linux-kernel - [PATCH] drm: Fix for GEM buffers with write-combine memory

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Date:   Fri, 28 May 2021 00:03:34 +0100
From:   Paul Cercueil <paul@...pouillou.net>
To:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>
Cc:     list@...ndingux.net, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, Paul Cercueil <paul@...pouillou.net>,
        Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Subject: [PATCH] drm: Fix for GEM buffers with write-combine memory

The previous commit wrongly assumed that dma_mmap_wc() could be replaced
by pgprot_writecombine() + dma_mmap_pages(). It did work on my setup,
but did not work everywhere.

Use dma_mmap_wc() when the buffer has the write-combine cache attribute,
and dma_mmap_pages() when it has the non-coherent cache attribute.

Signed-off-by: Paul Cercueil <paul@...pouillou.net>
Reported-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Fixes: cf8ccbc72d61 ("drm: Add support for GEM buffers backed by non-coherent memory")
---
 drivers/gpu/drm/drm_gem_cma_helper.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
index 235c7a63da2b..4c3772651954 100644
--- a/drivers/gpu/drm/drm_gem_cma_helper.c
+++ b/drivers/gpu/drm/drm_gem_cma_helper.c
@@ -514,13 +514,17 @@ int drm_gem_cma_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
 
 	cma_obj = to_drm_gem_cma_obj(obj);
 
-	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
-	if (!cma_obj->map_noncoherent)
-		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
+	if (cma_obj->map_noncoherent) {
+		vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
+
+		ret = dma_mmap_pages(cma_obj->base.dev->dev,
+				     vma, vma->vm_end - vma->vm_start,
+				     virt_to_page(cma_obj->vaddr));
+	} else {
+		ret = dma_mmap_wc(cma_obj->base.dev->dev, vma, cma_obj->vaddr,
+				  cma_obj->paddr, vma->vm_end - vma->vm_start);
 
-	ret = dma_mmap_pages(cma_obj->base.dev->dev,
-			     vma, vma->vm_end - vma->vm_start,
-			     virt_to_page(cma_obj->vaddr));
+	}
 	if (ret)
 		drm_gem_vm_close(vma);
 
-- 
2.30.2