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

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

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@xxxxxxx>
> > > > > > >
> > > > > > > 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)