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

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Message-ID: <605bf442-41f9-49d9-a4b7-e889ff800734@suse.de>
Date: Wed, 12 Jun 2024 15:03:41 +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>,
 Javier Martinez Canillas <javierm@...hat.com>,
 "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 12.06.24 um 10:45 schrieb Daniel Vetter:
> On Tue, Jun 11, 2024 at 11:29:58AM +0200, Thomas Zimmermann wrote:
>> 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].
> I think when you have an iommu dma_alloc just allocates pages, and uses
> the iommu to make it all contiguous for the device, and vmalloc for the
> kernel. So it's not allocated with vmalloc(), just ends up getting mapped
> into the vmalloc range.

Thanks, that makes sense. As I mentioned in another reply, it might make 
sense to protect smem_start with an opt-in flag for only that single 
driver that needs it.

Best regards
Thomas

> -Sima
>
>
>> [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)
>>

-- 
--
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)