linux-kernel - [PATCH] drm: gem_cma_helper.c: Allow importing of contiguous scatterlists with nents

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Message-Id: <20171101141419.3180-1-Liviu.Dudau@arm.com>
Date:   Wed,  1 Nov 2017 14:14:19 +0000
From:   Liviu Dudau <Liviu.Dudau@....com>
To:     Brian Starkey <brian.starkey@....com>
Cc:     Mali DP Maintainers <malidp@...s.arm.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        DRI-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        Liviu Dudau <liviu.dudau@....com>
Subject: [PATCH] drm: gem_cma_helper.c: Allow importing of contiguous scatterlists with nents > 1

drm_gem_cma_prime_import_sg_table() will fail if the number of entries
in the sg_table > 1. However, you can have a device that uses an IOMMU
engine and can map a discontiguous buffer with multiple entries that
have consecutive sg_dma_addresses, effectively making it contiguous.
Allow for that scenario by testing the entries in the sg_table for
contiguous coverage.

Reviewed-by: Brian Starkey <brian.starkey@....com>
Signed-off-by: Liviu Dudau <liviu.dudau@....com>
---

Hi,

This patch is the only change I need in order to be able to use existing
IOMMU domain infrastructure with the Mali DP driver. I have tested the
patch and I know it works correctly for my setup, but I would like to get
some comments on whether I am on the right path or if CMA really wants to
see an sg_table with only one entry.

Best regards,
Liviu

 drivers/gpu/drm/drm_gem_cma_helper.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c
index 020e7668dfaba..43b179212052d 100644
--- a/drivers/gpu/drm/drm_gem_cma_helper.c
+++ b/drivers/gpu/drm/drm_gem_cma_helper.c
@@ -482,8 +482,26 @@ drm_gem_cma_prime_import_sg_table(struct drm_device *dev,
 {
 	struct drm_gem_cma_object *cma_obj;
 
-	if (sgt->nents != 1)
-		return ERR_PTR(-EINVAL);
+	if (sgt->nents != 1) {
+		/* check if the entries in the sg_table are contiguous */
+		dma_addr_t next_addr = sg_dma_address(sgt->sgl);
+		struct scatterlist *s;
+		unsigned int i;
+
+		for_each_sg(sgt->sgl, s, sgt->nents, i) {
+			/*
+			 * sg_dma_address(s) is only valid for entries
+			 * that have sg_dma_len(s) != 0
+			 */
+			if (!sg_dma_len(s))
+				continue;
+
+			if (sg_dma_address(s) != next_addr)
+				return ERR_PTR(-EINVAL);
+
+			next_addr = sg_dma_address(s) + sg_dma_len(s);
+		}
+	}
 
 	/* Create a CMA GEM buffer. */
 	cma_obj = __drm_gem_cma_create(dev, attach->dmabuf->size);
-- 
2.14.3