linux-kernel - Re: [PATCH v2 31/37] drm: rcar-du: Add support for the nomodeset kernel parameter

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Message-ID: <1b1a5dc1-e3cd-c7ce-426b-cc330ae3ed81@suse.de>
Date:   Fri, 28 Jan 2022 12:26:03 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        Javier Martinez Canillas <javierm@...hat.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 31/37] drm: rcar-du: Add support for the nomodeset
 kernel parameter

Hi

Am 28.01.22 um 12:04 schrieb Laurent Pinchart:
> Hi Thomas,
> 
> On Fri, Jan 28, 2022 at 11:46:49AM +0100, Thomas Zimmermann wrote:
>> Am 28.01.22 um 11:34 schrieb Laurent Pinchart:
>>> On Fri, Jan 28, 2022 at 10:33:21AM +0100, Thomas Zimmermann wrote:
>>>> Am 28.01.22 um 10:13 schrieb Kieran Bingham:
>>>>> Quoting Javier Martinez Canillas (2021-12-17 00:37:46)
>>>>>> According to disable Documentation/admin-guide/kernel-parameters.txt, this
>>>>>> parameter can be used to disable kernel modesetting.
>>>>>>
>>>>>> DRM drivers will not perform display-mode changes or accelerated rendering
>>>>>> and only the system framebuffer will be available if it was set-up.
>>>>>
>>>>> What is the 'system framebuffer' in this instance? Reading
>>>>> https://www.kernel.org/doc/Documentation/admin-guide/kernel-parameters.txt
>>>>> it sounds like that means anything already set up by the bootloader.
>>>>
>>>> Exactly this.
>>>>
>>>>>> But only a few DRM drivers currently check for nomodeset, make this driver
>>>>>> to also support the command line parameter.
>>>>>>
>>>>>> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
>>>>>> ---
>>>>>>
>>>>>> (no changes since v1)
>>>>>>
>>>>>>     drivers/gpu/drm/rcar-du/rcar_du_drv.c | 3 +++
>>>>>>     1 file changed, 3 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_drv.c b/drivers/gpu/drm/rcar-du/rcar_du_drv.c
>>>>>> index 5a8131ef81d5..982e450233ed 100644
>>>>>> --- a/drivers/gpu/drm/rcar-du/rcar_du_drv.c
>>>>>> +++ b/drivers/gpu/drm/rcar-du/rcar_du_drv.c
>>>>>> @@ -701,6 +701,9 @@ static struct platform_driver rcar_du_platform_driver = {
>>>>>>     
>>>>>>     static int __init rcar_du_init(void)
>>>>>>     {
>>>>>> +       if (drm_firmware_drivers_only())
>>>>>> +               return -ENODEV;
>>>>>> +
>>>>>
>>>>> This will completely disable all control of the display device when
>>>>> nomodeset is enabled.
>>>>>
>>>>> Is there any requirement for us to support outputting to the display if
>>>>> it was previously set up? presumably without setting or changing any
>>>>> modes, but simply allowing the existing frame to be updated?
>>>>
>>>> There's no requirement for your driver. We just want a parameter where
>>>> we can conveniently disable most of DRM's drivers and reduce it to a
>>>> minimum. Helps distributions to provide a simple fallback mode.  Most
>>>> PCI-based drivers already support that. Now we're added it to the other
>>>> drivers as well.
>>>>
>>>>>
>>>>> I think the implication is that 'firmware drivers' would mean a display
>>>>> could be updated through some firmware interface, which we won't have
>>>>> ... so it seems reasonable to accept that this whole driver can be
>>>>> disabled in that instance.
>>>>
>>>> It cannot be 'mode-setted'. We get a pre-configured framebuffer from the
>>>> firmware or bootloader. Whatever we draw there shows up on the screen.
>>>
>>> I doubt that's going to work as you expect, clocks and regulators will
>>> get disabled at boot if not used by any driver.
>>
>> Simpledrm and simplefb attach to these firmware framebuffers. Both
>> drivers look at the device tree nodes to acquire the relevant clocks and
>> regulators.
> 
> How about clocks and regulators for the ancillary devices, such as
> encoders, or in the R-Car case, the external composer handled by the
> vsp1 driver (in drivers/media/platform/vsp1) ?
> 
> This approach may work fine on x86 desktop systems, but for ARM-based
> devices, the situation is usually more complex.

Well, in that case the problem has always been there. We don't make it 
worse.

Best regards
Thomas

> 
>>>>> Reading your mail that brought this thread up in my inbox, I think
>>>>> you've already hit merge on this, so don't worry about adding a tag in
>>>>> that instance, but I think this is ok.
>>>>>
>>>>> Reviewed-by: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
>>>>>
>>>>>>            rcar_du_of_init(rcar_du_of_table);
>>>>>>     
>>>>>>            return platform_driver_register(&rcar_du_platform_driver);
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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