Re: [PATCH] drm: gem: add an option for supporting the dma-coherent hardware.

[Sui Jingfeng <suijingfeng@xxxxxxxxxxx>]

Hi,  welcome to discussion.


I have limited skills in manipulating English.

It may not express what I'm really means in the short time.

Part of word in the sentence may not as accurate as your.

Well, please don't misunderstand, I'm not doing the rude to you.

I will explain it with more details.

See below:


On 2023/6/7 20:09, Paul Cercueil wrote:
> Hi Sui,
>
> Le mercredi 07 juin 2023 à 18:30 +0800, Sui Jingfeng a écrit :
> > Hi,
> >
> >
> > On 2023/6/7 17:36, Paul Cercueil wrote:
> > > Hi Sui,
> > >
> > > Le mercredi 07 juin 2023 à 13:30 +0800, Sui Jingfeng a écrit :
> > > > The single map_noncoherent member of struct drm_gem_dma_object
> > > > may
> > > > not
> > > > sufficient for describing the backing memory of the GEM buffer
> > > > object.
> > > >
> > > > Especially on dma-coherent systems, the backing memory is both
> > > > cached
> > > > coherent for multi-core CPUs and dma-coherent for peripheral
> > > > device.
> > > > Say architectures like X86-64, LoongArch64, Loongson Mips64, etc.
> > > >
> > > > Whether a peripheral device is dma-coherent or not can be
> > > > implementation-dependent. The single map_noncoherent option is
> > > > not
> > > > enough
> > > > to reflect real hardware anymore. For example, the Loongson
> > > > LS3A4000
> > > > CPU
> > > > and LS2K2000/LS2K1000 SoC, peripheral device of such hardware
> > > > platform
> > > > allways snoop CPU's cache. Doing the allocation with
> > > > dma_alloc_coherent
> > > > function is preferred. The return buffer is cached, it should not
> > > > using
> > > > the default write-combine mapping. While with the current
> > > > implement,
> > > > there
> > > > no way to tell the drm core to reflect this.
> > > >
> > > > This patch adds cached and coherent members to struct
> > > > drm_gem_dma_object.
> > > > which allow driver implements to inform the core. Introducing new
> > > > mappings
> > > > while keeping the original default behavior unchanged.
> > > Did you try to simply set the "dma-coherent" property to the
> > > device's
> > > node?
> > But this approach can only be applied for the device driver with DT
> > support.
> >
> > X86-64, Loongson ls3a4000 mips64, Loongson ls3a5000 CPU typically do
> > not
> > have DT support.
> >
> > They using ACPI to pass parameter from the firmware to Linux kernel.
> >
> > You approach will lost the effectiveness on such a case.
> Well, I don't really know how ACPI handles it - but it should just be a
> matter of setting dev->dma_coherent. That's basically what the DT code
> does.
>
> Some MIPS boards set it in their setup code for instance.
This is a *strategy*, not a *mechanism*.

In this case, DT is just used to describing the hardware.

(It is actually a hardware feature describing language, the granularity 
is large)

It does not changing the state of the hardware.

It's your platform firmware or kernel setting up code who actually do 
such a things.


It's just that it works on *one* platform, it does not guarantee it will 
works on others.

While my patch is trying to create a *mechanism* which could probably

works on all platform.


It is based the patch you have already commuted.

Thanks for your excellent contribution.


> > >   From what I understand if you add that property then Linux will
> > > use DMA
> > > coherent memory even though you use dma_alloc_noncoherent() and the
> > > sync_single_for_cpu() / sync_single_for_device() are then NOPs.
> > Please do not mitigate the problems with confusing method.
> >
> >
> > This approach not only tend to generate confusion but also
> > implement-dependent
> >
> > and arch-dependent. It's definitely problematic.
> >
> >
> > How does the dma_alloc_coherent/dma_alloc_noncoherent is a ARCH
> > specific
> > thing.
> >
> > Dependent on how does the arch_dma_ops is implemented.
> >
> >
> > The definition of the coherent on different ARCH has different
> > meanings.
> >
> > The definition of the wirte-combine on different ARCH has different
> > meanings.
> >
> >
> > The wirte-combine(uncache acceleration) on mips is non dma-coherent.
> It is dma-coherent on Ingenic SoCs.
It is dma-coherent ? How does it achieve it?


