Re: [PATCH v3 6/8] drm/shmem-helper: Provide a vmap function for short-term mappings

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Jan 7, 2021 at 11:28 AM Thomas Zimmermann <tzimmermann@suse.de> wrote:
>
> Hi Daniel
>
> Am 11.12.20 um 10:50 schrieb Daniel Vetter:
> [...]
> >> +/**
> >> + * drm_gem_shmem_vmap_local - Create a virtual mapping for a shmem GEM object
> >> + * @shmem: shmem GEM object
> >> + * @map: Returns the kernel virtual address of the SHMEM GEM object's backing
> >> + *       store.
> >> + *
> >> + * This function makes sure that a contiguous kernel virtual address mapping
> >> + * exists for the buffer backing the shmem GEM object.
> >> + *
> >> + * The function is called with the BO's reservation object locked. Callers must
> >> + * hold the lock until after unmapping the buffer.
> >> + *
> >> + * This function can be used to implement &drm_gem_object_funcs.vmap_local. But
> >> + * it can also be called by drivers directly, in which case it will hide the
> >> + * differences between dma-buf imported and natively allocated objects.
> >> + *
> >> + * Acquired mappings should be cleaned up by calling drm_gem_shmem_vunmap_local().
> >> + *
> >> + * Returns:
> >> + * 0 on success or a negative error code on failure.
> >> + */
> >> +int drm_gem_shmem_vmap_local(struct drm_gem_object *obj, struct dma_buf_map *map)
> >> +{
> >> +    struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
> >> +    int ret;
> >> +
> >> +    dma_resv_assert_held(obj->resv);
> >> +
> >> +    ret = mutex_lock_interruptible(&shmem->vmap_lock);
> >
> > This bites. You need to check for shmem->import_attach and call
> > dma_buf_vmap_local directly here before you take any shmem helper locks.
> > Real fix would be to replace both vmap_lock and pages_lock with dma_resv
> > lock, but that's more work. Same for vunmap_local
>
> This comment confuses me. AFAICT vmap_lock protects vmap_use_count. Why
> does this exact code work in drm_gem_shmem_vmap() but not in _local() ?

You'd hold the dma_resv lock both inside and outside of the
->vmap_lock mutex. Which deadlocks. For the _vmap version we more or
less mandate that the caller doens't hold the dma_resv lock already,
hence why we can get away with that.
-Daniel

> Best regards
> Thomas
>
> >
> > With that fixed on the helper part of this patch:
> >
> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> >
> >
> >> +    if (ret)
> >> +            return ret;
> >> +    ret = drm_gem_shmem_vmap_locked(shmem, map);
> >> +    mutex_unlock(&shmem->vmap_lock);
> >> +
> >> +    return ret;
> >> +}
> >> +EXPORT_SYMBOL(drm_gem_shmem_vmap_local);
> >> +
> >>   static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
> >>                                      struct dma_buf_map *map)
> >>   {
> >> @@ -366,7 +406,7 @@ static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
> >>      drm_gem_shmem_put_pages(shmem);
> >>   }
> >>
> >> -/*
> >> +/**
> >>    * drm_gem_shmem_vunmap - Unmap a virtual mapping fo a shmem GEM object
> >>    * @shmem: shmem GEM object
> >>    * @map: Kernel virtual address where the SHMEM GEM object was mapped
> >> @@ -389,6 +429,33 @@ void drm_gem_shmem_vunmap(struct drm_gem_object *obj, struct dma_buf_map *map)
> >>   }
> >>   EXPORT_SYMBOL(drm_gem_shmem_vunmap);
> >>
> >> +/**
> >> + * drm_gem_shmem_vunmap_local - Unmap a virtual mapping fo a shmem GEM object
> >> + * @shmem: shmem GEM object
> >> + * @map: Kernel virtual address where the SHMEM GEM object was mapped
> >> + *
> >> + * This function cleans up a kernel virtual address mapping acquired by
> >> + * drm_gem_shmem_vmap_local(). The mapping is only removed when the use count
> >> + * drops to zero.
