Re: [PATCH v1 1/8] drm/etnaviv: Using the size_t variable to store the number of pages

[Sui Jingfeng]

Hi

On 2023/7/17 17:43, Lucas Stach wrote:
> Hi Jingfeng,
>
> Am Freitag, dem 23.06.2023 um 18:08 +0800 schrieb Sui Jingfeng:
> > From: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
> >
> > Because the etnaviv_gem_new_private() function receives the size_t argument
> > for the number of pages. And the number of pages should be unsigned.
> >
> > Note that Most 32-bit architectures use "unsigned int" size_t,
> > and all 64-bit architectures use "unsigned long" size_t.
> > So, let's keep the argument and parameter consistent.
> This explanation doesn't add up. npages is just that: a number of
> pages. Why would it make sense to use size_t here?

Because the 'size' variable in the etnaviv_gem_prime_import_sg_table() 
function is declared

as size_t type. On 64-bit machine, size_t is actually is 64-bit wide and 
it is *unsigned*.

While 'int' is actually 32-bit wide(at both 32-bit system and 64-bit 
system) and it is *signed*,

So, my point (argument) is that


1) This patch help to avoid the unnecessary 64 bit to 32 bit conversion.

2) The kvmalloc_array() function also accept  size_t type (see the 
prototype of  kvmalloc_array function include/linux/slab.h)


So my patch do helps to keep the code style consistent.


> If you want to be consistent I would have expected this change to
> switch things to unsigned int,

This may introduce a truncate operation (from a 64-bit to 32-bit), which 
is necessary.

And when you pass the 'npages' parameter to kvmalloc_array() function,

The compiler probably has to do the integer promotion (from a 32-bit to 
64-bit) for you.


> as you did in the second patch of this
> series.
>
> Regards,
> Lucas
>
> > Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c | 5 ++---
> >   1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c b/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> > index 3524b5811682..b003481adc2b 100644
> > --- a/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> > +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> > @@ -114,7 +114,8 @@ struct drm_gem_object *etnaviv_gem_prime_import_sg_table(struct drm_device *dev,
> >   {
> >   	struct etnaviv_gem_object *etnaviv_obj;
> >   	size_t size = PAGE_ALIGN(attach->dmabuf->size);
> > -	int ret, npages;
> > +	size_t npages = size / PAGE_SIZE;
> > +	int ret;
> >   
> >   	ret = etnaviv_gem_new_private(dev, size, ETNA_BO_WC,
> >   				      &etnaviv_gem_prime_ops, &etnaviv_obj);
> > @@ -123,8 +124,6 @@ struct drm_gem_object *etnaviv_gem_prime_import_sg_table(struct drm_device *dev,
> >   
> >   	lockdep_set_class(&etnaviv_obj->lock, &etnaviv_prime_lock_class);
> >   
> > -	npages = size / PAGE_SIZE;
> > -
> >   	etnaviv_obj->sgt = sgt;
> >   	etnaviv_obj->pages = kvmalloc_array(npages, sizeof(struct page *), GFP_KERNEL);
> >   	if (!etnaviv_obj->pages) {