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

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Date:   Fri, 11 Jan 2019 17:49:15 +0800
From:   CK Hu <ck.hu@...iatek.com>
To:     Daniel Vetter <daniel@...ll.ch>
CC:     Noralf Trønnes <noralf@...nnes.org>,
        Frank Wunderlich <frank-w@...lic-files.de>,
        David Airlie <airlied@...ux.ie>,
        "Alexander Ryabchenko" <d3adme4t@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Re: [PATCH] drm/mediatek: Add MTK Framebuffer-Device (mt7623)

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.

Regards,
CK

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