linux-kernel - Re: [PATCH] drm/panfrost:modify 'break' to 'continue' to traverse the circulation

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Date:   Tue, 22 Jun 2021 09:31:09 +0800
From:   Chunyou Tang <tangchunyou@....com>
To:     Steven Price <steven.price@....com>
Cc:     tomeu.vizoso@...labora.com, airlied@...ux.ie,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        alyssa.rosenzweig@...labora.com,
        ChunyouTang <tangchunyou@...becorp.cn>
Subject: Re: [PATCH] drm/panfrost:modify 'break' to 'continue' to traverse
 the circulation

Hi Steve,
	I make a mistake about the code branch,I will test it later,
thinks for your reply.

Chunyou

于 Mon, 21 Jun 2021 11:45:18 +0100
Steven Price <steven.price@....com> 写道:

> On 19/06/2021 04:09, Chunyou Tang wrote:
> > Hi Steve,
> > 	1,
> > from
> > https://lore.kernel.org/lkml/31644881-134a-2d6e-dddf-e658a3a8176b@arm.com/
> > I can see what your sent,I used a wrong email address,Now it
> > correct. 2,
> >>> Unless I'm mistaken the situation where some mappings may be NULL
> >>> is caused by the loop in panfrost_lookup_bos() not completing
> >>> successfully
> >>> (panfrost_gem_mapping_get() returning NULL). In this case if
> >>> mappings[i]
> >>> is NULL then all following mappings must also be NULL. So 'break'
> >>> allows
> >>> us to skip the later ones. Admittedly the performance here isn't
> >>> important so I'm not sure it's worth the optimisation, but AIUI
> >>> this code isn't actually wrong.
> > 
> > from panfrost_lookup_bos(),you can see:
> >         for (i = 0; i < job->bo_count; i++) {
> >                 struct panfrost_gem_mapping *mapping;
> > 
> >                 bo = to_panfrost_bo(job->bos[i]);
> >                 ICUBE_DEBUG_PRINTK("panfrost bo gem handle=0x%x
> >                 is_dumb=%d\n", bo->gem_handle, bo->is_dumb);
> >                 if (!bo->is_dumb) {
> >                        mapping = panfrost_gem_mapping_get(bo, priv);
> >                        if (!mapping) {
> >                                 ret = -EINVAL;
> >                                 break;
> >                        }
> > 
> >                         atomic_inc(&bo->gpu_usecount);
> >                         job->mappings[i] = mapping;
> >                 } else {
> >                         atomic_inc(&bo->gpu_usecount);
> >                         job->mappings[i] = NULL;
> >                 }
> >         }
> 
> This code isn't upstream - in drm-misc/drm-misc-next (and all mainline
> kernels from what I can tell) this doesn't have any "is_dumb" test.
> Which branch are you using?
> 
> > if bo->is_dumb is TRUE,the job->mappings[i] will set to NULL,and the
> > while will be continue,so if job->mappings[i] is NULL,the following
> > can not be NULL.
> 
> I agree that with the above code the panfrost_job_cleanup() would need
> changing. But we don't (currently) have this code upstream, so this
> change doesn't make sense upstream.
> 
> Thanks,
> 
> Steve
> 
> > 	3,
> > I've had this problem in our project,the value of is_dumb like
> > these: 0
> > 0
> > 0
> > 1
> > 0
> > 0
> > 0
> > so,when job->mappings[i] is NULL,we can not break the while in 
> > panfrost_job_cleanup().
> > 
> > thanks
> > Chunyou
> > 
> > 于 Fri, 18 Jun 2021 13:43:25 +0100
> > Steven Price <steven.price@....com> 写道:
> > 
> >> On 17/06/2021 09:04, ChunyouTang wrote:
> >>> From: ChunyouTang <tangchunyou@...becorp.cn>
> >>>
> >>> The 'break' can cause 'Memory manager not clean during takedown'
> >>>
> >>> It cannot use break to finish the circulation,it should use
> >>>
> >>> continue to traverse the circulation.it should put every mapping
> >>>
> >>> which is not NULL.
> >>
> >> You don't appear to have answered my question about whether you've
> >> actually seen this happen (and ideally what circumstances). In my
> >> previous email[1] I explained why I don't think this is needed. You
> >> need to convince me that I've overlooked something.
> >>
> >> Thanks,
> >>
> >> Steve
> >>
> >> [1]
> >> https://lore.kernel.org/r/31644881-134a-2d6e-dddf-e658a3a8176b%40arm.com
> >>
> >>> Signed-off-by: ChunyouTang <tangchunyou@...becorp.cn>
> >>> ---
> >>>  drivers/gpu/drm/panfrost/panfrost_job.c | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c
> >>> b/drivers/gpu/drm/panfrost/panfrost_job.c index
> >>> 6003cfeb1322..52bccc1d2d42 100644 ---
> >>> a/drivers/gpu/drm/panfrost/panfrost_job.c +++
> >>> b/drivers/gpu/drm/panfrost/panfrost_job.c @@ -281,7 +281,7 @@
> >>> static void panfrost_job_cleanup(struct kref *ref) if
> >>> (job->mappings) { for (i = 0; i < job->bo_count; i++) {
> >>>  			if (!job->mappings[i])
> >>> -				break;
> >>> +				continue;
> >>>  
> >>>  			atomic_dec(&job->mappings[i]->obj->gpu_usecount);
> >>>  			panfrost_gem_mapping_put(job->mappings[i]);
> >>>
> > 
> >