Date:   Thu, 6 Feb 2020 09:17:41 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Philippe Mathieu-Daudé <f4bug@...at.org>
cc:     Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paulburton@...nel.org>,
        James Hogan <jhogan@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        Laurent Vivier <laurent@...ier.eu>,
        Thomas Bogendoerfer <tbogendoerfer@...e.de>
Subject: Re: [PATCH 1/3] fbdev/g364fb: Fix build failure

On Wed, 5 Feb 2020, Philippe Mathieu-Daudé wrote:

> On Sun, Feb 2, 2020 at 3:41 AM Finn Thain <fthain@...egraphics.com.au> wrote:
> >
> > This patch resolves these compiler errors and warnings --
> >
> >   CC      drivers/video/fbdev/g364fb.o
> > drivers/video/fbdev/g364fb.c: In function 'g364fb_cursor':
> > drivers/video/fbdev/g364fb.c:137:9: error: 'x' undeclared (first use in this function)
> > drivers/video/fbdev/g364fb.c:137:9: note: each undeclared identifier is reported only once for each function it appears in
> > drivers/video/fbdev/g364fb.c:137:7: error: implicit declaration of function 'fontwidth' [-Werror=implicit-function-declaration]
> > drivers/video/fbdev/g364fb.c:137:23: error: 'p' undeclared (first use in this function)
> > drivers/video/fbdev/g364fb.c:137:38: error: 'y' undeclared (first use in this function)
> > drivers/video/fbdev/g364fb.c:137:7: error: implicit declaration of function 'fontheight' [-Werror=implicit-function-declaration]
> > drivers/video/fbdev/g364fb.c: In function 'g364fb_init':
> > drivers/video/fbdev/g364fb.c:233:24: error: 'fbvar' undeclared (first use in this function)
> > drivers/video/fbdev/g364fb.c:234:24: error: 'xres' undeclared (first use in this function)
> 
> 18 years unnoticed...
> 

More likely, it was noticed by those without the skills or time to get it 
fixed upstream.

Those with the hardware skills and platform knowledge to be affected by an 
obscure bug aren't necessarily also capable of fixing a kernel bug, 
sending a patch upstream and getting it past code review.

Getting a patch into the Linux kernel is itself a lot of work, unless 
you've had years of experience with that constantly changing process 
(which varies significantly between subsystems).

Kernel developers are only human and do accidentally introduce breakage in 
their work (as contributors) while ironically (as reviewers) they raise 
the bar for random fixes from users not versed in the 10000+ lines of 
Documentation/process/*.rst

Broken code does not mean zero potential users or zero frustrated users 
yet I often hear kernel developers disingenuously claim that it does.
They have an incentive to make that claim and often there's no-one reading 
the mailing lists to push back.

> > drivers/video/fbdev/g364fb.c:201:14: warning: unused variable 'j' [-Wunused-variable]
> > drivers/video/fbdev/g364fb.c:197:25: warning: unused variable 'pal_ptr' [-Wunused-variable]
> >
> > The MIPS Magnum framebuffer console now works when tested in QEMU.
> >
> > Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> 
> This commit is the kernel 'git origin' import, not the proper reference.
> 
> The actual change is between v2.5.17/2.5.19:
> https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/diff/drivers/video/g364fb.c?id=b30e6e183a728923267
> Date: 2002-05-22 07:52:33...
> 

I think the driver was broken before that point. I didn't compile it, but 
it seems that "xres" still has no declaration.

BTW, is it wise to cite a sha-1 that doesn't exist in mainline?

$ git show b30e6e183a728923267
fatal: ambiguous argument 'b30e6e183a728923267': unknown revision or path 
not in the working tree.

I'd prefer to use a tag like this,
References: https://git.kernel.org/tglx/history/c/b30e6e183a728923267

FWIW, if you search for "tglx.history" in git log, you can find other 
stuff, like "Histroy Tree:" [sic] and "History-tree:". I don't know what 
the canonical form is.

> The same commit introduced the changes in g364fb_cursor(), which was
> implemented previous to v2.4.0 so it is hard to follow from there.
> 
> Nobody complains during 18 years so I doubt anyone care that
> g364fb_cursor() is removed.

Right. The .fb_cursor method is optional (not all framebuffers have a 
hardware cursor).

