Linux kernel：移植linux-4.20.9到正点原子imx6ull mini开发板

1、移植Linux内核

首先去下载一份Linux源码，然后将它解压编译：

tar xzf linux-4.20.9.tar.gz
cd linux-4.20.9/
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- imx_v6_v7_defconfig
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- -j96
ls -l arch/arm/boot/zImage arch/arm/boot/dts/imx6ull-14x14-evk.dtb # 编译生成镜像和设备树文件

将编译生成的zImage和imx6ull-14x14-evk.dtb通过tftp下载或者其他方式进行启动，查看现象（mmc中已经烧录过rootfs文件系统了）：

[    3.293373] mmcblk1boot0: mmc1:0001 8GTF4R partition 1 4.00 MiB
[    3.302758] mmcblk1boot1: mmc1:0001 8GTF4R partition 2 4.00 MiB
[    3.314072] NET: Registered protocol family 17
[    3.321206] mmcblk1rpmb: mmc1:0001 8GTF4R partition 3 512 KiB, chardev (244:0)
[    3.328954] can: controller area network core (rev 20170425 abi 9)
[    3.335794] NET: Registered protocol family 29
[    3.349613] can: raw protocol (rev 20170425)
[    3.354339] can: broadcast manager protocol (rev 20170425 t)
[    3.362870] can: netlink gateway (rev 20170425) max_hops=1
[    3.373898]  mmcblk1: p1 p2
[    3.378598] Key type dns_resolver registered
[    3.387168] cpu cpu0: Linked as a consumer to regulator.2
[    3.411398] cpu cpu0: Linked as a consumer to regulator.3
[    3.436852] Registering SWP/SWPB emulation handler
[    3.448656] Loading compiled-in X.509 certificates
[    3.553456] imx_thermal tempmon: Industrial CPU temperature grade - max:105C critical:100C passive:95C
[    3.571483] snvs_rtc 20cc000.snvs:snvs-rtc-lp: setting system clock to 2019-08-14 13:04:25 UTC (1565787865)
[    3.581967] cfg80211: Loading compiled-in X.509 certificates for regulatory database
[    3.598600] cfg80211: Loaded X.509 cert 'sforshee: 00b28ddf47aef9cea7'
[    3.606425] VSD_3V3: disabling
[    3.609999] ALSA device list:
[    3.613016]   No soundcards found.
[    3.618048] platform regulatory.0: Direct firmware load for regulatory.db failed with error -2
[    3.626818] platform regulatory.0: Falling back to syfs fallback for: regulatory.db
[    3.638888] VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
[    3.646416] Please append a correct "root=" boot option; here are the available partitions:
[    3.655085] 0100           65536 ram0
[    3.655091]  (driver?)
# 省略 ...
[    3.742420]  (driver?)
[    3.748727] 010f           65536 ram15
[    3.748733]  (driver?)
[    3.755000] b300         7634944 mmcblk1
[    3.755006]  driver: mmcblk
[    3.761925]   b301          131072 mmcblk1p1 5ee8f5c0-01
[    3.761931]
[    3.768851]   b302         7493632 mmcblk1p2 5ee8f5c0-02
[    3.768855]
[    3.775707] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[    3.784270] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) ]---

根据最后这两句log可以知道内核奔溃，说挂载根文件系统出错了：

[    3.775707] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[    3.784270] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) ]---

而根据以下log可以看到，说明MMC已经能识别出来了：

[    3.293373] mmcblk1boot0: mmc1:0001 8GTF4R partition 1 4.00 MiB
[    3.302758] mmcblk1boot1: mmc1:0001 8GTF4R partition 2 4.00 MiB
[    3.314072] NET: Registered protocol family 17
[    3.321206] mmcblk1rpmb: mmc1:0001 8GTF4R partition 3 512 KiB, chardev (244:0)

我们MMC设备中已经有文件系统了，但是却提示了以下log：

[    3.646416] Please append a correct "root=" boot option; here are the available partitions:

说是让我们设置好“root=xxx”来指定文件系统所在设备分区等来传递，先不管之前uboot环境变量是怎样设置来兼容多种启动方式的，我们重新进入u-boot来重新设置一个：

setenv bootargs 'console=ttymxc0,115200 root=/dev/mmcblk1p2 rootwait rw' # 可以不用急着saveenv
tftp 80800000 zImage;tftp 83000000 imx6ull-14x14-evk.dtb;bootz 80800000 - 83000000

