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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.

rcn-ee: add use_adma option: https://www.spinics.net/lists/linux-omap/msg137942.html

Signed-off-by: Will Newton <willn@resin.io>
Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
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willnewton authored and RobertCNelson committed Jun 22, 2017
1 parent ac973e3 commit de8372efaf5a0f16965aabc0f79a8a26625520e5
Showing with 6 additions and 3 deletions.
  1. +6 −3 drivers/mmc/host/omap_hsmmc.c
@@ -2979,9 +2979,12 @@ static int omap_hsmmc_probe(struct platform_device *pdev)
host->use_adma = true;
}
/* 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). */
if (host->use_adma)
mmc->max_segs = 512;
else
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 */

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