Re: [PATCH] drm/mediatek: Add MTK Framebuffer-Device (mt7623)

[CK Hu]

On Fri, 2019-01-11 at 17:07 +0100, Noralf TrÃnnes wrote:
> 
> Den 11.01.2019 11.25, skrev Daniel Vetter:
> > On Fri, Jan 11, 2019 at 05:49:15PM +0800, CK Hu wrote:
> >> Hi, Daniel:
> >>
> >> On Fri, 2019-01-11 at 10:20 +0100, Daniel Vetter wrote:
> >>> On Fri, Jan 11, 2019 at 11:20:09AM +0800, CK Hu wrote:
> >>>> Hi, Daniel:
> >>>>
> >>>> On Thu, 2019-01-10 at 21:02 +0100, Daniel Vetter wrote:
> >>>>> On Thu, Jan 10, 2019 at 08:01:37PM +0100, Frank Wunderlich wrote:
> >>>>>> Hi Daniel,
> >>>>>>
> >>>>>>>> Would be good to use the new generic fbdev emulation code here, for even
> >>>>>>>> less code. Or at least know why this isn't possible to use for mtk (and
> >>>>>>>> maybe address that in the core code). Hand-rolling fbdev code shouldn't be
> >>>>>>>> needed anymore.
> >>>>>>>
> >>>>>>> Back on the mailing list, no private replies please:
> >>>>>>
> >>>>>> i don't wanted to spam all people with dumb questions ;)
> >>>>>
> >>>>> There's no dumb questions, only insufficient documentation :-)
> >>>>>
> >>>>>>> For examples please grep for drm_fbdev_generic_setup(). There's also a
> >>>>>>> still in-flight series from Gerd Hoffmann to convert over bochs. That,
> >>>>>>> plus all the kerneldoc linked from there should get you started.
> >>>>>>> -Daniel
> >>>>>>
> >>>>>> this is one of google best founds if i search for drm_fbdev_generic_setup:
> >>>>>>
> >>>>>> https://lkml.org/lkml/2018/12/19/305
> >>>>>>
> >>>>>> not very helpful...
> >>>>>>
> >>>>>> so i tried kernel-doc
> >>>>>>
> >>>>>> https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html?highlight=drm_fbdev_generic_setup#c.drm_fbdev_generic_setup
> >>>>>>
> >>>>>> which is nice function-reference but i've found no generic workflow
> >>>>>>
> >>>>>> as the posted driver is "only" a driver ported from kernel 4.4 by Alexander, i don't know if this new framework can be used and which parts need to be changed. I only try to bring his code Mainline....
> >>>>>> Maybe CK Hu can help here because driver is originally from him and he knows internals. Or maybe you can help here?
> >>>>>>
> >>>>>> i personally make my first steps as spare-time kernel-developer :)
> >>>>>
> >>>>> There's a ton of in-kernel users of that function already, I meant you can
> >>>>> use those to serve as examples ... If those + the kerneldoc aren't
> >>>>> good enough, then we need to improve them.
> >>>>> -Daniel
> >>>>
> >>>> I've tried with drm_fbdev_generic_setup() and it works fine with simple
> >>>> modification. The patch is
> >>>>
> >>>> --- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> >>>> +++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> >>>> @@ -16,6 +16,7 @@
> >>>>  #include <drm/drm_atomic.h>
> >>>>  #include <drm/drm_atomic_helper.h>
> >>>>  #include <drm/drm_crtc_helper.h>
> >>>> +#include <drm/drm_fb_helper.h>
> >>>>  #include <drm/drm_gem.h>
> >>>>  #include <drm/drm_gem_cma_helper.h>
> >>>>  #include <drm/drm_of.h>
> >>>> @@ -379,6 +380,7 @@ static void mtk_drm_kms_deinit(struct drm_device
> >>>> *drm)
> >>>>         .gem_prime_get_sg_table = mtk_gem_prime_get_sg_table,
> >>>>         .gem_prime_import_sg_table = mtk_gem_prime_import_sg_table,
> >>>>         .gem_prime_mmap = mtk_drm_gem_mmap_buf,
> >>>> +       .gem_prime_vmap = mtk_drm_gem_prime_vmap,
> >>>>         .ioctls = mtk_ioctls,
> >>>>         .num_ioctls = ARRAY_SIZE(mtk_ioctls),
> >>>>         .fops = &mtk_drm_fops,
> >>>> @@ -416,6 +418,8 @@ static int mtk_drm_bind(struct device *dev)
> >>>>         if (ret < 0)
> >>>>                 goto err_deinit;
> >>>>
> >>>> +       drm_fbdev_generic_setup(drm, 32);
> >>>> +
> >>>>         return 0;
> >>>>
> >>>>
> >>>> But I implement .gem_prime_vmap with a workaround,
> >>>>
> >>>>
> >>>> --- a/drivers/gpu/drm/mediatek/mtk_drm_gem.c
> >>>> +++ b/drivers/gpu/drm/mediatek/mtk_drm_gem.c
> >>>> @@ -280,3 +280,8 @@ int mtk_gem_create_ioctl(struct drm_device *dev,
> >>>> void *data,
> >>>>         mtk_drm_gem_free_object(&mtk_gem->base);
> >>>>         return ret;
> >>>>  }
> >>>> +
> >>>> +void *mtk_drm_gem_prime_vmap(struct drm_gem_object *obj)
> >>>> +{
> >>>> +       return (void *)1;
> >>>> +}
> >>>>
> >>>> Current drm_fb_helper depends on drm_client to create fbdev. When
> >>>> drm_client create drm_client_buffer, it need to vmap to get kernel
> >>>> vaddr. But I think for fbdev, the vaddr is useless. Do you agree that I
> >>>> temporarily implement .gem_prime_vmap in such way?
> >>>
> >>> The vmap is used by fbcon, without that fbdev won't really work I think.
> >>> So I'm rather confused on what you tested ...
> >>>
> >>> Adding Noralf, he's the expert here.
> >>> -Daniel
> >>
> >> I test with fb_test [1], it is a user space program just open /dev/fb0
> >> and mmap it to user space. I does not turn on fbcon so everything looks
> >> well. If support fbcon is essential, I would implement vmap correctly.
> >> If it's not essential, I think I could return 0 when
> >> CONFIG_FRAMBUFFER_CONSOLE is defined.
> > 
> > I think fbdev emulation without working fbcon is fairly useless. And it
> > shouldn't be that much work to make it work, we have quite a few more
> > helers for gem bo nowadays.
> 
> If the virtual address is not set, fbcon can't be used and that means it
> has to be disabled using something like this in the driver Kconfig:
> 	depends on !FRAMEBUFFER_CONSOLE
> 
> Otherwise it will crash if someone tries to use it.
> 
> The problem here is that this driver doesn't map a virtual address for
> its buffers:
> 	mtk_gem->dma_attrs |= DMA_ATTR_NO_KERNEL_MAPPING;
> 
> Probably because of limited virtual address space.
> rockchip is in the same situation.
> 
> I'm aware of this shortcoming of the generic emulation, but I don't see
> how it can be solved without using somekind of flag attached to the dumb
> buffer creation request.

The virtual address space is limited so we do not map it by default. I
also see the shortcoming of the generic emulation, so I would refer to
rockchip to implement this mapping.

Regards,
CK

> 
> Noralf.