tftp下载镜像之后重新启动系统，这时发现能够正常启动了。

2、其他修改

2.1 网卡修改

参考网卡驱动系列文章和Documentation/devicetree/bindings/net/fsl-fec.txt文档来修改。

对于正点原子的Mini版开发板来说，只外接了一个网口（ENET2）：

所以修改dts如下（imx6ull-14x14-evk.dts里面include包含了imx6ul-14x14-evk.dtsi）：

--- a/arch/arm/boot/dts/imx6ul-14x14-evk.dtsi
+++ b/arch/arm/boot/dts/imx6ul-14x14-evk.dtsi
@@ -98,6 +98,7 @@};};+/*&fec1 {pinctrl-names = "default";pinctrl-0 = <&pinctrl_enet1>;
@@ -105,6 +106,7 @@phy-handle = <&ethphy0>;status = "okay";};
+*/&fec2 {pinctrl-names = "default";
@@ -117,15 +119,17 @@#address-cells = <1>;#size-cells = <0>;+/*ethphy0: ethernet-phy@2 {reg = <2>;micrel,led-mode = <1>;clocks = <&clks IMX6UL_CLK_ENET_REF>;clock-names = "rmii-ref";};
+*/ethphy1: ethernet-phy@1 {
-                       reg = <1>;
+                       reg = <1>; /* accordinig to hardware, ensure addr is 0x01 */micrel,led-mode = <1>;clocks = <&clks IMX6UL_CLK_ENET2_REF>;clock-names = "rmii-ref";

上面修改就是屏蔽fec1（ENET1）和PHY0节点，这时使用新的dtb文件进行启动，就只有一个能使用的ENET2了，之前无法正常使用的ENET1已经被我们屏蔽掉了，防止混淆视线。

此外，对于PHY驱动，一般会有两套驱动，一套是内核的通用驱动（Generic PHY），一套是PHY厂家针对自家芯片的驱动，通用PHY驱动能驱动一部分符合规范的芯片了，如果不能驱动起来，也可以使用厂家提供的特定的驱动，对应的驱动文件一般放在drivers/net/phy目录下，根据该目录下的Makefile指示的CONFIG_XXX_PHY去make menuconfig菜单里选中即可（本版本默认已配置了SMSC的PHY驱动）。

注意！Linux-4.20.9版本的内核里面默认没有配置PHY芯片的复位引脚，这个属性在上面文档中说是可选的，如果网络出现问题的话可以添加复位引脚的使用。

2.2 emmc

其实emmc并没有什么需要修改的，不过数据的位宽倒是可以调整一下的，默认填写的是4bit，硬件原理图：

而开发板是8bit数据带宽，所以，修改dts中的对应节点：

&usdhc2 {pinctrl-names = "default";pinctrl-0 = <&pinctrl_usdhc2>;no-1-8-v;keep-power-in-suspend;wakeup-source;bus-width = <8>; /* overlay it with 8 bit */status = "okay";
};

修改后重启系统查看信息：

root@IMX6ULL:~# cat /sys/kernel/debug/mmc1/ios
clock:          52000000 Hz
vdd:            21 (3.3 ~ 3.4 V)
bus mode:       2 (push-pull)
chip select:    0 (don't care)
power mode:     2 (on)
bus width:      3 (8 bits) /* 修改后变成8bit位宽 */
timing spec:    8 (mmc DDR52)
signal voltage: 0 (3.30 V)
driver type:    0 (driver type B)
root@IMX6ULL:~#

此外，还可以算下容量：

root@IMX6ULL:~# cat /sys/block/mmcblk1/size
15269888
root@IMX6ULL:~#

容量 = 15269888 block * 512Bytes/block = 7,818,182,656 Bytes = 7,634,944 KBytes = 7,456 MBytes = 7.28125 GBytes

这就对应于系统启动时的打印：

[    3.132548] mmcblk1: mmc1:0001 8GTF4R 7.28 GiB

2.3 SD卡测试

插上SD卡，查看打印：

[   30.547634] mmc0: Tuning failed, falling back to fixed sampling clock
[   30.574852] mmc0: new ultra high speed SDR104 SDHC card at address 59b4
[   30.612676] mmcblk0: mmc0:59b4 NCard 14.9 GiB
[   30.712609] mmc0: Tuning failed, falling back to fixed sampling clock
[   30.850448] mmc0: Tuning failed, falling back to fixed sampling clock
[   30.862775]  mmcblk0: p1
[   30.940431] mmc0: Tuning failed, falling back to fixed sampling clock
[   30.992448] mmc0: Tuning failed, falling back to fixed sampling clock
...

