drv: split DMA on mapping failure

By default the SW-IOMMU reserves 64MiB, but despite this it fails in allocating buffere bigger than 0.3MiB. We do not have control over the kernel code, and it is not clear how to debug it (e.g. see the current status of those 64MiB). For this reasons the most practical solution is to break the DMA transfer in smaller chunks until it works. We set the minimum chunk to PAGE_SIZE, because if it fails with this, really there is nothing we can do. Signed-off-by: Federico Vaga <federico.vaga@cern.ch>

drv: split DMA on mapping failure
By default the SW-IOMMU reserves 64MiB, but despite this it fails in allocating buffere bigger than 0.3MiB. We do not have control over the kernel code, and it is not clear how to debug it (e.g. see the current status of those 64MiB). For this reasons the most practical solution is to break the DMA transfer in smaller chunks until it works. We set the minimum chunk to PAGE_SIZE, because if it fails with this, really there is nothing we can do. Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
330caa51 · Federico Vaga · 9afa3d2e · 330caa51
Commit 330caa51 authored Sep 09, 2020 by Federico Vaga
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Showing with 29 additions and 17 deletions

fa-dma.c kernel/fa-dma.c +29 -17

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--- a/kernel/fa-dma.c
+++ b/kernel/fa-dma.c
@@ -379,25 +379,37 @@ static int zfad_dma_prep_slave_sg(struct dma_chan *dchan,
 	if (err)
 		goto err_to_pages;

-
-	max_segment_size = dma_get_max_seg_size(dchan->device->dev);
+	max_segment_size = min(zfad_block->block->datalen + PAGE_SIZE, /* PAGE aligned */
+			       (size_t)dma_get_max_seg_size(dchan->device->dev));
 	max_segment_size &= PAGE_MASK; /* to make alloc_table happy */
-	err = fa->sg_alloc_table_from_pages(&zfad_block->sgt, pages, nr_pages,
-					    offset_in_page(zfad_block->block->data),
-					    zfad_block->block->datalen,
-					    max_segment_size, GFP_KERNEL);
-	if (unlikely(err))
-		goto err_sgt;
-
-	sg_mapped = dma_map_sg(&fa->pdev->dev,
-			       zfad_block->sgt.sgl,
-			       zfad_block->sgt.nents,
-			       DMA_DEV_TO_MEM);
-	if (sg_mapped <= 0) {
-		err = sg_mapped ? sg_mapped : -ENOMEM;
+	/* Find something that fits in the [SW-]IOMMU */
+	do {
+		dev_dbg(&fa->pdev->dev, "DMA max segment %ld\n",
+			max_segment_size);
+		err = fa->sg_alloc_table_from_pages(&zfad_block->sgt, pages,
+						    nr_pages,
+						    offset_in_page(zfad_block->block->data),
+						    zfad_block->block->datalen,
+						    max_segment_size,
+						    GFP_KERNEL);
+		if (unlikely(err))
+			goto err_sgt;
+
+		sg_mapped = dma_map_sg(&fa->pdev->dev,
+				       zfad_block->sgt.sgl,
+				       zfad_block->sgt.nents,
+				       DMA_DEV_TO_MEM);
+		if (sg_mapped <= 0) {
+			sg_free_table(&zfad_block->sgt);
+			err = sg_mapped ? sg_mapped : -ENOMEM;
+			max_segment_size /= 2;
+		} else {
+			err = 0;
+			break;
+		}
+	} while (max_segment_size >= PAGE_SIZE);
+	if (err)
 		goto err_map;
-	}
-
 	/* Prepare the DMA transmisison */
 	tx = dmaengine_prep_slave_sg_ctx(dchan, zfad_block->sgt.sgl, sg_mapped,
 					 DMA_DEV_TO_MEM, 0, zfad_block->dma_ctx);