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

Sui Jingfeng <suijingfeng@xxxxxxxxxxx> · Wed, 7 Jun 2023 18:30:01 +0800

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.

 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.

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.


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.


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







