[PATCH 05/20] drm/meson: Use drm_fbdev_generic_setup()

[Neil Armstrong]

Hi Daniel,

On 13/09/2018 15:21, Daniel Vetter wrote:
> On Wed, Sep 12, 2018 at 01:06:07PM +0200, Noralf Trønnes wrote:
>>
>> Den 12.09.2018 12.57, skrev Noralf Trønnes:
>>> (Cc: Daniel Vetter)
>>>
>>
>> Somehow that CC was dropped somewhere after leaving email client.
>> Trying once more.
> 
> Yeah I just made myself unpopular. If you want SMEM_START, then you need
> to carry a local patch now. virt_to_pfn on the vmap should work. It's
> about 2 lines, including the change to drop HIDE_SMEM_START.

Would it be totally unacceptable to add such 2 line :
- enabled by a specific config depending on EXPERT and narrowed to specific platforms
- printing a big fat pr_warning_once when used
- with a big fat comment specifying when this code will be dropped and why we should not activate it

Neil
> 
> And if consensus is that hiding SMEM_START is not a nice idea, then I'm
> sure we can reintroduce it through some slightly unpretty backdoor, even
> with Noralf's generic code. So not really a good reason to reject the
> cleanup I think.
> -Daniel
> 
> 
>>> Den 12.09.2018 11.56, skrev Maxime Ripard:
>>>> On Wed, Sep 12, 2018 at 11:48:34AM +0200, Neil Armstrong wrote:
>>>>> Hi Noralf,
>>>>>
>>>>> On 08/09/2018 15:46, Noralf Trønnes wrote:
>>>>>> The CMA helper is already using the
>>>>>> drm_fb_helper_generic_probe part of
>>>>>> the generic fbdev emulation. This patch makes full use of the generic
>>>>>> fbdev emulation by using its drm_client callbacks. This means that
>>>>>> drm_mode_config_funcs->output_poll_changed and
>>>>>> drm_driver->lastclose are
>>>>>> now handled by the emulation code. Additionally fbdev
>>>>>> unregister happens
>>>>>> automatically on drm_dev_unregister().
>>>>>>
>>>>>> The drm_fbdev_generic_setup() call is put after
>>>>>> drm_dev_register() in the
>>>>>> driver. This is done to highlight the fact that fbdev emulation is an
>>>>>> internal client that makes use of the driver, it is not part of the
>>>>>> driver as such. If fbdev setup fails, an error is printed,
>>>>>> but the driver
>>>>>> succeeds probing.
>>>>> I can't really ack/nack this move, on one side it's great to have a
>>>>> generic fbdev emulation instead of the old fbdev code, but on the
>>>>> other side the Amlogic platform (like allwinner, samsung, rockchip,
>>>>> ...)  still relies on an Fbdev variant of the libMali that uses
>>>>> smem_start...
>>>>>
>>>>> I know it's dirty and fbdev based code should be legacy now, but I
>>>>> consider it like an user-space breakage...
>>>>>
>>>>> I'll be happy if ARM provided it's Mali vendors a Fbdev libMali that
>>>>> could use FBINFO_HIDE_SMEM_START and allocate BO's from the DRM
>>>>> driver, it won't be the case and it will never be the case until the
>>>>> Lima projects finalizes.
>>>>>
>>>>> So for me it's a no-go until Lima lands upstream in Linux *and* Mesa.
>>>> My feelings exactly. If the choice is between reducing the DRM driver
>>>> by a couple of dozens of lines or keeping the mali working, I'll pick
>>>> the latter, every single day.
>>>
>>> I don't know the reasoning for blocking smem_start other than what Daniel
>>> wrote in these commit messages:
>>>
>>> da6c7707caf3736c1cf968606bd97c07e79625d4
>>> fbdev: Add FBINFO_HIDE_SMEM_START flag
>>>
>>>   DRM drivers really, really, really don't want random userspace to
>>>   share buffer behind it's back, bypassing the dma-buf buffer sharing
>>>   machanism. For that reason we've ruthlessly rejected any IOCTL
>>>   exposing the physical address of any graphics buffer.
>>>
>>>   Unfortunately fbdev comes with that built-in. We could just set
>>>   smem_start to 0, but that means we'd have to hand-roll our own fb_mmap
>>>   implementation. For good reasons many drivers do that, but
>>>   smem_start/length is still super convenient.
>>>
>>>   Hence instead just stop the leak in the ioctl, to keep fb mmap working
>>>   as-is. A second patch will set this flag for all drm drivers.
>>>
>>>
>>> 6be8f3bd2c78915a9f3a058a346ae93068d35c01
>>> drm/fb: Stop leaking physical address
>>>
>>>   For buffer sharing, use dma-buf instead. We can't set smem_start to 0
>>>   unconditionally since that's used by the fbdev mmap default
>>>   implementation. And we have plenty of userspace which would like to
>>>   keep that working.
>>>
>>>   This might break legit userspace - if it does we need to look at a
>>>   case-by-cases basis how to handle that. Worst case I expect overrides
>>>   for only specific drivers, since anything remotely modern should be
>>>   using dma-buf/prime now (which is about 7 years old now for DRM
>>>   drivers).
>>>
>>>   This issue was uncovered because Noralf's rework to implement a
>>>   generic fb_probe also implements it's own fb_mmap callback. Which
>>>   means smem_start didn't have to be set anymore, which blew up some
>>>   blob in userspace rather badly.
>>>
>>>
>>> Noralf.
>>>
>>> _______________________________________________
>>> dri-devel mailing list
>>> dri-devel at lists.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>
>>
>