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

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Date:   Fri, 28 Jan 2022 10:33:21 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        Javier Martinez Canillas <javierm@...hat.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v2 31/37] drm: rcar-du: Add support for the nomodeset
 kernel parameter

Hi

Am 28.01.22 um 10:13 schrieb Kieran Bingham:
> Hi Javier,
> 
> 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.

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);
>> -- 
>> 2.33.1
>>

-- 
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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