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From: Jason Gunthorpe <jgg@nvidia.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: kvm@vger.kernel.org, "Rafael J. Wysocki" <rafael@kernel.org>
Subject: Re: [PATCH 03/10] driver core: Flow the return code from ->probe() through to sysfs bind
Date: Tue, 8 Jun 2021 11:03:09 -0300	[thread overview]
Message-ID: <20210608140309.GI1002214@nvidia.com> (raw)
In-Reply-To: <YL9twy33JvsaeWt7@kroah.com>

On Tue, Jun 08, 2021 at 03:16:51PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Jun 08, 2021 at 09:30:23AM -0300, Jason Gunthorpe wrote:
> > On Tue, Jun 08, 2021 at 08:47:19AM +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Jun 07, 2021 at 09:55:45PM -0300, Jason Gunthorpe wrote:
> > > > Currently really_probe() returns 1 on success and 0 if the probe() call
> > > > fails. This return code arrangement is designed to be useful for
> > > > __device_attach_driver() which is walking the device list and trying every
> > > > driver. 0 means to keep trying.
> > > > 
> > > > However, it is not useful for the other places that call through to
> > > > really_probe() that do actually want to see the probe() return code.
> > > > 
> > > > For instance bind_store() would be better to return the actual error code
> > > > from the driver's probe method, not discarding it and returning -ENODEV.
> > > 
> > > Why does that matter?  Why does it need to know this?
> > 
> > Proper return code to userspace are important. Knowing why the driver
> > probe() fails is certainly helpful for debugging. Is there are reason
> > to hide them? I think this is an improvement for sysfs bind.
> > 
> > Why this series needs it is because mdev has fixed sys uAPI at this point
> > that requires carring the return code from device driver probe() to
> > a mdev sysfs function.
> 
> What is mdev and what userspace tool requires such a userspace api to
> depend on this?

Were you able to see the cover letter? mdev is part of vfio, it is
very ugly, but it has a userspace ecosystem now.

> Tools doing manual bind/unbind from userspace are crazy, it's always
> been a "look at this neat hack!" type of thing.  To do it "right" you
> should always do it correctly within the kernel.

Which is what the later patches do for mdev, but the dual operation of
creating the struct device and connecting to its driver have a
historical requirement to return the error code from driver probe to
the userspace.

v1 of this just did a hacky approach inside mdev to achieve this but
Dan and CH thought it would be more widely useful so asked for this
series to allow the driver core to handle it. This did turn out fairly
nice so I tend to agree - but returning the error code is important.

> Random boolean flags as parameters are just as bad.

Sure, but the function needs different behavior depending on the call
site. 

An alternative is to rework all the call chains to somehow embed the
difference directly and I don't have a clear vision how to do that
that is any nicer than this.

