[PATCH v5 4/8] drm/cma-helper: Use the generic fbdev emulation

[John Stultz]

On Tue, Aug 21, 2018 at 7:59 AM, Noralf Trønnes <noralf at tronnes.org> wrote:
> Den 21.08.2018 10.44, skrev Daniel Vetter:
>> On Mon, Aug 20, 2018 at 11:44:56PM -0700, John Stultz wrote:
>>>
>>> Since we don't have a drm_gem_cma_object reference in
>>> drm_fb_helper_generic_probe(), I instead added:
>>>     fbi->fix.smem_start = page_to_phys(virt_to_page(fbi->screen_buffer));
>>>
>>> And that got things working!
>>>
>>> So yea, I wanted to figure out if we are just missing the line above
>>> in drm_fb_helper_generic_probe()? Or if the kirin driver should be
>>> setting the fix.smem_start in some other fashion?
>>
>> With the generic helpers we can't use the generic fb_mmap() implementation
>> in fbdev/core/fbmem.c anymore. Instead Noralf added a generic fb_mmap
>> fb_ops callback in drm_fbdev_fb_mmap, which you're supposed to use. Can
>> you pls double-check that this is wired up correctly for kirin?
>>
>> At least I don't see any other place where we use smem_start in the fbdev
>> code. It does get copied to userspace, but userspace should never use
>> that.
>
>
> I googled 'FBIOGET_FSCREENINFO smem_start' and came across info[1]
> about the Mali blob using it and HiKey seems to have a Mali GPU:
>
> 00:17 <ssvb> BorgCuba:
> the mali blob is supposed to open the framebuffer device,
> get "smem_start" and try to map this memory via MALI_IOC_MEM_MAP_EXT ioctl
>
> Could this be the problem here?

It does look like that's the case. Looking for smem_start usage, for
Android its the gralloc code that fetches it:
   https://android.googlesource.com/device/linaro/hikey/+/master/gralloc/framebuffer_device.cpp#353
and then puts it into the private_handle_t:
   https://android.googlesource.com/device/linaro/hikey/+/master/gralloc/framebuffer_device.cpp#384

Which I assume then gets used later in the opaque mali blob (which
then results in a similarly opaque hang).

While I'm perfectly understanding that folks are not interested as its
related to mali (which is fine, I can carry a patch - working to move
away from fbdev emulation anyway), I am wondering if it might cause
trouble for other users, as smem_start was previously set and now its
not, so it seems likely to break userspace examples like:
   https://www.linuxtv.org/downloads/v4l-dvb-apis-old/osd.html

Or are those such old legacy they don't really count?

thanks
-john