As far as I know,  there is a write buffer within the mips cpu.

typically 64 byte,  but it is not cache. It will gather the CPU write 
access,

When a peripheral device do the DMA, how does you platform guarantee

the data in the CPU write buffer has been already arrived at (or flushed 
out to)

the system RAM?


Does the  peripheral device snoop the CPU's write buffer,

or it need manually flush the write buffer with SYNC instruction?

> > But on arm, It seem that wirte-combine is coherent. (guaranteed by
> > arch
> > implement).
> >
> >
> > I also heard using dma_alloc_coherent  to allocation the buffer for
> > the
> > non-coherent doesn't hurt, but the reverse is not true.
> >
> >
> > But please do not create confusion.
> >
> > software composite is faster because better cacheusing rate and
> >
> > cache is faster to read.
> >
> > It is faster because it is cached, not because it is non-coherent.
> >
> > non-coherent is arch thing and/or driver-side thing,
> >
> > it is a side effect of  using the cached mapping.
> Yes, I know that.
>
> > It should left to driver to handle such a side effect. The device
> > driver
> >
> > know their device, so its the device driver's responsibility to
> > maintain
> > the coherency.  On loongson platform, we don't need to call
> > drm_fb_dma_sync_non_coherent() function, Its already guaranteed by
> > hardware.
> I understand. What I'm saying, is that you should be able to set
> dma_obj->map_noncoherent (which would arguably be better named
> "map_cached",

My point is that the word *cached* reflect the nature,

dma-coherent or dma-noncoherent is secondary.

We are all on the way to pursue the performance.

In the end, it is the cache give us the speed.


Why not we credit the cache hardware inside of the CPU?

> but that's a different problem). Then the GEM code would
> end up calling dma_alloc_noncoherent(), which will give you *cached*
> memory. Then as long as dev->dma_coherent = true,
> drm_fb_dma_sync_non_coherent() should be a NOP - so you wouldn't
> pointlessly sync/invalidate the caches.
>
> And I disagree with you, the driver shouldn't handle such things.

You already handle the side effect of such things, See below:


```

   if (ingenic_drm_map_noncoherent(ipu->master))
        drm_fb_dma_sync_non_coherent(ipu->drm, oldstate, newstate);

```

By the way,  Ingenic is the only driver in the drivers/gpu/drm/ that 
handle such things

so far.

>   The
> fact that it is better to use cached memory or uncached with write-
> combine really is platform-specific and not something that the driver
> should be aware of.

But the fact is that,  It is drm/ingenic tell the drm core,  some SoC is 
prefer cached,

but unable to enforce the coherent. So that it need  flush the cache 
manually.

What do you meant by saying that the driver should not be aware of ?

