Re: [PATCH 3/3] drm/mediatek: Use drm_gem_cma_object instead of mtk_drm_gem_obj

[CK Hu <ck.hu@mediatek.com>]

Hi, Daniel:

On Mon, 2018-10-29 at 10:16 +0100, Daniel Vetter wrote:
> On Mon, Oct 29, 2018 at 11:11:16AM +0800, CK Hu wrote:
> > Hi,Daniel:
> > 
> > On Fri, 2018-10-26 at 12:21 +0200, Daniel Vetter wrote:
> > > On Fri, Oct 26, 2018 at 03:22:03PM +0800, CK Hu wrote:
> > > > After adding dma_dev in struct drm_device and
> > > > drm_gem_cma_dumb_create_no_kmap(), drm_gem_cma_object could replace
> > > > mtk_drm_gem_obj, so use drm_gem_cma_object instead of mtk_drm_gem_obj to
> > > > reduce redundant code.
> > > > 
> > > > Signed-off-by: CK Hu <ck.hu@mediatek.com>
> > > 
> > > A few questions/thoughts:
> > > 
> > > - Why do you need both drm_device->dev and drm_device->dma_dev? Can't you
> > >   just register the drm_device with the right struct device?
> > > 
> > 
> > In [1], mmsys is the drm driver and ovl0 and ovl1 is the sub device
> > which has dma function.
> > In this drm, there are two crtc and each one is comprised of many
> > component.
> > This is an example of mt8173:
> > 
> > crtc0: ovl0, color0, aal, od, rdma0, ufoe, dsi0
> > crtc1: ovl1, color1, gamma, rdma1, dpi0
> > 
> > In the device node of ovl0 and ovl1, there is a 'iommus' parameter in
> > it, so use dma_alloc_xxx() and dma_map_xxx() with that device would get
> > iova rather than pa. I don't think it's a good idea to register ovl0 or
> > ovl1 as drm device because each one is just a component in a pipeline.
> > mmsys controls the clock and routing of multi-media system which include
> > this drm system, so it's better to register mmsys as drm device. Maybe
> > we could move 'iommus' parameter from ovl device to mmsys device, so the
> > dma device changes from ovl device to mmsys device. I'm not sure this
> > would be a good choice, how do you think?
> > 
> > [1]
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/boot/dts/mediatek/mt8173.dtsi?h=v4.19
> 
> Ah ok. But if you have 2 blocks that make up the overall drm device, why
> don't you need to switch at runtime between them? I.e. buffer allocated
> for crtc0 needs to be dma-mapped to crtc0, buffer allocated to crtc1 needs
> to be dma-mapped on crtc1?
> 
> And if they're both the exact same iommu, then imo it would make indeed
> sense to move the iommu attribute up. Since your current code cant'
> actually handle truly separate dma-mappings. And neither can your patch
> series here handled separate iommu for crtc0 and crtc1.

Yes, they're the exact same iommu. So I would move iommu attribute up.

> 
> > > - You don't use drm_gem_prime_import_dev, so prime import isn't using the
> > >   right device either.
> > 
> > Yes, you are right. I'm not familiar with whore drm core, so I start to
> > modify what Mediatek drm use. But this function still works for the drm
> > device that itself is dma device. If one day there is a drm device which
> > itself is not a dma device and need this function, send a patch to
> > modify this function and test it with that drm device. If you want me to
> > modify all in advance, I'm ok but need others to test it because
> > Mediatek drm driver does not use them.
> 
> I meant to say that mediatek should use drm_gem_prime_import_dev, but
> currently isn't using that. And your patch series here doesn't fix that
> either. So there's more bugs left in this area.

Great, you find a bug. My test only include export but not import. This
would take time to generate import-test environment.

> 
> > > - exynos seems to have the same or at least similar issue, stronger case
> > >   for your patches if you can solve both.
> > 
> > I'm still Mediatek's employee. If I modify other company's driver and it
> > is not a MUST-BE for Mediatek, Mediatek may think I give contribution to
> > other company. So I've better not to modify exynos driver.
> 
> This isn't how upstream works :-)

OK, because now I would not modify drm core, I would focus on Mediatek
drm driver first. If the modification of exynos driver is easy, I could
try. But if the modification of exynos is huge, I suggest that someone
who is familiar with exynos driver and have exynos platform to do it.

Regards,
CK
> 
> > > - I'd start out with using struct drm_gem_cma_object in mtk (similar to
> > >   what vc4 does), and then reusing as much as possible of the existing
> > >   helpers. And then looking later on what's still left (like the support
> > >   for leaving out the virtual mapping).
> > 
> > I'm not clear what vc4 does. It looks like that you want me to redefine
> > mtk_drm_gem_obj based on drm_gem_cma_object. So it would be like
> > 
> > struct mtk_drm_gem_obj {
> > 	struct drm_gem_cma_object base;
> > 	void *cookie;
> > 	unsigned long dma_attrs;
> > };
> > 
> > I could try to modify as this and see what have left.
> 
> Yup, that's my suggestion. Then we can look at what mtk can use unchanged
> from the core helpers. And what would need to change and so better
> evaluate whether it makes sense to do that.
> 
> I still think just moving the iommu is probably best.
> -Daniel