> >> + *
> >> + * The function is called with the BO's reservation object locked.
> >> + *
> >> + * This function can be used to implement &drm_gem_object_funcs.vmap_local.
> >> + * But it can also be called by drivers directly, in which case it will hide
> >> + * the differences between dma-buf imported and natively allocated objects.
> >> + */
> >> +void drm_gem_shmem_vunmap_local(struct drm_gem_object *obj, struct dma_buf_map *map)
> >> +{
> >> +    struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
> >> +
> >> +    dma_resv_assert_held(obj->resv);
> >> +
> >> +    mutex_lock(&shmem->vmap_lock);
> >> +    drm_gem_shmem_vunmap_locked(shmem, map);
> >> +    mutex_unlock(&shmem->vmap_lock);
> >> +}
> >> +EXPORT_SYMBOL(drm_gem_shmem_vunmap_local);
> >> +
> >>   struct drm_gem_shmem_object *
> >>   drm_gem_shmem_create_with_handle(struct drm_file *file_priv,
> >>                               struct drm_device *dev, size_t size,
> >> diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c b/drivers/gpu/drm/mgag200/mgag200_mode.c
> >> index 1dfc42170059..a33e28d4c5e9 100644
> >> --- a/drivers/gpu/drm/mgag200/mgag200_mode.c
> >> +++ b/drivers/gpu/drm/mgag200/mgag200_mode.c
> >> @@ -1552,22 +1552,32 @@ mgag200_handle_damage(struct mga_device *mdev, struct drm_framebuffer *fb,
> >>                    struct drm_rect *clip)
> >>   {
> >>      struct drm_device *dev = &mdev->base;
> >> +    struct drm_gem_object *obj = fb->obj[0];
> >>      struct dma_buf_map map;
> >>      void *vmap;
> >>      int ret;
> >>
> >> -    ret = drm_gem_shmem_vmap(fb->obj[0], &map);
> >> +    ret = dma_resv_lock(obj->resv, NULL);
> >>      if (drm_WARN_ON(dev, ret))
> >> -            return; /* BUG: SHMEM BO should always be vmapped */
> >> +            return;
> >> +    ret = drm_gem_shmem_vmap_local(obj, &map);
> >> +    if (drm_WARN_ON(dev, ret))
> >> +            goto err_dma_resv_unlock; /* BUG: SHMEM BO should always be vmapped */
> >>      vmap = map.vaddr; /* TODO: Use mapping abstraction properly */
> >>
> >>      drm_fb_memcpy_dstclip(mdev->vram, vmap, fb, clip);
> >>
> >> -    drm_gem_shmem_vunmap(fb->obj[0], &map);
> >> +    drm_gem_shmem_vunmap_local(obj, &map);
> >> +    dma_resv_unlock(obj->resv);
> >>
> >>      /* Always scanout image at VRAM offset 0 */
> >>      mgag200_set_startadd(mdev, (u32)0);
> >>      mgag200_set_offset(mdev, fb);
> >> +
> >> +    return;
> >> +
> >> +err_dma_resv_unlock:
> >> +    dma_resv_unlock(obj->resv);
> >>   }
> >>
> >>   static void
> >> diff --git a/drivers/gpu/drm/tiny/cirrus.c b/drivers/gpu/drm/tiny/cirrus.c
> >> index 561c49d8657a..58c694964148 100644
> >> --- a/drivers/gpu/drm/tiny/cirrus.c
> >> +++ b/drivers/gpu/drm/tiny/cirrus.c
> >> @@ -315,6 +315,7 @@ static int cirrus_fb_blit_rect(struct drm_framebuffer *fb,
> >>                             struct drm_rect *rect)
> >>   {
> >>      struct cirrus_device *cirrus = to_cirrus(fb->dev);
> >> +    struct drm_gem_object *obj = fb->obj[0];
> >>      struct dma_buf_map map;
> >>      void *vmap;