> Cheers,
> -Paul
>
> > > Cheers,
> > > -Paul
> > >
> > > > Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
> > > > ---
> > > >    drivers/gpu/drm/drm_fb_dma_helper.c       | 11 +++++------
> > > >    drivers/gpu/drm/drm_fbdev_dma.c           |  2 +-
> > > >    drivers/gpu/drm/drm_gem_dma_helper.c      | 20
> > > > ++++++++++++++++----
> > > >    drivers/gpu/drm/ingenic/ingenic-drm-drv.c |  5 ++++-
> > > >    drivers/gpu/drm/rcar-du/Kconfig           |  2 --
> > > >    drivers/gpu/drm/rcar-du/rcar_du_kms.c     |  4 +++-
> > > >    include/drm/drm_gem_dma_helper.h          |  7 +++++--
> > > >    7 files changed, 34 insertions(+), 17 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/drm_fb_dma_helper.c
> > > > b/drivers/gpu/drm/drm_fb_dma_helper.c
> > > > index 3b535ad1b07c..93ff05041192 100644
> > > > --- a/drivers/gpu/drm/drm_fb_dma_helper.c
> > > > +++ b/drivers/gpu/drm/drm_fb_dma_helper.c
> > > > @@ -106,16 +106,15 @@ dma_addr_t drm_fb_dma_get_gem_addr(struct
> > > > drm_framebuffer *fb,
> > > >    EXPORT_SYMBOL_GPL(drm_fb_dma_get_gem_addr);
> > > >    
> > > >    /**
> > > > - * drm_fb_dma_sync_non_coherent - Sync GEM object to non-
> > > > coherent
> > > > backing
> > > > - *     memory
> > > > + * drm_fb_dma_sync_non_coherent - Sync GEM object to cached
> > > > backing
> > > > memory
> > > >     * @drm: DRM device
> > > >     * @old_state: Old plane state
> > > >     * @state: New plane state
> > > >     *
> > > >     * This function can be used by drivers that use damage clips
> > > > and
> > > > have
> > > > - * DMA GEM objects backed by non-coherent memory. Calling this
> > > > function
> > > > - * in a plane's .atomic_update ensures that all the data in the
> > > > backing
> > > > - * memory have been written to RAM.
> > > > + * DMA GEM objects backed by cached memory. Calling this
> > > > function in
> > > > a
> > > > + * plane's .atomic_update ensures that all the data in the
> > > > backing
> > > > memory
> > > > + * have been written to RAM.
> > > >     */
> > > >    void drm_fb_dma_sync_non_coherent(struct drm_device *drm,
> > > >                                     struct drm_plane_state
> > > > *old_state,
> > > > @@ -131,7 +130,7 @@ void drm_fb_dma_sync_non_coherent(struct
> > > > drm_device *drm,
> > > >    
> > > >           for (i = 0; i < finfo->num_planes; i++) {
> > > >                   dma_obj = drm_fb_dma_get_gem_obj(state->fb, i);
> > > > -               if (!dma_obj->map_noncoherent)
> > > > +               if (dma_obj->cached && dma_obj->coherent)
> > > >                           continue;
> > > >    
> > > >                   daddr = drm_fb_dma_get_gem_addr(state->fb,
> > > > state, i);
> > > > diff --git a/drivers/gpu/drm/drm_fbdev_dma.c
> > > > b/drivers/gpu/drm/drm_fbdev_dma.c
> > > > index d86773fa8ab0..49fe9b284cc8 100644
> > > > --- a/drivers/gpu/drm/drm_fbdev_dma.c
> > > > +++ b/drivers/gpu/drm/drm_fbdev_dma.c
> > > > @@ -131,7 +131,7 @@ static int
> > > > drm_fbdev_dma_helper_fb_probe(struct
> > > > drm_fb_helper *fb_helper,
> > > >    
> > > >           /* screen */
> > > >           info->flags |= FBINFO_VIRTFB; /* system memory */
> > > > -       if (dma_obj->map_noncoherent)
> > > > +       if (dma_obj->cached)
> > > >                   info->flags |= FBINFO_READS_FAST; /* signal
> > > > caching
> > > > */
> > > >           info->screen_size = sizes->surface_height * fb-
> > > > > pitches[0];
> > > >           info->screen_buffer = map.vaddr;
> > > > diff --git a/drivers/gpu/drm/drm_gem_dma_helper.c
> > > > b/drivers/gpu/drm/drm_gem_dma_helper.c
> > > > index 870b90b78bc4..dec1d512bdf1 100644
> > > > --- a/drivers/gpu/drm/drm_gem_dma_helper.c
