Re: [PATCH] drm/komeda: Added AFBC support for komeda driver

[james qian wang (Arm Technology China)]

On Fri, May 24, 2019 at 03:12:26PM +0300, Ville Syrjälä wrote:
> On Fri, May 24, 2019 at 11:10:09AM +0000, Brian Starkey wrote:
> > Hi,
> > 
> > On Tue, May 21, 2019 at 09:45:58AM +0100, james qian wang (Arm Technology China) wrote:
> > > On Thu, May 16, 2019 at 09:57:49PM +0800, Ayan Halder wrote:
> > > > On Thu, Apr 04, 2019 at 12:06:14PM +0100, james qian wang (Arm Technology China) wrote:
> > > > >  
> > > > > +static int
> > > > > +komeda_fb_afbc_size_check(struct komeda_fb *kfb, struct drm_file *file,
> > > > > +			  const struct drm_mode_fb_cmd2 *mode_cmd)
> > > > > +{
> > > > > +	struct drm_framebuffer *fb = &kfb->base;
> > > > > +	const struct drm_format_info *info = fb->format;
> > > > > +	struct drm_gem_object *obj;
> > > > > +	u32 alignment_w = 0, alignment_h = 0, alignment_header;
> > > > > +	u32 n_blocks = 0, min_size = 0;
> > > > > +
> > > > > +	obj = drm_gem_object_lookup(file, mode_cmd->handles[0]);
> > > > > +	if (!obj) {
> > > > > +		DRM_DEBUG_KMS("Failed to lookup GEM object\n");
> > > > > +		return -ENOENT;
> > > > > +	}
> > > > > +
> > > > > +	switch (fb->modifier & AFBC_FORMAT_MOD_BLOCK_SIZE_MASK) {
> > > > > +	case AFBC_FORMAT_MOD_BLOCK_SIZE_32x8:
> > > > > +		alignment_w = 32;
> > > > > +		alignment_h = 8;
> > > > > +		break;
> > > > > +	case AFBC_FORMAT_MOD_BLOCK_SIZE_16x16:
> > > > > +		alignment_w = 16;
> > > > > +		alignment_h = 16;
> > > > > +		break;
> > > > > +	default:
> > > > Can we have something like a warn here ?
> > > 
> > > will add a WARN here.
> > > 
> > 
> > I think it's better not to. fb->modifier comes from
> > userspace, so a malicious app could spam us with WARNs, effectively
> > dos-ing the system. -EINVAL should be sufficient.
> 
> Should probably check that the entire modifier+format is
> actually valid. Otherwise you risk passing on a bogus
> modifier deeper into the driver which may trigger
> interesting bugs.
> 
> Also theoretically (however unlikely) some broken userspace
> might start to depend on the ability to create framebuffers
> with crap modifiers, which could later break if you change
> the way you handle the modifiers. Then you're stuck between
> the rock and hard place because you can't break existing
> userspace but you still want to change the way modifiers
> are handled in the kernel.
> 
> Best not give userspace too much rope IMO. Two ways to go about
> that:
> 1) drm_any_plane_has_format() (assumes your .format_mod_supported()
>    does its job properly)
> 2) roll your own 
> 
> -- 
> Ville Syrjälä
> Intel

Hi Brian & Ville:

komed has a format+modifier check before the fb size check.
and for komeda_fb_create, the first step is do the format+modifier
check, the size check is the furthur check after the such format
valid check. and the detailed fb_create is like:

struct drm_framebuffer *
komeda_fb_create(struct drm_device *dev, struct drm_file *file,
		 const struct drm_mode_fb_cmd2 *mode_cmd)
{
        ...
        /* Step 1: format+modifier valid check, if it can not be support,
         * get_format_caps will return a NULL ptr.
         */
	kfb->format_caps = komeda_get_format_caps(&mdev->fmt_tbl,
						  mode_cmd->pixel_format,
						  mode_cmd->modifier[0]);
	if (!kfb->format_caps) {
		DRM_DEBUG_KMS("FMT %x is not supported.\n",
			      mode_cmd->pixel_format);
		kfree(kfb);
		return ERR_PTR(-EINVAL);
	}

	drm_helper_mode_fill_fb_struct(dev, &kfb->base, mode_cmd);

        /* step 2, do the size check */
	if (kfb->base.modifier)
		ret = komeda_fb_afbc_size_check(kfb, file, mode_cmd);
	else
		ret = komeda_fb_none_afbc_size_check(mdev, kfb, file, mode_cmd);
	if (ret < 0)
		goto err_cleanup;

        ...
}

So theoretically, the WARN in step2 is redundant if get_format_caps
function has no problem. :). the WARN here is only for reporting
the kernel BUG or code inconsitent with format caps check and the
fb size check. And I agree, basically it will not happene.
@Brian, I'm Ok to remove it. :)

@Ville:
Basically komeda follow the way-1, but a little improvement for
matching komeda's requirement. for komeda which will do two level's
format+modifier check.
1). In fb_create, A roughly check to see if the format+modifier can be
    supported by current HW.
2). In plane_atomic_check: to see if the format+modifier can be
    supported for a specific layer and with a specific configuration (ROT)

This is a format valid check example for plane_check.
https://patchwork.freedesktop.org/patch/301140/?series=59747&rev=2

James