linux-kernel - Re: [PATCH 11/60] drm/gud: Add support for the nomodeset kernel parameter

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Date:   Thu, 16 Dec 2021 09:20:42 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Noralf Trønnes <noralf@...nnes.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 11/60] drm/gud: Add support for the nomodeset kernel
 parameter

Hi

Am 15.12.21 um 22:37 schrieb Noralf Trønnes:
> 
> 
> Den 15.12.2021 01.59, skrev Javier Martinez Canillas:
>> 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 systewm system framebuffer will be available if it was set-up.
>>
>> 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>
>> ---
>>
> 
> I don't understand why this is applicable to USB drivers, there's no way
> the firmware can setup a framebuffer and continue pushing pixels over
> USB when Linux has been given control over the USB bus?
> 
> The same argument goes for the SPI drivers in drm/tiny/ as well.

The intended semantics of the option is to disable every display output 
except for the buffer provided by the firmware.

With USB it still would still disable the driver. That's useful if only 
for debugging. There are also systems with hard-wired USB displays where 
one cannot just unplug the adapter.

Admittedly, USB graphics is a bit of an odd use case, but neither is it 
too far fetched IMHO.

Best regards
Thomas

> 
> Noralf.
> 
>>   drivers/gpu/drm/gud/gud_drv.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/gud/gud_drv.c b/drivers/gpu/drm/gud/gud_drv.c
>> index 3f9d4b9a1e3d..4d253d249512 100644
>> --- a/drivers/gpu/drm/gud/gud_drv.c
>> +++ b/drivers/gpu/drm/gud/gud_drv.c
>> @@ -446,6 +446,9 @@ static int gud_probe(struct usb_interface *intf, const struct usb_device_id *id)
>>   	u32 *formats;
>>   	int ret, i;
>>   
>> +	if (drm_firmware_drivers_only())
>> +		return -ENODEV;
>> +
>>   	ret = usb_find_bulk_out_endpoint(intf->cur_altsetting, &bulk_out);
>>   	if (ret)
>>   		return ret;
>>

-- 
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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