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Date:   Fri, 11 Jan 2019 11:25:03 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     CK Hu <ck.hu@mediatek.com>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Noralf =?iso-8859-1?Q?Tr=F8nnes?= <noralf@tronnes.org>,
        Frank Wunderlich <frank-w@public-files.de>,
        David Airlie <airlied@linux.ie>,
        Alexander Ryabchenko <d3adme4t@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@lists.infradead.org>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: Re: [PATCH] drm/mediatek: Add MTK Framebuffer-Device (mt7623)
Message-ID: <20190111102503.GU21184@phenom.ffwll.local>
Mail-Followup-To: CK Hu <ck.hu@mediatek.com>,
        Noralf =?iso-8859-1?Q?Tr=F8nnes?= <noralf@tronnes.org>,
        Frank Wunderlich <frank-w@public-files.de>,
        David Airlie <airlied@linux.ie>,
        Alexander Ryabchenko <d3adme4t@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Philipp Zabel <p.zabel@pengutronix.de>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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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.
-Daniel

> 
> Regards,
> CK
> 
> [1]
> https://android.googlesource.com/platform/system/extras/+/master/tests/framebuffer/fb_test.c
> 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch