Re: [PATCH] drm/gma500: Fixup fbdev stolen size usage evaluation

[Paul Kocialkowski]

Hi,

On Wed 13 Nov 19, 11:04, Patrik Jakobsson wrote:
> On Tue, Nov 12, 2019 at 4:50 PM Paul Kocialkowski
> <paul.kocialkowski@xxxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > On Tue 12 Nov 19, 16:11, Paul Kocialkowski wrote:
> > > Hi,
> > >
> > > On Tue 12 Nov 19, 11:20, Patrik Jakobsson wrote:
> > > > On Thu, Nov 7, 2019 at 4:30 PM Paul Kocialkowski
> > > > <paul.kocialkowski@xxxxxxxxxxx> wrote:
> > > > >
> > > > > psbfb_probe performs an evaluation of the required size from the stolen
> > > > > GTT memory, but gets it wrong in two distinct ways:
> > > > > - The resulting size must be page-size-aligned;
> > > > > - The size to allocate is derived from the surface dimensions, not the fb
> > > > >   dimensions.
> > > > >
> > > > > When two connectors are connected with different modes, the smallest will
> > > > > be stored in the fb dimensions, but the size that needs to be allocated must
> > > > > match the largest (surface) dimensions. This is what is used in the actual
> > > > > allocation code.
> > > > >
> > > > > Fix this by correcting the evaluation to conform to the two points above.
> > > > > It allows correctly switching to 16bpp when one connector is e.g. 1920x1080
> > > > > and the other is 1024x768.
> > > > >
> > > > > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@xxxxxxxxxxx>
> > > > > ---
> > > > >  drivers/gpu/drm/gma500/framebuffer.c | 8 ++++++--
> > > > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/gma500/framebuffer.c b/drivers/gpu/drm/gma500/framebuffer.c
> > > > > index 218f3bb15276..90237abee088 100644
> > > > > --- a/drivers/gpu/drm/gma500/framebuffer.c
> > > > > +++ b/drivers/gpu/drm/gma500/framebuffer.c
> > > > > @@ -462,6 +462,7 @@ static int psbfb_probe(struct drm_fb_helper *helper,
> > > > >                 container_of(helper, struct psb_fbdev, psb_fb_helper);
> > > > >         struct drm_device *dev = psb_fbdev->psb_fb_helper.dev;
> > > > >         struct drm_psb_private *dev_priv = dev->dev_private;
> > > > > +       unsigned int fb_size;
> > > > >         int bytespp;
> > > > >
> > > > >         bytespp = sizes->surface_bpp / 8;
> > > > > @@ -471,8 +472,11 @@ static int psbfb_probe(struct drm_fb_helper *helper,
> > > > >         /* If the mode will not fit in 32bit then switch to 16bit to get
> > > > >            a console on full resolution. The X mode setting server will
> > > > >            allocate its own 32bit GEM framebuffer */
> > > > > -       if (ALIGN(sizes->fb_width * bytespp, 64) * sizes->fb_height >
> > > > > -                       dev_priv->vram_stolen_size) {
> > > > > +       fb_size = ALIGN(sizes->surface_width * bytespp, 64) *
> > > > > +                 sizes->surface_height;
> > > > > +       fb_size = ALIGN(fb_size, PAGE_SIZE);
> > > > > +
> > > > > +       if (fb_size > dev_priv->vram_stolen_size) {
> > > >
> > > > psb_gtt_alloc_range() already aligns by PAGE_SIZE for us. Looks like
> > > > we align a couple of times extra for luck. This needs cleaning up
> > > > instead of adding even more aligns.
> > >
> > > I'm not sure this is really for luck. As far as I can see, we need to do it
> > > properly for this size estimation since it's the final size that will be
> > > allocated (and thus needs to be available in whole).
> 
> Ok now I understand what you meant. Actually vram_stolen_size is
> always page aligned so fb_size doesn't need any page alignment here.

I'm a bit confused here, what about the case where:
unaligned fb_size < dev_priv->vram_stolen_size but
aligned fb_size > dev_priv->vram_stolen_size ?

Granted, it's a corner case, but I don't follow the logic of comparing aligned
and unaligned sizes: it feels a bit like comparing two values of different
units.

