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

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jun 12, 2024 at 10:37:14AM +0200, Thomas Zimmermann wrote:
> Hi Javier
> 
> Am 12.06.24 um 09:49 schrieb Javier Martinez Canillas:
> > Thomas Zimmermann <tzimmermann@suse.de> writes:
> > 
> > Hello Thomas,
> > 
> > > Hi
> > > 
> > > Am 10.06.24 um 10:47 schrieb Thomas Zimmermann:
> > > > 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?
> > > > 
> > > > > [    3.536043] ------------[ cut here ]------------
> > > > > [    3.540716] virt_to_phys used for non-linear address:
> > > > > 000000007fc4f540 (0xffff800086001000)
> > > > > [    3.552628] WARNING: CPU: 4 PID: 61 at arch/arm64/mm/physaddr.c:12
> > > > > __virt_to_phys+0x68/0x98
> > > > > [    3.565455] Modules linked in:
> > > > > [    3.568525] CPU: 4 PID: 61 Comm: kworker/u12:5 Not tainted
> > > > > 6.6.23-06226-g4986cc3e1b75-dirty #250
> > > > > [    3.577310] Hardware name: NXP i.MX95 19X19 board (DT)
> > > > > [    3.582452] Workqueue: events_unbound deferred_probe_work_func
> > > > > [    3.588291] pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS
> > > > > BTYPE=--)
> > > > > [    3.595233] pc : __virt_to_phys+0x68/0x98
> > > > > [    3.599246] lr : __virt_to_phys+0x68/0x98
> > > > > [    3.603276] sp : ffff800083603990
> > > > > [    3.677939] Call trace:
> > > > > [    3.680393]  __virt_to_phys+0x68/0x98
> > > > > [    3.684067]  drm_fbdev_dma_helper_fb_probe+0x138/0x238
> > > > > [    3.689214] __drm_fb_helper_initial_config_and_unlock+0x2b0/0x4c0
> > > > > [    3.695385]  drm_fb_helper_initial_config+0x4c/0x68
> > > > > [    3.700264]  drm_fbdev_dma_client_hotplug+0x8c/0xe0
> > > > > [    3.705161]  drm_client_register+0x60/0xb0
> > > > > [    3.709269]  drm_fbdev_dma_setup+0x94/0x148
> > > > > 
> > > > > So add a check 'is_vmalloc_addr'.
> > > > > 
> > > > > Fixes: b79fe9abd58b ("drm/fbdev-dma: Implement fbdev emulation for
> > > > > GEM DMA helpers")
> > > > > Signed-off-by: Peng Fan <peng.fan@nxp.com>
> > > > Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
> > > I'm taking back my r-b. The memory is expected to by be physically
> > > contiguous and vmalloc() won't guarantee that.
> > > 
> > Agreed.
> 
> These smem_ fields are clearly designed for PCI BARs of traditional graphics
> cards. So can we even assume contiguous memory for DMA? That was my
> assumption, but with IOMMUs it might not be the case. Fbdev-dma only sets
> smem_start to support a single old userspace driver. Maybe we should further
> restrict usage of this field by making it opt-in for each driver. Best
> regards Thomas

We could make it all conditional on CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM, and
remove the FBINFO_HIDE_SMEM_START flag. The reason I've done the flag is
that with the old fb_mmap code we had to always fill out smem_start to
make mmap work. But now that the various drm fbdev helpers have all their
own mmap implementation, we could make this a lot cleaner.

If I haven't missed anything, that is.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch