Linux Kernel Platform Development: Challenges and Insights

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Linux Kernel Platform Development:
Challenges and Insights
Sam Protsenko
Software Engineer
February 10, 2018

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Agenda
1. Role Overview
2. War Stories and Learnings
• Story #1: Board bring-up
• Story #2: Migration to a new kernel
• Story #3: Hardware fixing

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3
Role Overview

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Tasks
• Board bring-up
• Porting kernel (and bootloader) to a new board
- Writing the BSP
- Drivers development
• Migrating to a new kernel version
• Bug fixing
• Boot time optimizations
• Upstreaming
• Sometimes:
- Bootloader related work
- Low-level work in rootfs (Android, OpenEmbedded, etc.)
- Hardware debugging

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Tasks
Make board minimally functional,
step by step, for all required
components. Make sure it works.
• Check power and reset lines
• Check clock signals
• Debug SoC issues (caches, etc.)
• Configure pin muxes
• Bring-up RAM (DDR), serial
console (UART), peripheral
(eMMC, USB, LCD, etc.)
Board bring-up

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Tasks
Make kernel and bootloader fully
functional on a new board.
• Bootloader bring-up
• Device Tree file
• Configuration file
• SoC support
• Write missing peripheral drivers
Porting kernel to a new board

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Tasks
Modify board related files to work in
a new kernel version.
• Sometimes company doesn’t
want to upstream its kernel
• Migration work:
- board files -> device tree
- new API for kernel frameworks
- config options
- sometimes back-porting is needed
• Automated approach exists: see
“Prequel” by Julia Lawall
Migrating to a new kernel version

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Components and Tools
• Linux kernel
• Bootloader (U-Boot, UEFI/edk2)
• RootFS (Android, Debian, OpenEmbedded,
BusyBox)
• C language
• OS principles
• Microcontrollers basics (+schematics)
• Linux user experience
Components
• Software
- toolchain (ARM GCC): gcc, gdb, ld, as
- binutils (add2line, strings, objdump, nm)
- Git
- Linux CLI tools
- board specific and task specific tools
(fastboot, dfu-util, omapconf, minicom, adb)
• Hardware
- JTAG
- logical analyzer
- oscilloscope
- soldering iron
Tools
Knowledge

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War Stories and Learnings

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Story #1: NOR flash support + XIP
boot

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Story #1: NOR flash support and XIP boot
Task highlights:
• Provide NOR flash chip support:
- in kernel
- in U-Boot
• Provide XIP boot:
- for U-Boot
- for kernel

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Overview: NOR flash
• NOR flash is XIP capable (eXecution In Place)
• XIP from NOR is often used for reducing the boot time
• Can be useful in automotive and mobile areas
• It’s possible to run U-Boot and kernel from NOR flash
(Android or Linux rootfs is stored on eMMC)
• Concerns:
- drivers (GPMC, MTD, CFI, ELM)
- Device Tree definition
- timings
- U-Boot/kernel configuration for XIP

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Overview: NAND flash wiring

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Overview: NOR flash wiring

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Overview: NOR and NAND flash comparison
Parameter NOR NAND
Capacity Small (64 KB - 512 MB) Huge (16 MB - 1 TB)
XIP (code execution) Yes No
Block erase time Very slow (1 sec) Fast (500 μsec)
Write time Slow (10 μsec / word) Fast (200 μsec / page)
Read time Fast (100 nsec / word) Fast (50 μsec / page)
Erase cycle range 10,000 to 100,000 1,000,000 with ECC
Access method Random Sequential
Price High (4x / MB) Very low (x / MB)

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Overview: Connection to SoC

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Sub-task 1: Timings
● GPMC controller must be programmed with correct timings
● Timings must be calculated from NOR chip characteristics
● There are a lot of them
● Usually board manufacturer should provide you with correct timings
● If NOR flash hangs - most likely timings are wrong

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Sub-task 1: Timings
• cs-on
• cs-rd-off
• cs-wr-off
• oe-on
• oe-off
• we-on
• we-off
• rd-access
And much more else timings...
GPMC: Timings and parameters

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Sub-task 1: Timings
• CONFIG1
• CONFIG2
• CONFIG3
• CONFIG4
• CONFIG5
• CONFIG6
GPMC: Registers
Register example:

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Sub-task 1: Timings
● Conversion tool was developed:
https://gitlab.com/joeskb7/gpmc-timings
Usage:
./gpmc-timings -p CONFIG1..CONFIG6
./gpmc-timings -y <file>
-y file - yield registers values from file
-p registers - parse and print registers values
Conversion tool

