Hi Laurent,
On Sat, Jul 09, 2016 at 10:29:03PM +0300, Laurent Pinchart wrote:
> Hi Sakari,
>
> On Monday 09 May 2016 16:16:26 Sakari Ailus wrote:
> > Laurent Pinchart wrote:
> > > On Wednesday 04 May 2016 16:09:51 Sakari Ailus wrote:
> > >> Refactor copying the IOCTL argument structs from the user space and back,
> > >> in order to reduce code copied around and make the implementation more
> > >> robust.
> > >>
> > >> As a result, the copying is done while not holding the graph mutex.
> > >>
> > >> Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> > >> ---
> > >> since v2:
> > >>
> > >> - Remove function to calculate maximum argument size, replace by a char
> > >> array of 256 or kmalloc() if that's too small.
> > >>
> > >> drivers/media/media-device.c | 194 ++++++++++++++++---------------------
> > >> 1 file changed, 94 insertions(+), 100 deletions(-)
> > >>
> > >> diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> > >> index 9b5a88d..0797e4b 100644
> > >> --- a/drivers/media/media-device.c
> > >> +++ b/drivers/media/media-device.c
>
> [snip]
>
> > >> @@ -453,10 +432,24 @@ static long __media_device_ioctl(
> > >>
> > >> info = &info_array[_IOC_NR(cmd)];
> > >>
> > >> + if (_IOC_SIZE(info->cmd) > sizeof(__karg)) {
> > >> + karg = kmalloc(_IOC_SIZE(info->cmd), GFP_KERNEL);
> > >> + if (!karg)
> > >> + return -ENOMEM;
> > >> + }
> > >> +
> > >> + info->arg_from_user(karg, arg, cmd);
> > >> +
> > >> mutex_lock(&dev->graph_mutex);
> > >> - ret = info->fn(dev, arg);
> > >> + ret = info->fn(dev, karg);
> > >> mutex_unlock(&dev->graph_mutex);
> > >>
> > >> + if (!ret)
> > >
> > > How about if (!ret && info->arg_to_user) instead, and getting rid of
> > > copy_arg_to_user_nop() ?
> >
> > I thought of that, but I decided to optimise the common case --- which
> > is that the argument is copied back and forth. Not copying the argument
> > back is a very special case, we use it for a single compat IOCTL.
> >
> > That said, we could use it for the proper ENUM_LINKS as well. Still that
> > does not change what's normal.
>
> We're talking about one comparison and one branching instruction (that will
> not be taken in the common case). Is that micro-optimization really worth it
> in an ioctl path that is not that performance-critical ? If you think it is,
> could you analyse what the impact of the copy_arg_to_user_nop() function on
> cache locality is for the common case ? ;-)
I sense a certain amount of insistence in your arguments. Fine, I'll change
it.
You might want to send a patch removing video_device_release_empty() as
well. :-)
--
Regards,
Sakari Ailus
e-mail: sakari.ailus@xxxxxx XMPP: sailus@xxxxxxxxxxxxxx
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