Re: [patch] VRAM detection in controlfb

On Thu, 8 Jun 2000, Ryuichi Oikawa wrote:

From: Geert Uytterhoeven <redacted>

quoted

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

Can't /dev/fb mmap handle this case?  I just thought I saw some page
handling code which adds extra page in fbmem.c. I'm using this for /dev/mem:

  int pages, trailer_page = 0;

  if(basePhys & (PAGE_SIZE - 1))
    ++trailer_page;
  pages = (memSize + PAGE_SIZE - 1)/PAGE_SIZE + trailer_page;
  *baseVirt = (pointer)mmap(0, pages*PAGE_SIZE, PROT_READ|PROT_WRITE,
			    MAP_SHARED, fd, basePhys & PAGE_MASK);
  *baseVirt += basePhys & (PAGE_SIZE - 1);

quoted

the formula from my previous posting.

From a first look, that looks very similar to my formula.

quoted

quoted

But looking through the XF4 fbdev support code I saw two more mistakes
causing potential problem. One is a confusion of virtual screen width
with screen pitch:

fbdev.c
	pScrn->displayWidth = pScrn->virtualX; /* FIXME: might be wrong */

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'.

 Ah, I see, but I simply cut & pasted it.... where this comes?

quoted

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?).

Yes, it is used calculate video memory address from the x, y coordinate.
How is displayWidth used for planar architecture? Is setting
displayWidth == xres_virtual necessary to work properly?

For non-interleaved bitplanes, the same formula as for chunky pixels is valid.
For interleaved bitplanes, displayWidth == xres_virtual (assumed you can
display the full xres_virtual).

quoted

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

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

Yes. But ilbm (interleaved bitplanes) still has to use a different formula.

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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