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Sub-task 1: Timings
&gpmc {
nor@0,0 {
gpmc,cs-on-ns = <0>;
gpmc,cs-rd-off-ns = <50>;
gpmc,cs-wr-off-ns = <57>;
gpmc,oe-on-ns = <0>;
gpmc,oe-off-ns = <50>;
...
};
};
Timings in Device Tree

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Sub-task 2: Clocks and DPLLs
● GPMC clock must be enabled and be of correct frequency
● Check correct value (calculated for NOR timings)
● Check actual value (from kernel DebugFS)
● Fix clock if needed
● Clock derivation path can be seen from SoC TRM

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GPMC_FCLK

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L3MAIN1_L3_GICLK = L3_ICLK

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L3_ICLK

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CORE_X2_CLK

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● Enable CONFIG_COMMON_CLK_DEBUG
● Mount DebugFS (in example below, it’s mounted to /d)
● Print clocks frequencies:
/d/clk/.../dpll_core_x2_ck/dpll_core_h12x2_ck # cat clk_rate
532000000
/d/clk/.../dpll_core_x2_ck/dpll_core_h12x2_ck/l3_iclk_div # cat clk_rate
532000000
But we see from TRM that L3_ICLK must be CORE_X2_CLK / 2
Check clock freq in DebugFS

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l3_iclk_div: l3_iclk_div {
#clock-cells = <0>;
- compatible = "fixed-factor-clock";
+ compatible = "ti,divider-clock";
+ ti,max-div = <2>;
};
Fix the clock

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● Print clocks frequencies:
/d/clk/.../dpll_core_x2_ck/dpll_core_h12x2_ck # cat clk_rate
532000000
/d/clk/.../dpll_core_x2_ck/dpll_core_h12x2_ck/l3_iclk_div # cat clk_rate
266000000
So GPMC functional clock frequency now is 266 MHz, which is a desired frequency.
Check clock freq in DebugFS after fix

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Sub-task 3: Kernel support
● NOR flash and eMMC were sharing the lines (found from schematic)
● So there was only 512 KiB of NOR flash accessible in kernel
● Means no kernel XIP
● But we can still:
○ read/write NOR from kernel
○ do U-Boot XIP
● NOR support wasn’t ready in our kernel at the time (k3.8)
● But it was ready in mainline kernel (3.11)

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● Patches often have dependencies
● It’s hard to find all actual dependencies between branches
● Kernel maintainers have dedicated trees/branches for features like GPMC
● Use smart cherry-picking
Backporting patches

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Cherry-pick like a pro

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nor@0,0 {
/* We can only access 512 KiB from kernel */
partition@0 {
label = "boot"; /* u-boot.bin */
reg = <0x00000000 0x00060000>; /* 384 KiB */
};
partition@1 {
label = "env"; /* U-Boot environment */
reg = <0x00060000 0x00020000>; /* 128 KiB */
};
};
NOR partitions

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# flash_erase /dev/mtd/mtd0 0 0
# cat mtd0-to.img >/dev/mtd/mtd0
# cat /dev/mtd/mtd0 >mtd0-from.img
MTD utils and MTD char devices read/write

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Sub-task 4: U-Boot XIP
● For XIP boot we don’t need MLO (U-Boot SPL), only u-boot.bin
● Boot sequence:
○ U-Boot is started by ROM code
○ U-Boot copies all files needed to RAM (dtb, kernel, ramdisk)
○ U-Boot switches to eMMC (e.g. via GPIO line)
○ U-Boot starts kernel from RAM; now we are not in XIP boot anymore
● Problem: U-Boot hangs when running XIP from NOR

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● U-Boot is too big to figure out why it’s not working
● We can’t even be sure XIP can work at all
● Let’s shrink U-Boot to minimal code and see if it works
● XIP “Hello world” baremetal firmware was implemented:
○ start assembler code (shrinked)
○ linker script (minimal: text/rodata/data/bss, all in SRAM)
○ BSS clear
○ enable UART clock + dependency clocks
○ pin muxes
○ naiive UART driver
○ print “Hello world” to UART (serial console)
https://gitlab.com/joeskb7/dra7xx-hello
Approach the problem

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● Minimal app worked, but U-Boot won’t work
● Now we know NOR flash + XIP is functional and there is
some bug in U-Boot
● Use JTAG to investigate the issue
Investigation

