linux-kernel - Re: simple-framebuffer enquire

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Date:   Tue, 26 Jun 2018 15:38:50 +0200
From:   Michael Nazzareno Trimarchi <michael@...rulasolutions.com>
To:     Hans de Goede <hdegoede@...hat.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 Tue, Jun 26, 2018 at 3:36 PM, Hans de Goede <hdegoede@...hat.com> wrote:
> 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.
>

Thank you very much for this lesson ;). I will try to document better
after my tour ;)

Michael

> Regards,
>
> Hans



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| Michael Nazzareno Trimarchi                     Amarula Solutions BV |
| COO  -  Founder                                      Cruquiuskade 47 |
| +31(0)851119172                                 Amsterdam 1018 AM NL |
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