linux-kernel - Re: [PATCH] agp: remove read/write stubs

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Date:	Wed, 9 Jul 2014 08:52:52 +0200
From:	Mathias Krause <minipli@...glemail.com>
To:	Dave Airlie <airlied@...il.com>
Cc:	David Airlie <airlied@...ux.ie>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] agp: remove read/write stubs

On 9 July 2014 00:48, Dave Airlie <airlied@...il.com> wrote:
> On 2 July 2014 16:15, Mathias Krause <minipli@...glemail.com> wrote:
>> On 15 June 2014 23:02, Mathias Krause <minipli@...glemail.com> wrote:
>>> The VFS layer handles those in the very same way, if unset. No need for
>>> additional stubs.
>>>
>>> Signed-off-by: Mathias Krause <minipli@...glemail.com>
>>> Cc: Alexander Viro <viro@...iv.linux.org.uk>
>>> ---
>>>  drivers/char/agp/frontend.c |   15 ---------------
>>>  1 file changed, 15 deletions(-)
>>>
>>> diff --git a/drivers/char/agp/frontend.c b/drivers/char/agp/frontend.c
>>> index b29703324e..09f17eb734 100644
>>> --- a/drivers/char/agp/frontend.c
>>> +++ b/drivers/char/agp/frontend.c
>>> @@ -710,19 +710,6 @@ static int agp_open(struct inode *inode, struct file *file)
>>>         return 0;
>>>  }
>>>
>>> -
>>> -static ssize_t agp_read(struct file *file, char __user *buf,
>>> -                       size_t count, loff_t * ppos)
>>> -{
>>> -       return -EINVAL;
>>> -}
>>> -
>>> -static ssize_t agp_write(struct file *file, const char __user *buf,
>>> -                        size_t count, loff_t * ppos)
>>> -{
>>> -       return -EINVAL;
>>> -}
>>> -
>>>  static int agpioc_info_wrap(struct agp_file_private *priv, void __user *arg)
>>>  {
>>>         struct agp_info userinfo;
>>> @@ -1047,8 +1034,6 @@ static const struct file_operations agp_fops =
>>>  {
>>>         .owner          = THIS_MODULE,
>>>         .llseek         = no_llseek,
>>> -       .read           = agp_read,
>>> -       .write          = agp_write,
>>>         .unlocked_ioctl = agp_ioctl,
>>>  #ifdef CONFIG_COMPAT
>>>         .compat_ioctl   = compat_agp_ioctl,
>>> --
>>> 1.7.10.4
>>>
>>
>> Ping. Any objections, comments? David?
>
> Are you sure VFS handles this the same way? can you point me where it
> checks ->read and returns -EINVAL, I'm probably just missing it in my
> quick look over fs/read_write.c

The VFS code changed during the last merge window so the ->read and
->write tests are no longer directly visible. The presence of a ->read
/ ->write function is now remembered at open(2) time and stored in
->f_mode as a flag.

The corresponding commit for the VFS change is 7f7f25e82d "replace
checking for ->read/->aio_read presence with check in ->f_mode".
Despite its subject, it also handles ->write / ->aio_write. The flags
are set in fs/open:do_dentry_open():

    if ((f->f_mode & FMODE_READ) &&
         likely(f->f_op->read || f->f_op->aio_read || f->f_op->read_iter))
        f->f_mode |= FMODE_CAN_READ;
    if ((f->f_mode & FMODE_WRITE) &&
         likely(f->f_op->write || f->f_op->aio_write || f->f_op->write_iter))
        f->f_mode |= FMODE_CAN_WRITE;

With that change in mind, the redundancy of the agp_read() and
agp_write() functions can be seen for read(2) in vfs_read():

    if (!(file->f_mode & FMODE_CAN_READ))
        return -EINVAL;

..and for write(2) in vfs_write():

    if (!(file->f_mode & FMODE_CAN_WRITE))
        return -EINVAL;

The other entry points (readv / writev / compat wrappers) have been
changed accordingly, so those handle a missing ->read / ->write
callback in the same way: returning -EINVAL.

Regards,
Mathias
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