SPI mode for SD card?

[xiongyw]

I have a vu13p-based card, which does not have a SD card slot. The exported IO ports is very limited. For adding SD card interface, I am planning to use the 6 control signals for one of the QSFP28 interface. The 6 QSFP control signals are (due to level-shifters' settings on the board, I guess):

• two inputs
• two outputs
• two inouts

Apparently there is no enough directional ports for 4-bit SD mode (it seems that at least 5 bidirectional ports needed, cmd+data). So I am thinking that probably SPI mode for SD is a route to go (sacrificing performance).

My question is, for a Verilog newbee like me, what's the estimated effort for modifying the current sd controller (particularly the RTL code) for supporting SPI mode? Or some guidelines for such a modification please?

Thanks!

[eugene-tarassov]

The SD card controller supports 1-bit mode. It is selected by bus-width parameter in the device tree, see board/vc707/bootrom.dts - change to bus-width = <1>.

Also, it is possible to boot Linux using JTAG for u-boot, and NFS share for rootfs. This workflow is not well documented, but it does work for boards without SD card slot, like VCU1525 or Alveo boards.

[xiongyw]

Thanks for your reply. That's a good news. Yes, I also noticed that the controller support 1-bit mode, and the driver will set the control register according to dts settings.

Thanks again!