> There is also no need to align for psbfb_create() since it also takes
> care of this.
> 
> > >
> > > For the other times there is explicit alignment, they seem justified too:
> > > - in psb_gem_create: it is common to pass the aligned size when creating the
> > >   associated GEM object with drm_gem_object_init, even though it's probably not
> > >   crucial given that this is not where allocation actually happens;
> > > - in psbfb_create: the full size is apparently only really used to memset 0
> > >   the allocated buffer. I think this makes sense for security reasons (and not
> > >   leak previous contents in the additional space required for alignment).
> 
> What I would prefer is to have a single place where the alignment is
> made so any hardware requirements would be transparent to the rest of
> the code.

Mhh, I thought that psbfb_create needs to be aware of the alignment in the
form of the pitch_lines variable to decide which 2d accel method can be used or
not (depending on associated alignment requirements). I guess this makes for
another reason to ditch the accelerated 2d accel support.

> Best would be if alignment is only made in psb_gtt_alloc_range() and
> then store the actual size in struct gtt_range. That way we can just
> pass along that value to memset() and drm_gem_object_init() without
> caring about how it is adjusted.
> 
> > >
> > > What strikes me however is that each call to psb_gtt_alloc_range takes the
> > > alignment as a parameter when it's really always PAGE_SIZE, so it should
> > > probably just be hardcoded in the call to allocate_resource.
> 
> This is a remnant from trying to add support for 2D and/or overlay
> planes (don't really remember). Doesn't matter if it stays or goes
> away.
> 
> > >
> > > What do you think?
> 
> I suppose most of this is outside the scope of what you're trying to
> do so we can just leave it as is and I can clean it up later.

I guess my main change here was to switch from sizes->fb_width/height to
sizes->surface_width/height anyway, yes. I can totally live without the
final PAGE_SIZE align for fb_size too (even though I think it makes sense).

Feel free to let me know what you'd like to receive as a v2 here and I'll do
that :)

> > >
> > > > Your size calculation looks correct and indeed makes my 1024x600 +
> > > > 1920x1080 setup actually display something, but for some reason I get
> > > > an incorrect panning on the smaller screen and stale data on the
> > > > surface only visible by the larger CRTC. Any idea what's going on?
> > >
> > > I'm not seeing this immediately, but I definitely have something strange
> > > after having printed more lines than the smallest display can handle or
> > > scrolling, where more than the actual size of the fb is used.
> > >
> > > Maybe this is related to using the PowerVR-accelerated fb ops, that aren't
> > > quite ready for this use case?
> > >
> > > I'll give it a try with psbfb_roll_ops and psbfb_unaccel_ops instead to see
> > > if it changes something for me. Maybe it would help for you too?
> >
> > Some quick feedback about that:
> > - psbfb_unaccel_ops gives a correct result where the scrolling area is bound
> >   to the smallest display;
> 
> Yes, this also works correctly for me.
> 
> > - psbfb_roll_ops gives a working scrolling but bound to the largest display
> >   (so the current shell line becomes invisible on the smallest one eventually);
> 
> It's not panning at all for me. I never really found gtt rolling to be
> useful. It's a neat trick but I didn't have a problem with console
> scrolling speed to begin with. I might just remove it.

Yeah, I also don't understand what the hype of accelerating fbdev ops is about.
I guess it could have been useful back when there were serious users of fbdev in
userspace (aka directfb) but that's not really where things are going today.
For console usage, I also find the software method fast enough.

> > - psbfb_ops gives the same issue as above and seems to add artifacts on top.
> 
> Did you try this on CDV? There's only 2D acceleration on Poulsbo and Oaktrail.

I tried this one on Poulsbo (the other gma500 platform I have around).

> >
> > There's probably limited interest in working on that aspect on our side though.
> > I'd be interested to know if it affects the issue you're seeing though.
> 
> Focus on your requirements and I'll look at the rest.

Sounds good to me, thanks a lot! I'll do according to what you'd like for a v2.

Cheers,

Paul

> -Patrik
> 
> >
> > Cheers,
> >
> > Paul
> >
> > > I suspect that the generic implementation is already bullet-proof for these
> > > kinds of use case.
> > >
> > > Cheers and thanks for the feedback,
> > >
> > > Paul
> > >
> > > >
> > > > >                  sizes->surface_bpp = 16;
> > > > >                  sizes->surface_depth = 16;
> > > > >          }
> > > > > --
> > > > > 2.23.0
> > > > >
> > >
> > > --
> > > Paul Kocialkowski, Bootlin
> > > Embedded Linux and kernel engineering
> > > https://bootlin.com
> >
> >
> >
> > --
> > Paul Kocialkowski, Bootlin
> > Embedded Linux and kernel engineering
> > https://bootlin.com

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

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