.
> > > > 
> > > > Add amdgpu_bo_kunmap() to that case to ensure the mapping is cleaned up.
> > > Mhm, good catch. But why do you want to do this in the first place?
> > I'm not sure I understand the question.
> > 
> > Any mapping of memory should be paired with an unmapping when no longer needed.
> > And this is supported by the call to amdgpu_bo_kunmap() in the other
> > non-default cases.
> > 
> > Do you believe the mapping is not needed?
> 
> No, the unmapping is not needed here. See the function amdgpu_bo_kmap(), it
> either creates the mapping or return the cached pointer.

Ah I missed that.  Thanks.

> 
> A call to amdgpu_bo_kunmap() is only done in a few places where we know that
> the created mapping most likely won't be needed any more. If that's not done
> the mapping is automatically destroyed when the BO is moved or freed up.
> 
> I mean good bug fix, but you seem to see this as some kind of prerequisite
> to some follow up work converting TTM to use kmap_local() which most likely
> won't work in the first place.

Sure.  I see now that it is more complicated than I thought but I never thought
of this as a strict prerequisite.  Just something I found while trying to
figure out how this works.

How much of a speed up is it to maintain the ttm_bo_map_kmap map type?  Could
this all be done with vmap and just remove the kmap stuff?

Ira

> 
> Regards,
> Christian.
> 
> > 
> > Ira
> > 
> > > Christian.
> > > 
> > > > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > > > 
> > > > ---
> > > > NOTE: It seems like this function could use a fair bit of refactoring
> > > > but this is the easiest way to fix the actual bug.
> > > > ---
> > > >    drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c | 1 +
> > > >    1 file changed, 1 insertion(+)
> > > > nice
> > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c
> > > > index 6f8de11a17f1..b3ffd0f6b35f 100644
> > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c
> > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c
> > > > @@ -889,6 +889,7 @@ static int amdgpu_uvd_cs_msg(struct amdgpu_uvd_cs_ctx *ctx,
> > > >    		return 0;
> > > >    	default:
> > > > +		amdgpu_bo_kunmap(bo);
> > > >    		DRM_ERROR("Illegal UVD message type (%d)!\n", msg_type);
> > > >    	}
>