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Date:   Wed, 5 Aug 2020 17:17:50 +0200
From:   gregkh <gregkh@linuxfoundation.org>
To:     "Eads, Gage" <gage.eads@intel.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@intel.com>,
        "Topel, Bjorn" <bjorn.topel@intel.com>
Subject: Re: [PATCH 04/20] dlb2: add device ioctl layer and first 4 ioctls
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On Wed, Aug 05, 2020 at 03:07:50PM +0000, Eads, Gage wrote:
> 
> 
> > -----Original Message-----
> > From: gregkh <gregkh@linuxfoundation.org>
> > Sent: Wednesday, August 5, 2020 1:46 AM
> > To: Eads, Gage <gage.eads@intel.com>
> > Cc: Arnd Bergmann <arnd@arndb.de>; linux-kernel@vger.kernel.org;
> > Karlsson, Magnus <magnus.karlsson@intel.com>; Topel, Bjorn
> > <bjorn.topel@intel.com>
> > Subject: Re: [PATCH 04/20] dlb2: add device ioctl layer and first 4 ioctls
> > 
> > On Tue, Aug 04, 2020 at 10:20:47PM +0000, Eads, Gage wrote:
> > > > > +/* [7:0]: device revision, [15:8]: device version */
> > > > > +#define DLB2_SET_DEVICE_VERSION(ver, rev) (((ver) << 8) | (rev))
> > > > > +
> > > > > +static int dlb2_ioctl_get_device_version(struct dlb2_dev *dev,
> > > > > +                                        unsigned long user_arg,
> > > > > +                                        u16 size)
> > > > > +{
> > > > > +       struct dlb2_get_device_version_args arg;
> > > > > +       struct dlb2_cmd_response response;
> > > > > +       int ret;
> > > > > +
> > > > > +       dev_dbg(dev->dlb2_device, "Entering %s()\n", __func__);
> > > > > +
> > > > > +       response.status = 0;
> > > > > +       response.id = DLB2_SET_DEVICE_VERSION(2, DLB2_REV_A0);
> > > > > +
> > > > > +       ret = dlb2_copy_from_user(dev, user_arg, size, &arg,
> > sizeof(arg));
> > > > > +       if (ret)
> > > > > +               return ret;
> > > > > +
> > > > > +       ret = dlb2_copy_resp_to_user(dev, arg.response, &response);
> > > >
> > > > Better avoid any indirect pointers. As you always return a constant
> > > > here, I think the entire ioctl command can be removed until you
> > > > actually need it. If you have an ioctl command that needs both
> > > > input and output, use _IOWR() to define it and put all arguments
> > > > into the same structure.
> > >
> > > I should've caught this in my earlier response, sorry. The device version
> > > command is intentionally the first in the user interface enum. My
> > > goal is for all device versions (e.g. DLB 1.0 in the future) to be accessible
> > > through a /dev/dlb%d node. To allow this, all drivers would support the
> > same
> > > device-version command as command 0, then the subsequent commands
> > can be
> > > tailored to that particular device. User-space would query the version first
> > > to determine which set of ioctl commands it needs to use.
> > >
> > > So even though the response is constant (for now), it must occupy
> > command 0 for
> > > this design to work.
> > 
> > "versions" for ioctls just do not work, please don't go down that path,
> > they should not be needed.  See the many different discussions about
> > this topic on lkml for other subsystem submissions if you are curious.
> > 
> 
> This approach is based on VFIO's modular ioctl design, which has a different
> API for Type1 vs. SPAPR IOMMUs. Similarly a DLB driver could have a different
> API for each device version (but each API would be fixed, not versioned). I
> didn't see any concerns on lkml over VFIO when it was originally submitted -- though
> that was 8 years ago, perhaps the community's feelings have changed since then.

Fixed apis for device types is usually the better way to go.  See the
review comments on the nitro_enclaves driver submission a few weeks ago
for the full details.

greg k-h