linux-kernel - Re: [PATCH v3 06/11] staging/android: turn fence_info into a _

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Date:	Thu, 4 Feb 2016 11:05:42 -0200
From:	Gustavo Padovan <gustavo.padovan@...labora.co.uk>
To:	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc:	Gustavo Padovan <gustavo@...ovan.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	dri-devel@...ts.freedesktop.org,
	Daniel Stone <daniels@...labora.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Riley Andrews <riandrews@...roid.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Rob Clark <robdclark@...il.com>,
	Greg Hackmann <ghackmann@...gle.com>,
	John Harrison <John.C.Harrison@...el.com>
Subject: Re: [PATCH v3 06/11] staging/android: turn fence_info into a __u64
 pointer

2016-02-04 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>:

> Op 03-02-16 om 21:09 schreef Gustavo Padovan:
> > Hi Maarten,
> >
> > 2016-02-03 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>:
> >
> >> Op 03-02-16 om 14:25 schreef Gustavo Padovan:
> >>> From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> >>>
> >>> Turn sync_fence_info into __u64 type enable us to extend the struct in the
> >>> future without breaking the ABI.
> >>>
> >>> v2: use type __u64 for fence_info
> >>>
> >>> v3: fix commit message to reflect the v2 change
> >>>
> >>> Signed-off-by: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> >>> ---
> >>>  drivers/staging/android/sync.c      | 2 +-
> >>>  drivers/staging/android/uapi/sync.h | 2 +-
> >>>  2 files changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/staging/android/sync.c b/drivers/staging/android/sync.c
> >>> index 2ab0c20..8425457 100644
> >>> --- a/drivers/staging/android/sync.c
> >>> +++ b/drivers/staging/android/sync.c
> >>> @@ -525,7 +525,7 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
> >>>  	if (info->status >= 0)
> >>>  		info->status = !info->status;
> >>>  
> >>> -	len = sizeof(struct sync_file_info);
> >>> +	len = sizeof(struct sync_file_info) - sizeof(__u64);
> >>>  
> >>>  	for (i = 0; i < sync_file->num_fences; ++i) {
> >>>  		struct fence *fence = sync_file->cbs[i].fence;
> >>> diff --git a/drivers/staging/android/uapi/sync.h b/drivers/staging/android/uapi/sync.h
> >>> index a0cf357..e649953 100644
> >>> --- a/drivers/staging/android/uapi/sync.h
> >>> +++ b/drivers/staging/android/uapi/sync.h
> >>> @@ -54,7 +54,7 @@ struct sync_file_info {
> >>>  	char	name[32];
> >>>  	__s32	status;
> >>>  
> >>> -	__u8	sync_fence_info[0];
> >>> +	__u64	sync_fence_info;
> >>>  };
> >>>  
> >>>  #define SYNC_IOC_MAGIC		'>'
> >> This still doesn't do what you expect it to.
> >>
> >> I think this is what you want is for userspace to do:
> >>
> >> struct sync_file_info info;
> >>
> >> info.flags = info.num_fences = 0;
> >> ioctl(fd, SYNC_IOC_FENCE_INFO, &info);
> >> if (info.num_fences) {
> >> info.sync_fence_info = (uintptr)kcalloc(info.num_fences, sizeof(struct sync_fence_info));
> >> ioctl(fd, SYNC_IOC_FENCE_INFO, &info);
> >> }
> >>
> >> Maybe userspace could preallocate the max in advance and set num_fences higher,
> >>
> >> kernel would do something like:
> >>
> >> num_fences = min(info.num_fences, sync->num_fences);
> >> struct sync_fence_info array[num_fences];
> >>
> >> info.num_fences = sync->num_fences;
> >> if (num_fences &&
> >>     copy_to_user((void * __user)(unsigned long)info.sync_fence_info, array, num_fences  * sizeof(array)))
> >>  return -EFAULT;
> > If we are going to call IOCTL twice I would actually have a new IOCTL only
> > to fetch sync_fence_info.
> >
> > First we would call
> >
> > ioctl(fd, SYNC_IOC_FILE_INFO, &info);
> >
> > where info is:
> >
> > struct sync_file_info {    
> >         char    name[32];  
> >         __s32   status;    
> >         __u32   flags;     
> >         __u32   num_fences;
> > };
> >
> > then we would allocate a buffer with
> >
> > size = info.num_fences * sizeof(struct sync_fence_info)
> >
> > and call the new ioctl
> >
> > ioctl(fd, SYNC_IOC_SYNC_FENCE_INFO, sync_fence_info);
> >
> > This looks like a cleaner solution and doesn't break ABI. What do you
> > think?
> I think it's good taste that userspace specifies the size of the buffer it passes, so former feels more clean to me,
> since you need to pass num_fences anyway.

Just to clarify, userspace specifies the size of the buffer in the
solution I proposed. It would be

	size = info.num_fences * sizeof(struct sync_fence_info)

	sync_fence_info = malloc(size);

	ioctl(fd, SYNC_IOC_SYNC_FENCE_INFO, sync_fence_info);