On Tue, May 19, 2015 at 08:51:54AM +0200, Geert Uytterhoeven wrote: > On Tue, May 19, 2015 at 6:01 AM, Vinod Koul <vinod.koul@xxxxxxxxx> wrote: > > On Fri, May 15, 2015 at 03:46:27PM +0200, Geert Uytterhoeven wrote: > > > > am ccing LKML, perhaps this needs wider discussion.. > > > >> Several drivers reuse mapped scatterlists, and modify sg_dma_len(sg) to > >> match the actual number of bytes they want to transfer. > >> > >> Hence during driver shutdown, dma_unmap_sg() is called with a scatterlist > >> that has modified lengths, compared to the original passed to dma_map_sg(). > >> If CONFIG_DMA_API_DEBUG=y, this causes warnings like: > >> > >> WARNING: CPU: 0 PID: 20 at lib/dma-debug.c:1103 check_unmap+0x24c/0x85c() > >> rcar-dmac e6700000.dma-controller: DMA-API: device driver frees > >> DMA memory with different size [device address=0x000000006e15f000] > >> [map size=4096 bytes] [unmap size=3 bytes] > >> > >> The warning can be silenced by restoring the original sg_dma_len() just before > >> calling dma_unmap_sg(), but it looks like no driver actually does that. > > Right, but should the drivers map with one length and then modify the > > length later on, why not map only the size that is required... > > This is mostly done in serial drivers, which set up an initial mapping, and > reuse it with different lengths, depending on the amount of data to transfer > at that time. > > > So driver should always unmap and remap with the right length whenever it is > > required to change the length. Perhaps a dma_remap_sg() API to do so > > properly. > > > > I do not think modifying length directly should be encouraged... > > In cases where there's only a single segment, I think it's better to use > dma_map_single() instead of dma_map_sg(). > Then the actual length to transfer can easily be passed to > dmaengine_prep_slave_single(), right? Sounds right to me -- ~Vinod -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
