linux-kernel - Re: [PATCH v3 2/4] drm/qxl: unpin release objects

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Message-ID: <20210122133545.acloe4ytgp6r4iql@sirius.home.kraxel.org>
Date:   Fri, 22 Jan 2021 14:35:45 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVER FOR QXL VIRTUAL GPU" 
        <virtualization@...ts.linux-foundation.org>,
        "open list:DRM DRIVER FOR QXL VIRTUAL GPU" 
        <spice-devel@...ts.freedesktop.org>,
        Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH v3 2/4] drm/qxl: unpin release objects

On Fri, Jan 22, 2021 at 09:13:42AM +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 20.01.21 um 12:12 schrieb Gerd Hoffmann:
> > Balances the qxl_create_bo(..., pinned=true, ...);
> > call in qxl_release_bo_alloc().
> > 
> > Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
> > ---
> >   drivers/gpu/drm/qxl/qxl_release.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/drivers/gpu/drm/qxl/qxl_release.c b/drivers/gpu/drm/qxl/qxl_release.c
> > index 0fcfc952d5e9..add979cba11b 100644
> > --- a/drivers/gpu/drm/qxl/qxl_release.c
> > +++ b/drivers/gpu/drm/qxl/qxl_release.c
> > @@ -166,6 +166,7 @@ qxl_release_free_list(struct qxl_release *release)
> >   		entry = container_of(release->bos.next,
> >   				     struct qxl_bo_list, tv.head);
> >   		bo = to_qxl_bo(entry->tv.bo);
> > +		bo->tbo.pin_count = 0; /* ttm_bo_unpin(&bo->tbo); */
> 
> This code looks like a workaround or a bug.
> 
> AFAICT the only place with pre-pinned BO is qdev->dumb_shadow_bo. Can you
> remove the pinned flag entirely and handle pinning as part of
> dumb_shadow_bo's code.

No, the release objects are pinned too, and they must be
pinned (qxl commands are in there, and references are
placed in the qxl rings, so allowing them to roam is
a non-starter).

> if (pin_count)
>     ttm_bo_unpin();
> WARN_ON(pin_count); /* should always be 0 now */

Well, the pin_count is 1 at this point.
No need for the if().

Just calling ttm_bo_unpin() here makes lockdep unhappy.

Not calling ttm_bo_unpin() makes ttm_bo_release() throw
a WARN() because of the pin.

Clearing pin_count (which is how ttm fixes things up
in the error path) works.

I'm open to better ideas.

take care,
  Gerd