Re: [PATCH] drm/fbdev-dma: fix getting smem_start

[Thomas Zimmermann <tzimmermann@suse.de>]

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@nxp.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)