linux-kernel - Re: [PATCH v1 3/8] drm/etnaviv: Drop the second argument of the etnaviv_gem_new

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Date:   Tue, 18 Jul 2023 10:12:25 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     suijingfeng <suijingfeng@...ngson.cn>,
        Sui Jingfeng <sui.jingfeng@...ux.dev>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     loongson-kernel@...ts.loongnix.cn, etnaviv@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/8] drm/etnaviv: Drop the second argument of the
 etnaviv_gem_new_impl()

Hi Jingfeng,

Am Dienstag, dem 18.07.2023 um 02:34 +0800 schrieb suijingfeng:
> Hi,  Lucas
> 
> 
> Thanks for you guidance!
> 
> 
> On 2023/7/17 17:51, Lucas Stach wrote:
> > Hi Jingfeng,
> > 
> > Am Freitag, dem 23.06.2023 um 18:08 +0800 schrieb Sui Jingfeng:
> > > From: Sui Jingfeng <suijingfeng@...ngson.cn>
> > > 
> > > Because it is not used by the etnaviv_gem_new_impl() function,
> > > no functional change.
> > > 
> > I think it would make sense to move into the opposite direction: in
> > both callsites of etnaviv_gem_new_impl we immediately call
> > drm_gem_object_init with the size.
> 
> Really?
> 
> But there are multiple call path to the etnaviv_gem_new_impl() function.
> 
> 
> Code path 1 (PRIME):
> 
> > - etnaviv_gem_prime_import_sg_table()
> 
> > --  etnaviv_gem_new_private()
> 
> > --- etnaviv_gem_new_impl(dev, size, flags, ops, &obj)
> 
> > --- drm_gem_private_object_init(dev, obj, size)
> 
> 
> Code path 2 (USERPTR):
> 
> > - etnaviv_gem_new_userptr()
> 
> > --  etnaviv_gem_new_private()
> 
> > --- etnaviv_gem_new_impl(dev, size, flags, ops, &obj)
> 
> > --- drm_gem_private_object_init(dev, obj, size)
> 
> 
> Code path 3 (construct a GEM buffer object for the user-space):
> 
> > - etnaviv_ioctl_gem_new()
> 
> > -- etnaviv_gem_new_handle()
> 
> > --- etnaviv_gem_new_impl(dev, size, flags, &etnaviv_gem_shmem_ops, &obj);
> 
> > ---  drm_gem_object_init(dev, obj, size);
> 
> 
> If I understand this correctly:
> 
> 
> Code path 1 is for cross device (and cross driver) buffer-sharing,
> 
> Code path 2 is going to share the buffer the userspace,
> 
> 
> *Only* the code path 3 is to construct a GEM buffer object for the 
> user-space the userspace,
> 
> that is say, *only* the code path 3 need to do the backing memory 
> allocation work for the userspace.
> 
> thus it need to call drm_gem_object_init() function, which really the 
> shmem do the backing memory
> 
> allocation.
> 
> 
> The code path 1 and the code path 2 do not need the kernel space 
> allocate the backing memory.
> 
> Because they are going to share the buffer already allocated by others.
> 
> thus, code path 2 and code path 3 should call drm_gem_private_object_init(),
> 
> *not* the drm_gem_object_init() function.
> 
> 
> When import buffer from the a specific KMS driver,
> 
> then etnaviv_gem_prime_import_sg_table() will be called.
> 
> 
> I guess you means that drm_gem_private_object_init() (not the 
> drm_gem_object_init() function)here ?
> 
> 
> > A better cleanup would be to make
> > use of the size parameter and move this object init call into
> > etnaviv_gem_new_impl.
> 
> If I following you guidance, how do I differentiate the cases
> 
> when to call drm_gem_private_object_init() not drm_gem_object_init() ?
> 
> and when call drm_gem_object_init() not drm_gem_private_object_init()?
> 
> 
> I don't think you are right here.
> 
Yes, clearly I was not taking into account the differences between
drm_gem_private_object_init and drm_gem_object_init properly. Please
disregard my comment, this patch is good as-is.

Regards,
Lucas

> > 
> > Regards,
> > Lucas
> > 
> > > Signed-off-by: Sui Jingfeng <suijingfeng@...ngson.cn>
> > > ---
> > >   drivers/gpu/drm/etnaviv/etnaviv_gem.c | 7 +++----
> > >   1 file changed, 3 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> > > index b5f73502e3dd..be2f459c66b5 100644
> > > --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> > > +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> > > @@ -542,7 +542,7 @@ static const struct drm_gem_object_funcs etnaviv_gem_object_funcs = {
> > >   	.vm_ops = &vm_ops,
> > >   };
> > >   
> > > -static int etnaviv_gem_new_impl(struct drm_device *dev, u32 size, u32 flags,
> > > +static int etnaviv_gem_new_impl(struct drm_device *dev, u32 flags,
> > >   	const struct etnaviv_gem_ops *ops, struct drm_gem_object **obj)
> > >   {
> > >   	struct etnaviv_gem_object *etnaviv_obj;
> > > @@ -591,8 +591,7 @@ int etnaviv_gem_new_handle(struct drm_device *dev, struct drm_file *file,
> > >   
> > >   	size = PAGE_ALIGN(size);
> > >   
> > > -	ret = etnaviv_gem_new_impl(dev, size, flags,
> > > -				   &etnaviv_gem_shmem_ops, &obj);
> > > +	ret = etnaviv_gem_new_impl(dev, flags, &etnaviv_gem_shmem_ops, &obj);
> > >   	if (ret)
> > >   		goto fail;
> > >   
> > > @@ -627,7 +626,7 @@ int etnaviv_gem_new_private(struct drm_device *dev, size_t size, u32 flags,
> > >   	struct drm_gem_object *obj;
> > >   	int ret;
> > >   
> > > -	ret = etnaviv_gem_new_impl(dev, size, flags, ops, &obj);
> > > +	ret = etnaviv_gem_new_impl(dev, flags, ops, &obj);
> > >   	if (ret)
> > >   		return ret;
> > >   
>