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

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Message-ID: <20210619110923.00001c64@163.com>
Date:   Sat, 19 Jun 2021 11:09:23 +0800
From:   Chunyou Tang <tangchunyou@....com>
To:     Steven Price <steven.price@....com>
Cc:     robh@...nel.org, tomeu.vizoso@...labora.com,
        alyssa.rosenzweig@...labora.com, airlied@...ux.ie, daniel@...ll.ch,
        ChunyouTang <tangchunyou@...becorp.cn>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/panfrost:modify 'break' to 'continue' to traverse
 the circulation

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;
                }
        }
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.

	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]);
> >