很明显，sd卡用不了。参考公版修改usdhc1节点：

--- a/arch/arm/boot/dts/imx6ul.dtsi
+++ b/arch/arm/boot/dts/imx6ul.dtsi
@@ -856,6 +856,10 @@<&clks IMX6UL_CLK_USDHC1>,<&clks IMX6UL_CLK_USDHC1>;clock-names = "ipg", "ahb", "per";
+                               assigned-clocks = <&clks IMX6UL_CLK_USDHC1_SEL>, <&clks IMX6UL_CLK_USDHC1>;
+                               assigned-clock-parents = <&clks IMX6UL_CLK_PLL2_PFD2>;
+                               assigned-clock-rates = <0>, <132000000>;
+                               fsl,tuning-step= <2>;bus-width = <4>;status = "disabled";

修改后下载重新启动，正常使用：

root@IMX6ULL:~# [   80.197467] mmc0: host does not support reading read-only switch, assuming write-enable
[   80.256790] mmc0: new ultra high speed SDR104 SDHC card at address 59b4
[   80.274353] mmcblk0: mmc0:59b4 NCard 14.9 GiB
[   80.301858]  mmcblk0: p1
[   81.341775] FAT-fs (mmcblk0p1): Volume was not properly unmounted. Some data may be corrupt. Please run fsck.root@IMX6ULL:~# mount -t vfat /dev/mmcblk0p1 /mnt/SDCard/
root@IMX6ULL:~#
root@IMX6ULL:~# ls -l /mnt/SDCard/
total 32
drwxrwx--- 2 root disk 32768 Mar 19  2021 'System Volume Information'
-rwxrwx--- 1 root disk     0 Mar 19  2021 hello.txt
root@IMX6ULL:~#
root@IMX6ULL:~# umount /dev/mmcblk0p1
root@IMX6ULL:~#
root@IMX6ULL:~# ls -l /mnt/SDCard/
total 0
root@IMX6ULL:~#

2.4 U盘测试

root@IMX6ULL:/# [  878.667578] usb 1-1: new high-speed USB device number 4 using ci_hdrc
[  878.913337] usb-storage 1-1:1.0: USB Mass Storage device detected
[  878.921623] scsi host0: usb-storage 1-1:1.0
[  879.942648] scsi 0:0:0:0: Direct-Access     Kingston DataTraveler 3.0      PQ: 0 ANSI: 6
[  879.969810] sd 0:0:0:0: [sda] 60437492 512-byte logical blocks: (30.9 GB/28.8 GiB)
[  879.993424] sd 0:0:0:0: [sda] Write Protect is off
[  879.999646] sd 0:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[  880.027679]  sda: sda1
[  880.051462] sd 0:0:0:0: [sda] Attached SCSI removable disk
[  880.915533] FAT-fs (sda1): Volume was not properly unmounted. Some data may be corrupt. Please run fsck.root@IMX6ULL:/#
root@IMX6ULL:/# mount /dev/sda1 /mnt/
root@IMX6ULL:/# ls -l /mnt/
total 16
drwxrwx--- 2 root disk 16384 May 29  2021 'System Volume Information'
-rwxrwx--- 1 root disk     0 May 29  2021 test.txt
root@IMX6ULL:/#
root@IMX6ULL:/# umount /dev/sda1
root@IMX6ULL:/# ls -l /mnt/
total 0
root@IMX6ULL:/#

2.5 LCD

看了下linux-4.20.9内核，imx6ull的LCD不是使用传统的framebuffer框架了，根据 Documentation/devicetree/bindings/display/mxsfb.txt说明，使用了新的DRM框架，它的节点如下：

lcdif: lcdif@21c8000 {compatible = "fsl,imx6ul-lcdif", "fsl,imx28-lcdif";reg = <0x021c8000 0x4000>;interrupts = <GIC_SPI 5 IRQ_TYPE_LEVEL_HIGH>;clocks = <&clks IMX6UL_CLK_LCDIF_PIX>,<&clks IMX6UL_CLK_LCDIF_APB>,<&clks IMX6UL_CLK_DUMMY>;clock-names = "pix", "axi", "disp_axi";status = "disabled";
};&lcdif {assigned-clocks = <&clks IMX6UL_CLK_LCDIF_PRE_SEL>;assigned-clock-parents = <&clks IMX6UL_CLK_PLL5_VIDEO_DIV>;pinctrl-names = "default";pinctrl-0 = <&pinctrl_lcdif_dat&pinctrl_lcdif_ctrl>;status = "okay";port {display_out: endpoint {remote-endpoint = <&panel_in>;};};
};panel {compatible = "lg,lg4573";backlight = <&backlight_display>;port {panel_in: endpoint {remote-endpoint = <&display_out>;};};};

默认情况下背光能亮，但屏幕显示只有一小部分。找了好久不知道正点原子4.3英寸800x480的LCD采用的哪个IC，后来通过grep -rn "lg,lg4573" drivers/找到公版使用的驱动在drivers/gpu/drm/panel/目录下，继续grep -rn "compatible" drivers/gpu/drm/panel/找了下都支持哪些，然后就试了几个，发现最接近能使用的就只有sii,43wvf1g属性，这方面的驱动后面再看看如何工作的吧，暂时用着先。（对比了下硬件原理图和dts中LCD的pinctrl配置是一样的）

2.6 其他

其他驱动估计还是要改的，目前先到这里，有需要再补充。