Date:   Thu, 4 Jul 2019 10:20:13 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Wu Hao <hao.wu@...el.com>
Cc:     Moritz Fischer <mdf@...nel.org>, linux-fpga@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Zhang Yi Z <yi.z.zhang@...el.com>,
        Xu Yilun <yilun.xu@...el.com>, Alan Tull <atull@...nel.org>
Subject: Re: [PATCH 06/15] fpga: dfl: fme: add
 DFL_FPGA_FME_PORT_RELEASE/ASSIGN ioctl support.

On Thu, Jul 04, 2019 at 02:31:06PM +0800, Wu Hao wrote:
> On Thu, Jul 04, 2019 at 07:39:27AM +0200, Greg KH wrote:
> > On Thu, Jul 04, 2019 at 07:30:58AM +0800, Wu Hao wrote:
> > > On Wed, Jul 03, 2019 at 08:07:53PM +0200, Greg KH wrote:
> > > > On Thu, Jun 27, 2019 at 05:49:42PM -0700, Moritz Fischer wrote:
> > > > > From: Wu Hao <hao.wu@...el.com>
> > > > > 
> > > > > In order to support virtualization usage via PCIe SRIOV, this patch
> > > > > adds two ioctls under FPGA Management Engine (FME) to release and
> > > > > assign back the port device. In order to safely turn Port from PF
> > > > > into VF and enable PCIe SRIOV, it requires user to invoke this
> > > > > PORT_RELEASE ioctl to release port firstly to remove userspace
> > > > > interfaces, and then configure the PF/VF access register in FME.
> > > > > After disable SRIOV, it requires user to invoke this PORT_ASSIGN
> > > > > ioctl to attach the port back to PF.
> > > > > 
> > > > >  Ioctl interfaces:
> > > > >  * DFL_FPGA_FME_PORT_RELEASE
> > > > >    Release platform device of given port, it deletes port platform
> > > > >    device to remove related userspace interfaces on PF, then
> > > > >    configures PF/VF access mode to VF.
> > > > > 
> > > > >  * DFL_FPGA_FME_PORT_ASSIGN
> > > > >    Assign platform device of given port back to PF, it configures
> > > > >    PF/VF access mode to PF, then adds port platform device back to
> > > > >    re-enable related userspace interfaces on PF.
> > > > 
> > > > Why are you not using the "generic" bind/unbind facility that userspace
> > > > already has for this with binding drivers to devices?  Why a special
> > > > ioctl?
> > > 
> > > Hi Greg,
> > > 
> > > Actually we think it should be safer that making the device invisble than
> > > just unbinding its driver. Looks like user can try to rebind it at any
> > > time and we don't have any method to stop them.
> > 
> > Why do you want to "stop" the user from doing something?  They asked to
> > do it, why prevent it?  If they ask to do something foolish, well, they
> > get to keep the pieces :)
> 
> Actually this is for SRIOV support, as we are moving FPGA accelerator from
> PF to VF, so we don't want users to see the FPGA accelerator from PF any
> more. We can't allow user to touch same FPGA accelerator from both PF and
> VF side (it leads to hardware erros).

Ick, ok, this needs to be documented really well then.

> > > > > --- a/include/uapi/linux/fpga-dfl.h
> > > > > +++ b/include/uapi/linux/fpga-dfl.h
> > > > > @@ -176,4 +176,36 @@ struct dfl_fpga_fme_port_pr {
> > > > >  
> > > > >  #define DFL_FPGA_FME_PORT_PR	_IO(DFL_FPGA_MAGIC, DFL_FME_BASE + 0)
> > > > >  
> > > > > +/**
> > > > > + * DFL_FPGA_FME_PORT_RELEASE - _IOW(DFL_FPGA_MAGIC, DFL_FME_BASE + 1,
> > > > > + *					struct dfl_fpga_fme_port_release)
> > > > > + *
> > > > > + * Driver releases the port per Port ID provided by caller.
> > > > > + * Return: 0 on success, -errno on failure.
> > > > > + */
> > > > > +struct dfl_fpga_fme_port_release {
> > > > > +	/* Input */
> > > > > +	__u32 argsz;		/* Structure length */
> > > > > +	__u32 flags;		/* Zero for now */
> > > > > +	__u32 port_id;
> > > > > +};
> > > > 
> > > > meta-comment, why do all of your structures for ioctls have argsz?  You
> > > > "know" the size of the structure already, it's part of the ioctl
> > > > definition.  You shouldn't need to also set it again, right?  Otherwise
> > > > ALL Linux ioctls would need something crazy like this.
> > > 
> > > Actually we followed the same method as vfio.
> > 
> > vfio is a protocol on "the wire", right?  Not an ioctl.
> > 
> > > The major purpose should be extendibility, as we really need this to
> > > be sth long term maintainable.
> > 
> > You can't change ioctl structure sizes at any time.
> > 
> > > It really helps, if we add some new members for extentions/enhancement
> > > under the same ioctl.
> > 
> > You don't do that.
> > 
> > > I don't think everybody needs this, but my consideration here is if
> > > newer generations of hardware/specs come with some extentions, I still
> > > hope we can resue these IOCTLs as much as we could, instead of
> > > creating more new ones.
> > 
> > You create new ones, like everyone else does, as you can not change old
> > code.  By trying to "version" structures like this, it's just going to
> > be a nightmare.
> 
> Actually i learned this from vfio code here, it's not trying to "version"
> structures, let me copy the comments from vfio header file. It should be
> more clear than above short description from me.
> 
>  "include/uapi/linux/vfio.h"
> 
>  /*
>   * The IOCTL interface is designed for extensibility by embedding the
>   * structure length (argsz) and flags into structures passed between
>   * kernel and userspace.  We therefore use the _IO() macro for these
>   * defines to avoid implicitly embedding a size into the ioctl request.
>   * As structure fields are added, argsz will increase to match and flag
>   * bits will be defined to indicate additional fields with valid data.
>   * It's *always* the caller's responsibility to indicate the size of
>   * the structure passed by setting argsz appropriately.
>   */
> 
>  For example.
> 
>  struct vfio_device_info {
>         __u32   argsz;
>         __u32   flags;
>  #define VFIO_DEVICE_FLAGS_RESET (1 << 0)        /* Device supports reset */
>  #define VFIO_DEVICE_FLAGS_PCI   (1 << 1)        /* vfio-pci device */
>  #define VFIO_DEVICE_FLAGS_PLATFORM (1 << 2)     /* vfio-platform device */
>  #define VFIO_DEVICE_FLAGS_AMBA  (1 << 3)        /* vfio-amba device */
>  #define VFIO_DEVICE_FLAGS_CCW   (1 << 4)        /* vfio-ccw device */
>  #define VFIO_DEVICE_FLAGS_AP    (1 << 5)        /* vfio-ap device */
>         __u32   num_regions;    /* Max region index + 1 */
>         __u32   num_irqs;       /* Max IRQ index + 1 */
> 
> Hope things could be more clear now. :)

That's nice for the vfio stuff, but you are just a "normal" driver here.
You want an ioctl that just does one thing, no arguments, no flags, no
anything.  No need for a size argument then at all.  These ioctls don't
even need a structure for them!

Don't try to be fancy, it's not needed, it's not like you are running
out of ioctl space...

thanks,

greg k-h

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