mmc: omap_hsmmc: Reduce max_segs for reliability

Reduce max_segs to 64, a value that allows allocation of an entire
EDMA descriptor list within a single page - EDMA descriptors
are 40 bytes and the header is much larger. This avoids doing a
higher order GFP_ATOMIC allocation in edma_prep_slave_sg
when setting up a transfer which can potentially fail due to
fragmentation under heavy I/O load.

The current value of 1024 is unusually high in comparison to
other mmc host drivers which mostly use values of between 1
and 256. The EDMA driver at present splits lists above 20
segments in any case so reducing the size of lists we pass to
it shouldn't add much overhead.
Signed-off-by: Will Newton <willn@resin.io>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/omap_hsmmc.c

@@ -2076,9 +2076,9 @@ static int omap_hsmmc_probe(struct platform_device *pdev)
 		host->dbclk = NULL;
 	}
 
-	/* Since we do only SG emulation, we can have as many segs
-	 * as we want. */
-	mmc->max_segs = 1024;
+	/* Set this to a value that allows allocating an entire descriptor
+	 * list within a page (zero order allocation). */
+	mmc->max_segs = 64;
 
 	mmc->max_blk_size = 512;       /* Block Length at max can be 1024 */
 	mmc->max_blk_count = 0xFFFF;    /* No. of Blocks is 16 bits */