linux-kernel - Re: [RFC/RFT PATCH] fbdev: q40fb: request memory region

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Message-ID: <CAMuHMdV3UvDHT0uu8oeiCGc9pURaLPDPmG-Fu9kC9H8DQyqRDw@mail.gmail.com>
Date: Fri, 21 Nov 2025 11:03:11 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Sukrut Heroorkar <hsukrut3@...il.com>
Cc: Helge Deller <deller@....de>, 
	"open list:FRAMEBUFFER LAYER" <linux-fbdev@...r.kernel.org>, 
	"open list:FRAMEBUFFER LAYER" <dri-devel@...ts.freedesktop.org>, open list <linux-kernel@...r.kernel.org>, 
	shuah@...nel.org, david.hunter.linux@...il.com, 
	linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [RFC/RFT PATCH] fbdev: q40fb: request memory region

Hi Sukrut,

CC linux-m68k

On Thu, 20 Nov 2025 at 19:03, Sukrut Heroorkar <hsukrut3@...il.com> wrote:
> The q40fb driver uses a fixed physical address but never reserves
> the corresponding I/O region. Reserve the range  as suggested in
> Documentation/gpu/todo.rst ("Request memory regions in all fbdev drivers").
>
> If the memory cannot be reserved, fail probe with -EBUSY to avoid
> conflicting with another user of the same address.
>
> Signed-off-by: Sukrut Heroorkar <hsukrut3@...il.com>

Thanks for your patch!

> ---
> Testing: This patch is sent as RFT since Q40 hardware is unavilable and
> QEMU does not emulated a Q40 platform. The change is therefore compile-tested
> only.

I would suggest not to apply this, unless it is tested on real
hardware.  It wouldn't be the first time an innocent-looking change like
this breaks a system. See e.g.
https://lore.kernel.org/all/Y5I2oQexHNdlIbsQ@shell.armlinux.org.uk

> --- a/drivers/video/fbdev/q40fb.c
> +++ b/drivers/video/fbdev/q40fb.c
> @@ -101,6 +101,13 @@ static int q40fb_probe(struct platform_device *dev)
>         info->par = NULL;
>         info->screen_base = (char *) q40fb_fix.smem_start;
>
> +       if (!request_mem_region(q40fb_fix.smem_start, q40fb_fix.smem_len,
> +                               "q40fb")) {
> +               dev_err(&dev->dev, "cannot reserve video memory at 0x%lx\n",
> +                       q40fb_fix.smem_start);
> +               return -EBUSY;
> +       }
> +
>         if (fb_alloc_cmap(&info->cmap, 256, 0) < 0) {
>                 framebuffer_release(info);
>                 return -ENOMEM;

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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