01:00.0 VGA compatible controller: ATI Technologies Inc RV350 AP [Radeon 9600]
mesa 7.0.1
xorg-ati 1:6.7.195
xorg 1:7.2


glClear ignore stencil mask while using xorg-ati driver (works fine with fglrx).
gl specifically state that:
            "The pixel ownership test, the scissor test,
            dithering, and the buffer writemasks affect the operation of glClear".


the following example demonstrate the problem. the expected rendering result is the intersection of the
first two rectangles. with ati driver nothing gets rendered.

void draw(void)
{
    glStencilMask(0xff);
    glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
    
    glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
    
    glStencilFunc(GL_ALWAYS, 0x01, 0x01);
    glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);
    glBegin(GL_POLYGON);
        glVertex2f(0, 0);
        glVertex2f(0, 100);
        glVertex2f(100, 100);
        glVertex2f(100, 0);
    glEnd();
    
    glStencilFunc(GL_ALWAYS, 0x01, 0x01);
    glStencilOp(GL_INCR, GL_INCR, GL_INCR);
    glBegin(GL_POLYGON);
        glVertex2f(50, 50);
        glVertex2f(50, 150);
        glVertex2f(150, 150);
        glVertex2f(150, 50);
    glEnd();
    
    glStencilMask(0x1);
    glClear(GL_STENCIL_BUFFER_BIT);
    
    glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
    glStencilFunc(GL_EQUAL, 0x02, 0x02);
    glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
    glColor3f(1, 0, 0);
    glBegin(GL_POLYGON);
        glVertex2f(0, 0);
        glVertex2f(0, 1000);
        glVertex2f(1000, 1000);
        glVertex2f(1000, 0);
    glEnd();
    
    glFlush();
}

static void init(int width, int height)
{
    glViewport(0, 0, width, height);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0, width, 0, height, -1, 1);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glClearColor(0, 0, 0, 0);
    glClearStencil(0);
    glEnable(GL_STENCIL_TEST);
    glStencilFunc(GL_ALWAYS, 0, 0);
    glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
}