add faq to boot from sdcard

Author: abertschi

content/docs/guides/faq.md

@@ -60,3 +60,24 @@ The firmware currently does not support dynamic RAM discovery and assumes 4GB in
 Some pointers to change this:
 [RMM](https://github.com/opencca/tf-rmm/blob/opencca/main/configs/rk3588_defcfg.cmake#L13), [TF-A](https://github.com/opencca/arm-trusted-firmware/blob/opencca/main/plat/rockchip/rk3588/opencca/rk3588_opencca.h#L71), [U-Boot](https://github.com/opencca/u-boot/commit/219910188adc59a6f17cc192044880ce76e8dc2a).
 {{< /details >}}
+
+
+{{< details "Q: I want to boot from SDMMC instead of eMMC" >}}  
+_October 2025_
+
+We recommend booting from eMMC. Compatible eMMC chips cost around 10 USD.
+
+https://de.aliexpress.com/item/1005007003959424.html?spm=a2g0o.order_list.order_list_main.35.4f2418023Ja2xZ&gatewayAdapt=glo2deu
+
+The prebuilt image used for the SysTEX evaluation boots from eMMC. In this build, the kernel option `OPENCCA_HW_DEBUG=y` is enabled, which disables SDMMC and enables the SWD interface. Since both SD and hardware debugging share the same pins you can either do hardware debugging or use SD card at the same time.
+
+You can see the relevant changes here:
+
+https://github.com/opencca/linux/compare/d8e18330ec8e59bc5262368961cd16aa044e0195...6d37f0d15c317804ceeb2d50354695d35783f0f6#diff-00edfaa8eb3606d42da7fb68c09687cd611602038ff3aa55436b2742d2d69bd2R3
+
+To fix this, you either:
+- boot from eMMC
+- or rebuild the kernel with `OPENCCA_HW_DEBUG=n`, and if needed, adjust the kernel cmd line arguments so the rootfs is loaded from SDMMC instead.
+  
+{{< /details >}}
+