linux-kernel - Re: simple-framebuffer enquire

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Date:   Tue, 26 Jun 2018 15:36:29 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Michael Nazzareno Trimarchi <michael@...rulasolutions.com>
Cc:     artlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-fbdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: simple-framebuffer enquire

Hi,

On 26-06-18 15:29, Michael Nazzareno Trimarchi wrote:
> Hi
> 
> to be more specific
> 
> On Tue, Jun 26, 2018 at 3:06 PM, Michael Nazzareno Trimarchi
> <michael@...rulasolutions.com> wrote:
>> Hi
>>
>> On Tue., 26 Jun. 2018, 12:01 pm Hans de Goede, <hdegoede@...hat.com> wrote:
>>>
>>> Hi,
>>>
>>> On 25-06-18 15:29, Michael Nazzareno Trimarchi wrote:
>>>> Hi Hans
>>>>
>>>> In order to let it even registered the simplefb I have added this
>>>> change. According on what I understand
>>>> from the code seems that this is the way to acquire memory with the
>>>> correct attribute
>>>>
>>>> diff --git a/drivers/video/fbdev/simplefb.c
>>>> b/drivers/video/fbdev/simplefb.c
>>>> index a3c44ec..7e61ce3 100644
>>>> --- a/drivers/video/fbdev/simplefb.c
>>>> +++ b/drivers/video/fbdev/simplefb.c
>>>> @@ -466,8 +466,8 @@ static int simplefb_probe(struct platform_device
>>>> *pdev)
>>>>
>>>>           info->fbops = &simplefb_ops;
>>>>           info->flags = FBINFO_DEFAULT | FBINFO_MISC_FIRMWARE;
>>>> -       info->screen_base = ioremap_wc(info->fix.smem_start,
>>>> -                                      info->fix.smem_len);
>>>> +       info->screen_base = arch_memremap_wb(info->fix.smem_start,
>>>> +                                            info->fix.smem_len);
>>>
>>> I'm not sure why you need this?  wb certainly is not optimal
>>> for a framebuffer, the existing wc mapping is really what you
>>> want.
>>>
>>
>> Well in this way raise a WARN and get a nice NULL on memory remap on imx6ull
>> SoC
>>
> 
> [    0.397484] WARNING: CPU: 0 PID: 1 at arch/arm/mm/ioremap.c:303
> __arm_ioremap_pfn_caller+0x80/0x1cc


This is causes by a mismatch in memory attributes, which means the
memory is already mapped by the kernel as regular RAM and may
already be used for other purposes by the kernel!

Memory used by a simplefb framebuffer must be reserved by the
bootloader, so that it does not get used by the kernel as regular
RAM. See e.g.:

http://git.denx.de/?p=u-boot.git;a=blob;f=drivers/video/sunxi/sunxi_display.c

Near the end of the file where the framebuffer RAM gets excluded from
the memory-range reported to the kernel as usable RAM. Note this relies
on the u-boot sunxi video code putting the framebuffer at the end of the
RAM.

Regards,

Hans