> >>      int idx, ret;
> >> @@ -323,9 +324,12 @@ static int cirrus_fb_blit_rect(struct drm_framebuffer *fb,
> >>      if (!drm_dev_enter(&cirrus->dev, &idx))
> >>              goto out;
> >>
> >> -    ret = drm_gem_shmem_vmap(fb->obj[0], &map);
> >> +    ret = dma_resv_lock(obj->resv, NULL);
> >>      if (ret)
> >>              goto out_dev_exit;
> >> +    ret = drm_gem_shmem_vmap_local(fb->obj[0], &map);
> >> +    if (ret)
> >> +            goto out_dma_resv_unlock;
> >>      vmap = map.vaddr; /* TODO: Use mapping abstraction properly */
> >>
> >>      if (cirrus->cpp == fb->format->cpp[0])
> >> @@ -345,9 +349,11 @@ static int cirrus_fb_blit_rect(struct drm_framebuffer *fb,
> >>      else
> >>              WARN_ON_ONCE("cpp mismatch");
> >>
> >> -    drm_gem_shmem_vunmap(fb->obj[0], &map);
> >>      ret = 0;
> >>
> >> +    drm_gem_shmem_vunmap_local(obj, &map);
> >> +out_dma_resv_unlock:
> >> +    dma_resv_unlock(obj->resv);
> >>   out_dev_exit:
> >>      drm_dev_exit(idx);
> >>   out:
> >> diff --git a/drivers/gpu/drm/tiny/gm12u320.c b/drivers/gpu/drm/tiny/gm12u320.c
> >> index 33f65f4626e5..b0c6e350f2b3 100644
> >> --- a/drivers/gpu/drm/tiny/gm12u320.c
> >> +++ b/drivers/gpu/drm/tiny/gm12u320.c
> >> @@ -265,11 +265,16 @@ static void gm12u320_copy_fb_to_blocks(struct gm12u320_device *gm12u320)
> >>      y1 = gm12u320->fb_update.rect.y1;
> >>      y2 = gm12u320->fb_update.rect.y2;
> >>
> >> -    ret = drm_gem_shmem_vmap(fb->obj[0], &map);
> >> +    ret = dma_resv_lock(fb->obj[0]->resv, NULL);
> >>      if (ret) {
> >> -            GM12U320_ERR("failed to vmap fb: %d\n", ret);
> >> +            GM12U320_ERR("failed to reserve fb: %d\n", ret);
> >>              goto put_fb;
> >>      }
> >> +    ret = drm_gem_shmem_vmap_local(fb->obj[0], &map);
> >> +    if (ret) {
> >> +            GM12U320_ERR("failed to vmap fb: %d\n", ret);
> >> +            goto unlock_resv;
> >> +    }
> >>      vaddr = map.vaddr; /* TODO: Use mapping abstraction properly */
> >>
> >>      if (fb->obj[0]->import_attach) {
> >> @@ -321,8 +326,11 @@ static void gm12u320_copy_fb_to_blocks(struct gm12u320_device *gm12u320)
> >>              if (ret)
> >>                      GM12U320_ERR("dma_buf_end_cpu_access err: %d\n", ret);
> >>      }
> >> +
> >> +unlock_resv:
> >> +    dma_resv_unlock(fb->obj[0]->resv);
> >>   vunmap:
> >> -    drm_gem_shmem_vunmap(fb->obj[0], &map);
> >> +    drm_gem_shmem_vunmap_local(fb->obj[0], &map);
> >>   put_fb:
> >>      drm_framebuffer_put(fb);
> >>      gm12u320->fb_update.fb = NULL;
> >> diff --git a/drivers/gpu/drm/udl/udl_modeset.c b/drivers/gpu/drm/udl/udl_modeset.c
> >> index 9d34ec9d03f6..46b55b4d03c2 100644
> >> --- a/drivers/gpu/drm/udl/udl_modeset.c
> >> +++ b/drivers/gpu/drm/udl/udl_modeset.c
> >> @@ -290,14 +290,18 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
> >>      else if ((clip.x2 > fb->width) || (clip.y2 > fb->height))
> >>              return -EINVAL;
> >>
> >> +    ret = dma_resv_lock(fb->obj[0]->resv, NULL);
> >> +    if (ret)
