如何在 u-boot 中设置此寄存器的位

Question

I am trying to figure out how "SRAM_CTL1_CFG" register's 4th and 5th bit is set using following statement in u-boot:

/* Map SRAM to EMAC */
   setbits_le32(&sram->ctrl1, 0x5 << 2);

http://git.denx.de/?p=u-boot.git;a=blob;f=drivers/net/sunxi_emac.c;h=11cd0ea06888ba8e271b0c10376306df5291a3e2;hb=HEAD#l503

As per datasheet if bit 4:5 of this "SRAM_CTL1_CFG" become "01" SRAM would be mapped to EMAC peripheral.

http://linux-sunxi.org/SRAM_Controller_Register_Guide

Shouldn't this setbits_le32(&sram->ctrl1, 0x5 << 2) be setbits_le32(&sram->ctrl1, 0x4 << 2) ?

Answer 1

So, the answer is that yes, (0x5 << 2), which is 20 decimal is 010100 in binary and we set '4:5' to '01' which is EMAC as well as setting bit 2 to 1. Setting bit 2 to 1 is currently a magic value that we're setting because it's being set (presumably, I didn't trace all of the history back) in the sources allwinner provides, without further explanation. Setting this to (0x4 << 2) would only set the claimed registers but is likely to have some hidden breakage on other boards where the magic value is required too.