Messages in this thread | | | Date | Wed, 5 Aug 2020 17:17:50 +0200 | From | gregkh <> | 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
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