Re: [RFC PATCH 3/3] drm: add dedicated minor for accelerator devices

[Michał Winiarski]

On Mon, Oct 24, 2022 at 08:43:58PM +0300, Oded Gabbay wrote:
> On Mon, Oct 24, 2022 at 6:21 PM Jeffrey Hugo <quic_jhugo@xxxxxxxxxxx> wrote:
> >
> > On 10/22/2022 3:46 PM, Oded Gabbay wrote:
> > > The accelerator devices are exposed to user-space using a dedicated
> > > major. In addition, they are represented in /dev with new, dedicated
> > > device char names: /dev/accel/accel*. This is done to make sure any
> > > user-space software that tries to open a graphic card won't open
> > > the accelerator device by mistake.
> > >
> > > The above implies that the minor numbering should be separated from
> > > the rest of the drm devices. However, to avoid code duplication, we
> > > want the drm_minor structure to be able to represent the accelerator
> > > device.
> > >
> > > To achieve this, we add a new drm_minor* to drm_device that represents
> > > the accelerator device. This pointer is initialized for drivers that
> > > declare they handle compute accelerator, using a new driver feature
> > > flag called DRIVER_COMPUTE_ACCEL. It is important to note that this
> > > driver feature is mutually exclusive with DRIVER_RENDER. Devices that
> > > want to expose both graphics and compute device char files should be
> > > handled by two drivers that are connected using the auxiliary bus
> > > framework.
> > >
> > > In addition, we define a different idr to handle the accelerators
> > > minors. This is done to make the minor's index be identical to the
> > > device index in /dev/. In most places, this is hidden inside the drm
> > > core functions except when calling drm_minor_acquire(), where I had to
> > > add an extra parameter to specify the idr to use (because the
> > > accelerators minors index and the drm primary minor index both begin
> > > at 0).
> > >
> > > Signed-off-by: Oded Gabbay <ogabbay@xxxxxxxxxx>
> > > ---
> > >   drivers/gpu/drm/drm_drv.c      | 171 +++++++++++++++++++++++++--------
> > >   drivers/gpu/drm/drm_file.c     |  69 +++++++++----
> > >   drivers/gpu/drm/drm_internal.h |   2 +-
> > >   drivers/gpu/drm/drm_sysfs.c    |  29 ++++--
> > >   include/drm/drm_device.h       |   3 +
> > >   include/drm/drm_drv.h          |   8 ++
> > >   include/drm/drm_file.h         |  21 +++-
> > >   7 files changed, 235 insertions(+), 68 deletions(-)
> >
> > Can we please add something to Documentation?  I know this leverages DRM
> > a lot, but I believe that a new subsystem should not be introduced
> > without documentation.  A lot of the info in the commit message is very
> > good, but should not be buried in the git log.
> >
> > Besides, imagine this has been in mainline for N years, and someone
> > completely new to the kernel wants to write an accel driver.  They
> > should be able to get started with something from Documentation that
> > at-least gives that person some insight into what to grep the code for.
> Agreed. The only reason I haven't done it at this stage was because I
> wanted to get an initial reaction to the code itself, see if the
> direction is accepted.
> I didn't want to write documentation and then completely re-write it.
> So I will do it for the next patch-set, once I collect everyone's
> feedback and I see there is a majority agreement.
> >
> > >
> > > diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> > > index b58ffb1433d6..c13701a8d4be 100644
> > > --- a/drivers/gpu/drm/drm_drv.c
> > > +++ b/drivers/gpu/drm/drm_drv.c
> > > @@ -56,6 +56,9 @@ MODULE_LICENSE("GPL and additional rights");
> > >   static DEFINE_SPINLOCK(drm_minor_lock);
> > >   static struct idr drm_minors_idr;
> > >
> > > +static DEFINE_SPINLOCK(accel_minor_lock);
> > > +static struct idr accel_minors_idr;
> >
> > IDR is deprecated.  XArray is the preferred mechanism.
> > Yes, there already is IDR here, but I believe we should not be adding
> > new uses.  Maybe at some point, the current IDR will be converted.  Also
> > with XArray, I think you don't need the spinlock since XArray has
> > internal locking already.
> ok, I wasn't aware. I don't have any problem replacing the idr to xarray.

The conversion is sitting on the mailinglist for a while now
(unfortunately, without much interest).
Perhaps you could help with reviewing it?
https://lore.kernel.org/dri-devel/20220911211443.581481-2-michal.winiarski@xxxxxxxxx/

-Michał

> 
> Thanks,
> Oded
>