linux-kernel - Re: [PATCH] drm/fbdev-dma: fix getting smem

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Date: Tue, 11 Jun 2024 11:29:58 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Peng Fan <peng.fan@....com>, "Peng Fan (OSS)" <peng.fan@....nxp.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, David Airlie <airlied@...il.com>,
 Daniel Vetter <daniel@...ll.ch>,
 Javier Martinez Canillas <javierm@...hat.com>
Cc: "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/fbdev-dma: fix getting smem_start

Hi

Am 11.06.24 um 10:23 schrieb Peng Fan:
>> Subject: Re: [PATCH] drm/fbdev-dma: fix getting smem_start
>>
>> Hi
>>
>> Am 11.06.24 um 03:00 schrieb Peng Fan:
>>>> Subject: Re: [PATCH] drm/fbdev-dma: fix getting smem_start
>>>>
>>>> Hi
>>>>
>>>> Am 04.06.24 um 10:03 schrieb Peng Fan (OSS):
>>>>> From: Peng Fan <peng.fan@....com>
>>>>>
>>>>> If 'info->screen_buffer' locates in vmalloc address space,
>>>>> virt_to_page will not be able to get correct results. With
>>>>> CONFIG_DEBUG_VM and CONFIG_DEBUG_VIRTUAL enabled on ARM64,
>>>> there is dump below:
>>>>
>>>> Which graphics driver triggers this bug?
>>> It is NXP i.MX95 DPU driver which is still in NXP downstream repo.
>> Which DRM memory manager does that driver use?
> DRM_GEM_DMA_DRIVER_OPS

So fbdev-dma would be correct. But with the gem-dma allocator, wouldn't 
you allocate from one of these dma_alloc_() calls at [1] ? How does the 
drivers end up with vmalloc'd fbdev memory? Specifically in the light of 
the docs at [2].

[1] 
https://elixir.bootlin.com/linux/v6.9/source/drivers/gpu/drm/drm_gem_dma_helper.c#L146
[2] 
https://elixir.bootlin.com/linux/v6.9/source/Documentation/core-api/dma-api-howto.rst#L124

Best regards
Thomas

>
>> I'm asking because if the driver allocates BOs via vmalloc(), should it really
>> use fbdev-dma?
> We use it for fbdev emulation. If drm_kms_helper.fbdev_emulation is 0,
> fbdev emulation could be disabled.
>
> Thanks
> Peng
>> Best regards
>> Thomas
>>
>>> Thanks,
>>> Peng.
>> --
>> --
>> Thomas Zimmermann
>> Graphics Driver Developer
>> SUSE Software Solutions Germany GmbH
>> Frankenstrasse 146, 90461 Nuernberg, Germany
>> GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman HRB
>> 36809 (AG Nuernberg)

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)