> > > > +++ b/drivers/gpu/drm/drm_gem_dma_helper.c
> > > > @@ -93,7 +93,11 @@ __drm_gem_dma_create(struct drm_device *drm,
> > > > size_t size, bool private)
> > > >                   drm_gem_private_object_init(drm, gem_obj, size);
> > > >    
> > > >                   /* Always use writecombine for dma-buf mappings
> > > > */
> > > > -               dma_obj->map_noncoherent = false;
> > > > +               /* FIXME: This is not always true, on some dma
> > > > coherent system,
> > > > +                * cached mappings should be preferred over
> > > > writecombine
> > > > +                */
> > > > +               dma_obj->cached = false;
> > > > +               dma_obj->coherent = false;
> > > >           } else {
> > > >                   ret = drm_gem_object_init(drm, gem_obj, size);
> > > >           }
> > > > @@ -143,7 +147,11 @@ struct drm_gem_dma_object
> > > > *drm_gem_dma_create(struct drm_device *drm,
> > > >           if (IS_ERR(dma_obj))
> > > >                   return dma_obj;
> > > >    
> > > > -       if (dma_obj->map_noncoherent) {
> > > > +       if (dma_obj->cached && dma_obj->coherent) {
> > > > +               dma_obj->vaddr = dma_alloc_coherent(drm->dev,
> > > > size,
> > > > +                                                   &dma_obj-
> > > > > dma_addr,
> > > > +                                                   GFP_KERNEL |
> > > > __GFP_NOWARN);
> > > > +       } else if (dma_obj->cached && !dma_obj->coherent) {
> > > >                   dma_obj->vaddr = dma_alloc_noncoherent(drm->dev,
> > > > size,
> > > >                                                          &dma_obj-
> > > > > dma_addr,
> > > >                                                         
> > > > DMA_TO_DEVICE,
> > > > @@ -153,6 +161,7 @@ struct drm_gem_dma_object
> > > > *drm_gem_dma_create(struct drm_device *drm,
> > > >                                                 &dma_obj-
> > > > > dma_addr,
> > > >                                                 GFP_KERNEL |
> > > > __GFP_NOWARN);
> > > >           }
> > > > +
> > > >           if (!dma_obj->vaddr) {
> > > >                   drm_dbg(drm, "failed to allocate buffer with
> > > > size
> > > > %zu\n",
> > > >                            size);
> > > > @@ -233,7 +242,10 @@ void drm_gem_dma_free(struct
> > > > drm_gem_dma_object
> > > > *dma_obj)
> > > >                           dma_buf_vunmap_unlocked(gem_obj-
> > > > > import_attach->dmabuf, &map);
> > > >                   drm_prime_gem_destroy(gem_obj, dma_obj->sgt);
> > > >           } else if (dma_obj->vaddr) {
> > > > -               if (dma_obj->map_noncoherent)
> > > > +               if (dma_obj->cached && dma_obj->coherent)
> > > > +                       dma_free_coherent(gem_obj->dev->dev,
> > > > dma_obj-
> > > > > base.size,
> > > > +                                         dma_obj->vaddr,
> > > > dma_obj-
> > > > > dma_addr);
> > > > +               else if (dma_obj->cached && !dma_obj->coherent)
> > > >                           dma_free_noncoherent(gem_obj->dev->dev,
> > > > dma_obj->base.size,
> > > >                                                dma_obj->vaddr,
> > > > dma_obj-
> > > > > dma_addr,
> > > >                                                DMA_TO_DEVICE);
> > > > @@ -532,7 +544,7 @@ int drm_gem_dma_mmap(struct
> > > > drm_gem_dma_object
> > > > *dma_obj, struct vm_area_struct *
> > > >           vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
> > > >           vm_flags_mod(vma, VM_DONTEXPAND, VM_PFNMAP);
> > > >    
> > > > -       if (dma_obj->map_noncoherent) {
> > > > +       if (dma_obj->cached) {
> > > >                   vma->vm_page_prot = vm_get_page_prot(vma-
