cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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cmap_xfbdev.rst (1992B)


      1==========================
      2Understanding fbdev's cmap
      3==========================
      4
      5These notes explain how X's dix layer uses fbdev's cmap structures.
      6
      7-  example of relevant structures in fbdev as used for a 3-bit grayscale cmap::
      8
      9    struct fb_var_screeninfo {
     10	    .bits_per_pixel = 8,
     11	    .grayscale      = 1,
     12	    .red =          { 4, 3, 0 },
     13	    .green =        { 0, 0, 0 },
     14	    .blue =         { 0, 0, 0 },
     15    }
     16    struct fb_fix_screeninfo {
     17	    .visual =       FB_VISUAL_STATIC_PSEUDOCOLOR,
     18    }
     19    for (i = 0; i < 8; i++)
     20	info->cmap.red[i] = (((2*i)+1)*(0xFFFF))/16;
     21    memcpy(info->cmap.green, info->cmap.red, sizeof(u16)*8);
     22    memcpy(info->cmap.blue, info->cmap.red, sizeof(u16)*8);
     23
     24-  X11 apps do something like the following when trying to use grayscale::
     25
     26    for (i=0; i < 8; i++) {
     27	char colorspec[64];
     28	memset(colorspec,0,64);
     29	sprintf(colorspec, "rgb:%x/%x/%x", i*36,i*36,i*36);
     30	if (!XParseColor(outputDisplay, testColormap, colorspec, &wantedColor))
     31		printf("Can't get color %s\n",colorspec);
     32	XAllocColor(outputDisplay, testColormap, &wantedColor);
     33	grays[i] = wantedColor;
     34    }
     35
     36There's also named equivalents like gray1..x provided you have an rgb.txt.
     37
     38Somewhere in X's callchain, this results in a call to X code that handles the
     39colormap. For example, Xfbdev hits the following:
     40
     41xc-011010/programs/Xserver/dix/colormap.c::
     42
     43  FindBestPixel(pentFirst, size, prgb, channel)
     44
     45  dr = (long) pent->co.local.red - prgb->red;
     46  dg = (long) pent->co.local.green - prgb->green;
     47  db = (long) pent->co.local.blue - prgb->blue;
     48  sq = dr * dr;
     49  UnsignedToBigNum (sq, &sum);
     50  BigNumAdd (&sum, &temp, &sum);
     51
     52co.local.red are entries that were brought in through FBIOGETCMAP which come
     53directly from the info->cmap.red that was listed above. The prgb is the rgb
     54that the app wants to match to. The above code is doing what looks like a least
     55squares matching function. That's why the cmap entries can't be set to the left
     56hand side boundaries of a color range.