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

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 12.06.24 um 17:00 schrieb Daniel Vetter:
> 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.

Enabling CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM would still crash the NXP 
driver. I think I'll add a flag to drm_fbdev_dma_setup() to set 
smem_start from within lima, which is the only driver that requires 
it.I'd like to remove CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM and all that, but 
I fear that it would break someone's setup. Best regards Thomas
>
> If I haven't missed anything, that is.
> -Sima

-- 
--
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)