> > > > > vm_flags);
> > > >    
> > > >                   ret = dma_mmap_pages(dma_obj->base.dev->dev,
> > > > diff --git a/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
> > > > b/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
> > > > index 5ec75e9ba499..a3df2f99a757 100644
> > > > --- a/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
> > > > +++ b/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
> > > > @@ -919,7 +919,10 @@ ingenic_drm_gem_create_object(struct
> > > > drm_device
> > > > *drm, size_t size)
> > > >           if (!obj)
> > > >                   return ERR_PTR(-ENOMEM);
> > > >    
> > > > -       obj->map_noncoherent = priv->soc_info->map_noncoherent;
> > > > +       if (priv->soc_info->map_noncoherent) {
> > > > +               obj->cached = true;
> > > > +               obj->coherent = false;
> > > > +       }
> > > >    
> > > >           return &obj->base;
> > > >    }
> > > > diff --git a/drivers/gpu/drm/rcar-du/Kconfig
> > > > b/drivers/gpu/drm/rcar-
> > > > du/Kconfig
> > > > index 53c356aed5d5..dddc70c08bdc 100644
> > > > --- a/drivers/gpu/drm/rcar-du/Kconfig
> > > > +++ b/drivers/gpu/drm/rcar-du/Kconfig
> > > > @@ -2,8 +2,6 @@
> > > >    config DRM_RCAR_DU
> > > >           tristate "DRM Support for R-Car Display Unit"
> > > >           depends on DRM && OF
> > > > -       depends on ARM || ARM64
> > > > -       depends on ARCH_RENESAS || COMPILE_TEST
> > > >           select DRM_KMS_HELPER
> > > >           select DRM_GEM_DMA_HELPER
> > > >           select VIDEOMODE_HELPERS
> > > > diff --git a/drivers/gpu/drm/rcar-du/rcar_du_kms.c
> > > > b/drivers/gpu/drm/rcar-du/rcar_du_kms.c
> > > > index adfb36b0e815..1142d51473e6 100644
> > > > --- a/drivers/gpu/drm/rcar-du/rcar_du_kms.c
> > > > +++ b/drivers/gpu/drm/rcar-du/rcar_du_kms.c
> > > > @@ -386,7 +386,9 @@ struct drm_gem_object
> > > > *rcar_du_gem_prime_import_sg_table(struct drm_device *dev,
> > > >           gem_obj->funcs = &rcar_du_gem_funcs;
> > > >    
> > > >           drm_gem_private_object_init(dev, gem_obj, attach-
> > > > > dmabuf-
> > > > > size);
> > > > -       dma_obj->map_noncoherent = false;
> > > > +
> > > > +       dma_obj->cached = false;
> > > > +       dma_obj->coherent = false;
> > > >    
> > > >           ret = drm_gem_create_mmap_offset(gem_obj);
> > > >           if (ret) {
> > > > diff --git a/include/drm/drm_gem_dma_helper.h
> > > > b/include/drm/drm_gem_dma_helper.h
> > > > index 8a043235dad8..585ce3d4d1eb 100644
> > > > --- a/include/drm/drm_gem_dma_helper.h
> > > > +++ b/include/drm/drm_gem_dma_helper.h
> > > > @@ -16,7 +16,9 @@ struct drm_mode_create_dumb;
> > > >     *       more than one entry but they are guaranteed to have
> > > > contiguous
> > > >     *       DMA addresses.
> > > >     * @vaddr: kernel virtual address of the backing memory
> > > > - * @map_noncoherent: if true, the GEM object is backed by non-
> > > > coherent memory
> > > > + * @cached: if true, the GEM object is backed by cached memory
> > > > + * @coherent: This option only meaningful when a GEM object is
> > > > cached.
> > > > + *            If true, Sync the GEM object for DMA access is not
> > > > required.
> > > >     */
> > > >    struct drm_gem_dma_object {
> > > >           struct drm_gem_object base;
> > > > @@ -26,7 +28,8 @@ struct drm_gem_dma_object {
> > > >           /* For objects with DMA memory allocated by GEM DMA */
> > > >           void *vaddr;
> > > >    
> > > > -       bool map_noncoherent;
> > > > +       bool cached;
> > > > +       bool coherent;
> > > >    };
> > > >    
> > > >    #define to_drm_gem_dma_obj(gem_obj) \

--
Jingfeng