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

[Daniel Vetter <daniel.vetter@ffwll.ch>]

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@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.

- 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@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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch