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

[Neil Armstrong]

Hi Daniel,

On 13/09/2018 16:55, Daniel Vetter wrote:
> On Thu, Sep 13, 2018 at 04:26:53PM +0200, Neil Armstrong wrote:
>> 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
> 
> module_param_debug auto-taints your kernel, I'd just go with that. Plus
> CONFIG_EXPERT (or CONFIG_BROKEN).

OK, I think you mean module_param_unsafe, but I see the point.

> 
> But yes, I think that's something I'll happily ack. Must be acked by Dave
> (and perhaps a few others too), since defacto this means we now suddenly
> care about closed source blobs. I'd say get someone from amd and a few of
> the driver maintainers on board with this (nouveau, Eric, Rob, Lucas,
> ...), to make sure it really represents community consensus.

I'll drop something, but I'm afraid a kernel won't have this hack, shouldn't this serie be delayed for a release ?

@Noaralf, do you have a branch somewhere I can base a work on ?

Neil

> 
> Cheers, Daniel
> 
>>
>> 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
>>>>>
>>>>
>>>
>>
>