[PATCH v2 1/9] drm: Do delayed switcheroo in drm_lastclose()

Alex Deucher alexdeucher at gmail.com
Mon Aug 12 18:47:59 UTC 2024 

On Mon, Aug 12, 2024 at 10:40 AM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> On Mon, Aug 12, 2024 at 12:41:39PM +0200, Thomas Zimmermann wrote:
> > Hi
> >
> > Am 12.08.24 um 12:18 schrieb Daniel Vetter:
> > > On Mon, Aug 12, 2024 at 11:23:44AM +0200, Daniel Vetter wrote:
> > > > On Mon, Aug 12, 2024 at 10:28:22AM +0200, Thomas Zimmermann wrote:
> > > > > Amdgpu and nouveau call vga_switcheroo_process_delayed_switch() from
> > > > > their lastclose callbacks. Call it from drm_lastclose(), so that the
> > > > > driver functions can finally be removed. Only PCI devices with enabled
> > > > > switcheroo do the delayed switching. The call has no effect on other
> > > > > hardware.
> > > > >
> > > > > v2:
> > > > > - move change to drm_lastclose() (Sima)
> > > > > - update docs for vga_switcheroo_process_delayed_switch()
> > > > >
> > > > > Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
> > > > A bit an aside: The entire vgaswitcheroo code is still a midlayer mess,
> > > > where the locking is at the wrong layers resulting in the can_switch check
> > > > potentially being racy. But that's a different can of worms.
> > > Ok I got a bit annoyed about this mess again, and I think I have a
> > > reasonable idea for how to address it. Not sure why this took a decade,
> > > and definitely only pick this up if you're really bored.
> >
> > No, definitely not. :) I don't think I have hardware for testing
> > vga_switcheroo. Does this still exist? It seemed to be a thing of the 2000s.
>
> Yeah good chances the old style switcheroo doesn't have many, if any users
> left ...

The power management aspect would be better handled by runtime pm.
That just leaves the output muxes.  We could arguably limit
vgaswitcheroo to just systems with the old mux controls.  If and when
we come up with a proper solution for display muxes, we could migrate
to that, but I'm not sure how many systems out there with the old mux
controls are still in use.  I'm not even sure amdgpu supports any
chips which used the old mux controls.  I think those were all chips
supported by radeon.

Alex

> -Sima
>
> >
> > Best regards
> > Thomas
> >
> > >
> > > - We add a new vga_switcheroo_client_tryget, which checks the current
> > >    state, and if it's on, increments a newly added refcount (which vgw
> > >    switheroo maintains). Otherwise it fails. Drivers call this from their
> > >    drm_driver->open hook. This check also allows us to drop the
> > >    layer-violating checks in drm_open_helper for drm_dev->dev_power_state.
> > >
> > > - That refcount is dropped with vga_switcheroo_client_put, called from
> > >    drm_driver->close. If the refcount drops to 0 this function also does
> > >    delayed switch processing.
> > >
> > > - All the can_switch callbacks get removed and instead the vgwswr code
> > >    directly consults its own refount.
> > >
> > > With this we don't have locking inversions anymore, and the old vgw
> > > switcheroo code works a lot more like the new mode based on runtime pm and
> > > power domains.
> > >
> > > With a bit more shuffling I think we can also ditch
> > > drm_driver->dev_power_state:
> > >
> > > - There's one in the intel backlight code, which is annoying, since it's
> > >    wants to know whether the current callchain is from a vga switcheroo
> > >    state change. But doable with a little helper.
> > >
> > > - Most others just want a vga_switcheroo_client_is_off() helper, which
> > >    should be easy. Some are even entirely redundant, at least from a cursor
> > >    callchain check. There's no races for these because they only matter
> > >    during system suspend, since you should not mix both runtime and classic
> > >    vgaswitcheroo logic. We might want some checks for that in that new
> > >    helper ...
> > >
> > > - The one in the fbdev code is annoying, because it's another race.
> > >    Ideally instead of that check it needs a call to
> > >    vga_switcheroo_client_tryget/put just around the call to restore modes
> > >    (we do not want fbdev to block state switches), but that probably means
> > >    wiring a new callback through drm_client to drivers.
> > >
> > > - Might have missed a special case ...
> > >
> > > Anyway, I got nerdsniped, had an idea, figured best to type it up. Maybe
> > > we want to add a link to this to todo.rst, I think we have a vgaswitcheroo
> > > entry already.
> > >
> > > Cheers, Sima
> > >
> > >
> > > > Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> > > >
> > > > > ---
> > > > >   drivers/gpu/drm/drm_file.c       | 4 ++++
> > > > >   drivers/gpu/vga/vga_switcheroo.c | 3 +--
> > > > >   2 files changed, 5 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> > > > > index 714e42b05108..513bef816ae9 100644
> > > > > --- a/drivers/gpu/drm/drm_file.c
> > > > > +++ b/drivers/gpu/drm/drm_file.c
> > > > > @@ -38,6 +38,7 @@
> > > > >   #include <linux/pci.h>
> > > > >   #include <linux/poll.h>
> > > > >   #include <linux/slab.h>
> > > > > +#include <linux/vga_switcheroo.h>
> > > > >   #include <drm/drm_client.h>
> > > > >   #include <drm/drm_drv.h>
> > > > > @@ -404,6 +405,9 @@ void drm_lastclose(struct drm_device * dev)
> > > > >         drm_dbg_core(dev, "driver lastclose completed\n");
> > > > >         drm_client_dev_restore(dev);
> > > > > +
> > > > > +       if (dev_is_pci(dev->dev))
> > > > > +               vga_switcheroo_process_delayed_switch();
> > > > >   }
> > > > >   /**
> > > > > diff --git a/drivers/gpu/vga/vga_switcheroo.c b/drivers/gpu/vga/vga_switcheroo.c
> > > > > index 365e6ddbe90f..18f2c92beff8 100644
> > > > > --- a/drivers/gpu/vga/vga_switcheroo.c
> > > > > +++ b/drivers/gpu/vga/vga_switcheroo.c
> > > > > @@ -926,8 +926,7 @@ static void vga_switcheroo_debugfs_init(struct vgasr_priv *priv)
> > > > >   /**
> > > > >    * vga_switcheroo_process_delayed_switch() - helper for delayed switching
> > > > >    *
> > > > > - * Process a delayed switch if one is pending. DRM drivers should call this
> > > > > - * from their ->lastclose callback.
> > > > > + * Process a delayed switch if one is pending.
> > > > >    *
> > > > >    * Return: 0 on success. -EINVAL if no delayed switch is pending, if the client
> > > > >    * has unregistered in the meantime or if there are other clients blocking the
> > > > > --
> > > > > 2.46.0
> > > > >
> > > > --
> > > > Daniel Vetter
> > > > Software Engineer, Intel Corporation
> > > > http://blog.ffwll.ch
> >
> > --
> > --
> > Thomas Zimmermann
> > Graphics Driver Developer
> > SUSE Software Solutions Germany GmbH
> > Frankenstrasse 146, 90461 Nuernberg, Germany
> > GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
> > HRB 36809 (AG Nuernberg)
> >
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

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