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

[Steven Price]

On 19/06/2021 04:09, Chunyou Tang wrote:
> Hi Steve,
> 	1,
> from
> https://lore.kernel.org/lkml/31644881-134a-2d6e-dddf-e658a3a8176b@xxxxxxx/
> 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
> 
> ÓÚ Fri, 18 Jun 2021 13:43:25 +0100
> Steven Price <steven.price@xxxxxxx> Đ´ľŔ:
> 
>> On 17/06/2021 09:04, ChunyouTang wrote:
>>> From: ChunyouTang <tangchunyou@xxxxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxxxx>
>>> ---
>>>  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]);
>>>
> 
>