> >> +            return ret;
> >> +
> >>      if (import_attach) {
> >>              ret = dma_buf_begin_cpu_access(import_attach->dmabuf,
> >>                                             DMA_FROM_DEVICE);
> >>              if (ret)
> >> -                    return ret;
> >> +                    goto out_dma_resv_unlock;
> >>      }
> >>
> >> -    ret = drm_gem_shmem_vmap(fb->obj[0], &map);
> >> +    ret = drm_gem_shmem_vmap_local(fb->obj[0], &map);
> >>      if (ret) {
> >>              DRM_ERROR("failed to vmap fb\n");
> >>              goto out_dma_buf_end_cpu_access;
> >> @@ -307,7 +311,7 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
> >>      urb = udl_get_urb(dev);
> >>      if (!urb) {
> >>              ret = -ENOMEM;
> >> -            goto out_drm_gem_shmem_vunmap;
> >> +            goto out_drm_gem_shmem_vunmap_local;
> >>      }
> >>      cmd = urb->transfer_buffer;
> >>
> >> @@ -320,7 +324,7 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
> >>                                     &cmd, byte_offset, dev_byte_offset,
> >>                                     byte_width);
> >>              if (ret)
> >> -                    goto out_drm_gem_shmem_vunmap;
> >> +                    goto out_drm_gem_shmem_vunmap_local;
> >>      }
> >>
> >>      if (cmd > (char *)urb->transfer_buffer) {
> >> @@ -336,8 +340,8 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
> >>
> >>      ret = 0;
> >>
> >> -out_drm_gem_shmem_vunmap:
> >> -    drm_gem_shmem_vunmap(fb->obj[0], &map);
> >> +out_drm_gem_shmem_vunmap_local:
> >> +    drm_gem_shmem_vunmap_local(fb->obj[0], &map);
> >>   out_dma_buf_end_cpu_access:
> >>      if (import_attach) {
> >>              tmp_ret = dma_buf_end_cpu_access(import_attach->dmabuf,
> >> @@ -345,6 +349,8 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
> >>              if (tmp_ret && !ret)
> >>                      ret = tmp_ret; /* only update ret if not set yet */
> >>      }
> >> +out_dma_resv_unlock:
> >> +    dma_resv_unlock(fb->obj[0]->resv);
> >>
> >>      return ret;
> >>   }
> >> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> >> index 434328d8a0d9..3f59bdf749aa 100644
> >> --- a/include/drm/drm_gem_shmem_helper.h
> >> +++ b/include/drm/drm_gem_shmem_helper.h
> >> @@ -114,7 +114,9 @@ void drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem);
> >>   int drm_gem_shmem_pin(struct drm_gem_object *obj);
> >>   void drm_gem_shmem_unpin(struct drm_gem_object *obj);
> >>   int drm_gem_shmem_vmap(struct drm_gem_object *obj, struct dma_buf_map *map);
> >> +int drm_gem_shmem_vmap_local(struct drm_gem_object *obj, struct dma_buf_map *map);
> >>   void drm_gem_shmem_vunmap(struct drm_gem_object *obj, struct dma_buf_map *map);
> >> +void drm_gem_shmem_vunmap_local(struct drm_gem_object *obj, struct dma_buf_map *map);
> >>
> >>   int drm_gem_shmem_madvise(struct drm_gem_object *obj, int madv);
> >>
> >> --
> >> 2.29.2
> >>
> >
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer
>


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