> And by removing it, you improve the kernel quality, so:
> Reviewed-by: Philippe Mathieu-Daudé <f4bug@...at.org>
> (Maybe remove the unhelpful 'Fixes' tag).
> 

In the past I've seen other reviewers of other patches say the same thing 
("Please remove Fixes: Linux 2.6.12-rc2"). I don't know why.

I used that tag because it indicates that the patch can be backported at 
least as far as the stated commit. Certainly the bug exists in Linux 
2.6.12-rc2, so the "Fixes" tag is meaningful in that sense.

> > Signed-off-by: Finn Thain <fthain@...egraphics.com.au>
> > ---
> >  drivers/video/fbdev/g364fb.c | 29 +++--------------------------
> >  1 file changed, 3 insertions(+), 26 deletions(-)
> >
> > diff --git a/drivers/video/fbdev/g364fb.c b/drivers/video/fbdev/g364fb.c
> > index 223896cc5f7d..fb26230a3c7b 100644
> > --- a/drivers/video/fbdev/g364fb.c
> > +++ b/drivers/video/fbdev/g364fb.c
> > @@ -108,7 +108,6 @@ static int g364fb_pan_display(struct fb_var_screeninfo *var,
> >  static int g364fb_setcolreg(u_int regno, u_int red, u_int green,
> >                             u_int blue, u_int transp,
> >                             struct fb_info *info);
> > -static int g364fb_cursor(struct fb_info *info, struct fb_cursor *cursor);
> >  static int g364fb_blank(int blank, struct fb_info *info);
> >
> >  static struct fb_ops g364fb_ops = {
> > @@ -119,28 +118,8 @@ static struct fb_ops g364fb_ops = {
> >         .fb_fillrect    = cfb_fillrect,
> >         .fb_copyarea    = cfb_copyarea,
> >         .fb_imageblit   = cfb_imageblit,
> > -       .fb_cursor      = g364fb_cursor,
> >  };
> >
> > -int g364fb_cursor(struct fb_info *info, struct fb_cursor *cursor)
> > -{
> > -
> > -       switch (cursor->enable) {
> > -       case CM_ERASE:
> > -               *(unsigned int *) CTLA_REG |= CURS_TOGGLE;
> > -               break;
> > -
> > -       case CM_MOVE:
> > -       case CM_DRAW:
> > -               *(unsigned int *) CTLA_REG &= ~CURS_TOGGLE;
> > -               *(unsigned int *) CURS_POS_REG =
> > -                   ((x * fontwidth(p)) << 12) | ((y * fontheight(p)) -
> > -                                                 info->var.yoffset);
> > -               break;
> > -       }
> > -       return 0;
> > -}
> > -
> >  /*
> >   *  Pan or Wrap the Display
> >   *
> > @@ -194,11 +173,9 @@ static int g364fb_setcolreg(u_int regno, u_int red, u_int green,
> >   */
> >  int __init g364fb_init(void)
> >  {
> > -       volatile unsigned int *pal_ptr =
> > -           (volatile unsigned int *) CLR_PAL_REG;
> >         volatile unsigned int *curs_pal_ptr =
> >             (volatile unsigned int *) CURS_PAL_REG;
> > -       int mem, i, j;
> > +       int mem, i;
> >
> >         if (fb_get_options("g364fb", NULL))
> >                 return -ENODEV;
> > @@ -230,8 +207,8 @@ int __init g364fb_init(void)
> >          */
> >         *(unsigned short *) (CURS_PAT_REG + 14 * 64) = 0xffff;
> >         *(unsigned short *) (CURS_PAT_REG + 15 * 64) = 0xffff;
> > -       fb_var.xres_virtual = fbvar.xres;
> > -       fb_fix.line_length = (xres / 8) * fb_var.bits_per_pixel;
> > +       fb_var.xres_virtual = fb_var.xres;
> > +       fb_fix.line_length = fb_var.xres_virtual * fb_var.bits_per_pixel / 8;
> >         fb_fix.smem_start = 0x40000000; /* physical address */
> >         /* get size of video memory; this is special for the JAZZ hardware */
> >         mem = (r4030_read_reg32(JAZZ_R4030_CONFIG) >> 8) & 3;
> > --
> > 2.24.1
> >
> 

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