Re: [RFC PATCH 1/2] drm/fb-helper: Bring back workaround for bugs of SDL 1.2

[Ivan Mironov <mironov.ivan@gmail.com>]

On Thu, 2018-12-27 at 13:00 +0100, Daniel Vetter wrote: 
> > +	/*
> > +	 * Workaround for SDL 1.2, which is known to be setting all pixel format
> > +	 * fields values to zero in some cases. We treat this situation as a
> > +	 * kind of "use some reasonable autodetected values".
> > +	 */
> > +	if (!var->red.offset     && !var->green.offset    &&
> > +	    !var->blue.offset    && !var->transp.offset   &&
> > +	    !var->red.length     && !var->green.length    &&
> > +	    !var->blue.length    && !var->transp.length   &&
> > +	    !var->red.msb_right  && !var->green.msb_right &&
> > +	    !var->blue.msb_right && !var->transp.msb_right) {
> > +		u8 depth;
> > +
> > +		/*
> > +		 * There is no way to guess the right value for depth when
> > +		 * bpp is 16 or 32. So we just restore the behaviour previously
> > +		 * introduced here by commit 785b93ef8c309. In fact, this was
> > +		 * implemented even earlier in various device drivers.
> > +		 */
> > +		switch (var->bits_per_pixel) {
> > +		case 16:
> > +			depth = 15;
> > +			break;
> > +		case 32:
> > +			depth = 24;
> > +			break;
> > +		default:
> > +			depth = var->bits_per_pixel;
> > +			break;
> > +		}
> 
> The guesswork here looks fishy. We should still have the drm-side format,
> and should use that.

This existed for a very long time until problematic commit was applied.
And there is a clear evidence that it was actually used by
applications.

> Otherwise the patches look good I think, but they
> need a Fixes: tag and cc: stable so backporters know what to do with
> these.
> 

I added "cc: stable" into the regression fix. Also added more info into
the commit messages. See the PATCH v1 in the mailing list.

Thanks.