Re: [patch] VRAM detection in controlfb

On Wed, 7 Jun 2000, Michael Schmitz wrote:

quoted

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Now you understand the fix is very simple:
-		fPtr->fboff = fPtr->fix.smem_len & (PAGE_SIZE-1);
+		fPtr->fboff = fPtr->fix.smem_start & (PAGE_SIZE-1);

That's not sufficient (perhaps it is in this case, though): both fix.smem_start
and fix.smem_len may be not page aligned. To catch all cases, you have to use
the formula from my previous posting.

Did that just get lost between XFree release 3 and 4, or did we have it
wrong all along? I wonder if that was what caused problems on some m68k
Mac video hardware ...

It should be correct in 3.3.x.

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Yes, this is wrong as the authur noted, but interestingly this code spreads
over the all drivers supporting fbdev, though I can't distinguish which is
the original :^)  Maybe polite solution is to fix each driver, but quick
fix will be

+	pScrn->displayWidth = fPtr->fix.line_width /
+				(fPtr->var.bits_per_pixel >> 3)

Typo: the field is called `line_length', not `line_width'.

And that formula is valid for chunky displays only, not for interleaved
bitplanes (I suppose pScrn->displayWidth is the width of one line in memory,
counted in pixel units?).

If line_length is 0, you must fallback to xres_virtual / bytes_per_pixel.

Is the displayWidth / xres_virtual synonymous to the line length (offset
between start of subsequent lines), or might displayWidth be used for
something else altogether? displayWidth seems a misnomer (on some
hardware, there's no video RAM between the end of one scan line and the
probably page aligned start of the next ...)?

At least in 3.3.6, X wasn't capable of handling displays where the offset to
the next line was not equal to what you'd expect from xres_virtual. The only
way to make such hardware work is to fake the virtual screen width
(displayWidth) by something derived from line_length.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds


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