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atmel_spi throughput improvement

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Don't insert (undesirable) delays between consecutive words (DLYBCT) or when
activating chipselects (DLYBS).

Removing the between-word delays improves the performance of bulk transfers
(such as mtd_dataflash, m25p80, mmc_spi) significantly.  In one test, the
improvement was a factor of more than eight!

(The large DLYBCT value came from the legacy at91 SPI driver, and it's not
clear why it used such a huge value.)

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Haavard Skinnemoen 2008-02-06 01:38:11 -08:00 committed by Linus Torvalds
parent 3c72426f05
commit 1eed29df47
1 changed files with 8 additions and 3 deletions
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11
drivers/spi/atmel_spi.c
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@ -490,9 +490,14 @@ static int atmel_spi_setup(struct spi_device *spi)
if (!(spi->mode & SPI_CPHA))
csr |= SPI_BIT(NCPHA);
/* TODO: DLYBS and DLYBCT */
csr |= SPI_BF(DLYBS, 10);
csr |= SPI_BF(DLYBCT, 10);
/* DLYBS is mostly irrelevant since we manage chipselect using GPIOs.
*
* DLYBCT would add delays between words, slowing down transfers.
* It could potentially be useful to cope with DMA bottlenecks, but
* in those cases it's probably best to just use a lower bitrate.
*/
csr |= SPI_BF(DLYBS, 0);
csr |= SPI_BF(DLYBCT, 0);
/* chipselect must have been muxed as GPIO (e.g. in board setup) */
npcs_pin = (unsigned int)spi->controller_data;