From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Ruder <andy@aeruder.net>
Date: Thu, 10 Apr 2014 13:41:03 -0500
Subject: [U-Boot] [PATCH RESEND-WITH-JUSTIFICATION] spi: soft_spi:
 Support NULL din/dout buffers
In-Reply-To: <1397146782-28966-1-git-send-email-andrew.ruder@elecsyscorp.com>
References: <1397146782-28966-1-git-send-email-andrew.ruder@elecsyscorp.com>
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I further justify this with a list of drivers that depend on half-duplex
SPI operation:

drivers/mmc/mmc_spi.c:          spi_xfer(spi, 2 * 8, tok, NULL, 0);
drivers/mtd/spi/eeprom_m95xxx.c:        if (spi_xfer(slave, 8, buf, NULL, SPI_XFER_BEGIN | SPI_XFER_END))
drivers/mtd/spi/sf_ops.c:               ret = spi_xfer(spi, 8, NULL, &status, 0);
drivers/net/e1000_spi.c:        return e1000_spi_xfer(hw, 8*sizeof(op), op, NULL, intr);
drivers/net/enc28j60.c: spi_xfer(enc->slave, 2 * 8, dout, NULL,
drivers/rtc/m41t94.c:   ret = spi_xfer(slave, 64, buf, NULL, SPI_XFER_BEGIN | SPI_XFER_END);

And an old email first highlighting this issue in 2010 with the soft_spi
driver:

http://article.gmane.org/gmane.comp.boot-loaders.u-boot/75839/match=soft_spi