Jason

  reply	other threads:[~2021-06-08 14:03 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-08  0:55 [PATCH 00/10] Allow mdev drivers to directly create the vfio_device Jason Gunthorpe
2021-06-08  0:55 ` [Intel-gfx] " Jason Gunthorpe
2021-06-08  0:55 ` [PATCH 01/10] driver core: Do not continue searching for drivers if deferred probe is used Jason Gunthorpe
2021-06-08  5:51   ` Christoph Hellwig
2021-06-08  6:44   ` Greg Kroah-Hartman
2021-06-08 12:16     ` Jason Gunthorpe
2021-06-08 13:13       ` Greg Kroah-Hartman
2021-06-08 13:53         ` Jason Gunthorpe
2021-06-08  7:35   ` Greg Kroah-Hartman
2021-06-08 12:17     ` Jason Gunthorpe
2021-06-08  0:55 ` [PATCH 02/10] driver core: Pull required checks into driver_probe_device() Jason Gunthorpe
2021-06-08  5:59   ` Christoph Hellwig
2021-06-08 12:21     ` Jason Gunthorpe
2021-06-08  0:55 ` [PATCH 03/10] driver core: Flow the return code from ->probe() through to sysfs bind Jason Gunthorpe
2021-06-08  6:07   ` Christoph Hellwig
2021-06-08 23:53     ` Jason Gunthorpe
2021-06-08  6:47   ` Greg Kroah-Hartman
2021-06-08 12:30     ` Jason Gunthorpe
2021-06-08 13:16       ` Greg Kroah-Hartman
2021-06-08 14:03         ` Jason Gunthorpe [this message]
2021-06-08  0:55 ` [PATCH 04/10] driver core: Don't return EPROBE_DEFER to userspace during " Jason Gunthorpe
2021-06-08  6:14   ` Christoph Hellwig
2021-06-08  7:37   ` Greg Kroah-Hartman
2021-06-08  0:55 ` [PATCH 05/10] driver core: Export device_driver_attach() Jason Gunthorpe
2021-06-08  6:19   ` Christoph Hellwig
2021-06-08 12:33     ` Jason Gunthorpe
2021-06-08  0:55 ` [PATCH 06/10] vfio/mdev: Remove CONFIG_VFIO_MDEV_DEVICE Jason Gunthorpe
2021-06-08  0:55   ` [Intel-gfx] " Jason Gunthorpe
2021-06-08  6:20   ` Christoph Hellwig
2021-06-08  6:20     ` [Intel-gfx] " Christoph Hellwig
2021-06-11 12:40   ` Cornelia Huck
2021-06-11 12:40     ` [Intel-gfx] " Cornelia Huck
2021-06-14 12:35     ` Jason Gunthorpe
2021-06-14 12:35       ` [Intel-gfx] " Jason Gunthorpe
2021-06-14 12:35       ` Jason Gunthorpe
2021-06-08  0:55 ` [PATCH 07/10] vfio/mdev: Allow the mdev_parent_ops to specify the device driver to bind Jason Gunthorpe
2021-06-08  6:21   ` Christoph Hellwig
2021-06-08  0:55 ` [PATCH 08/10] vfio/mtty: Convert to use vfio_register_group_dev() Jason Gunthorpe
2021-06-08  0:55 ` [PATCH 09/10] vfio/mdpy: " Jason Gunthorpe
2021-06-08  0:55 ` [PATCH 10/10] vfio/mbochs: " Jason Gunthorpe
2021-06-08  6:22 ` [PATCH 00/10] Allow mdev drivers to directly create the vfio_device Christoph Hellwig
2021-06-08  6:22   ` [Intel-gfx] " Christoph Hellwig
2021-06-14 14:34 ` Kirti Wankhede
2021-06-14 14:34   ` [Intel-gfx] " Kirti Wankhede
2021-06-14 14:34   ` Kirti Wankhede
2021-06-14 14:36   ` Jason Gunthorpe
2021-06-14 14:36     ` [Intel-gfx] " Jason Gunthorpe
2021-06-14 14:36     ` Jason Gunthorpe
2021-06-14 15:08 Allow mdev drivers to directly create the vfio_device (v2 / alternative) Christoph Hellwig
2021-06-14 15:08 ` [PATCH 03/10] driver core: Flow the return code from ->probe() through to sysfs bind Christoph Hellwig
2021-06-15  5:18   ` Greg Kroah-Hartman
2021-06-15  5:18     ` Greg Kroah-Hartman
2021-06-15 10:31   ` Cornelia Huck
2021-06-15 10:31     ` Cornelia Huck
2021-06-15 13:35 Allow mdev drivers to directly create the vfio_device (v3) Christoph Hellwig
2021-06-15 13:35 ` [PATCH 03/10] driver core: Flow the return code from ->probe() through to sysfs bind Christoph Hellwig
2021-06-17 14:22 Allow mdev drivers to directly create the vfio_device (v4) Christoph Hellwig
2021-06-17 14:22 ` [PATCH 03/10] driver core: Flow the return code from ->probe() through to sysfs bind Christoph Hellwig

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