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Root cause

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● git bisect-ing helped to find the actual bug
● We were given modified U-Boot with code like this
(in drivers/mmc/Makefile):
CFLAGS := $(subst -Os, -O0, $(CFLAGS)) -fPIC
Root cause (2)

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● So only MMC related files were built with -O0 -fPIC
● But other U-Boot files were build with -Os
● That led to NULL pointer dereference
● Removing that line fixed the problem
Fix

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Learnings:
● Always check mainline kernel and mailing lists for existing patch
● Git is your friend:
○ cherry-pick thoughtfully
○ bisect often helps to find the regression
● If some task is repeated often -- develop some tool or script
○ Share it if possible
● Isolating the problem helps to find the root cause
● JTAG is an ultimate tool for debugging

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Story #2: Updating MAX732x
driver

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Story #2: Updating MAX732x driver
Task highlights:
• Update MAX732x driver to use Device Tree
• Provide bindings documentation
• Make it functional and bug-free
• Upstream changes

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Overview: Chip description
• I2C I/O Expander
• Often used in automotive
• Have one-directional and
bidirectional ports
• Sends interrupt to SoC when input
event occurred

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Sub-task 1: Device tree support
#ifdef CONFIG_OF
static const struct of_device_id max732x_of_table[] = {
...
{ .compatible = "maxim,max7325" },
...
{ }
};
MODULE_DEVICE_TABLE(of, max732x_of_table);
#endif
OF Table

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static struct i2c_driver max732x_driver = {
.driver = {
.name = "max732x",
.owner = THIS_MODULE,
.of_match_table = of_match_ptr(max732x_of_table),
},
...
}
I2C driver struct

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Platform data doesn’t exist in case of OF provider.
- chip->irq_base = pdata->irq_base;
+ if (pdata)
+ chip->irq_base =
pdata->irq_base;
Check pdata

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Sub-task 2: Interrupt controller

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Sub-task 2: Interrupt controller
● GPIO framework is capable of handling IRQ controller code for you
● Remove IRQ domain code / manual IRQ calculation and use gpiochip_* API
config GPIO_MAX732X_IRQ
select GPIOLIB_IRQCHIP
#include <linux/gpio/driver.h>
gpiochip_add()
gpiochip_remove()
gpiochip_irqchip_add()
gpiochip_set_chained_irqchip()
GPIOLIB_IRQCHIP

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Sub-task 3: DT definition and bindings
● We want to I2C address of MAX7325 to be 0x5d
● As per datasheet:
Pin Line
----------
AD2 V+
AD0 V+
Check I2C address on schematics

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&i2c0 {
expander: max7325@5d {
compatible = "maxim,max7325";
reg = <0x5d>;
gpio-controller;
#gpio-cells = <2>;
};
};
DT Definition: disabled interrupts

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&i2c0 {
expander: max7325@5d {
compatible = "maxim,max7325";
reg = <0x5d>;
gpio-controller;
#gpio-cells = <2>;
interrupt-controller;
#interrupt-cells = <2>;
interrupt-parent = <&gpio4>;
interrupts = <29 IRQ_TYPE_EDGE_FALLING>;
};
};
DT Definition: enabled interrupts

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Sub-task 4: Upstreaming
$ git log --author="Protsenko" --oneline | grep max732
606f13e9efa0 gpio: max732x: Fix irq-events handler
68689dbf35e1 gpio: max732x: Add IRQF_SHARED to irq flags
67ddd32bfc9f gpio: max732x: Propagate wake-up setting to parent irq
controller
996bd13f28e6 gpio: max732x: Add DT binding documentation
09afa276d52e gpio: max732x: Fix possible deadlock
479f8a5744d8 gpio: max732x: Rewrite IRQ code to use irq_domain API
43c4bcf9425e gpio: max732x: Add device tree support
$ git log -- drivers/gpio/gpio-max732x.c
Patches in upstream

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Testing board: schematic

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Testing board: PCB

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Learnings:
● If in doubt -- check schematics
● Check out existing code in kernel for examples
● Check if there is existing framework for that problem
● Take responsibility (technical debt)
● Ask your customer about upstreaming

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Linus Torvalds shaves Bdale Garbee's beard

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Story #3: Flashing EEPROM

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Story #3: Flashing EEPROM
Task highlights:
• EEPROM chip wasn’t flashed with Board ID
• It led to 2 issues:
- U-Boot was unable to find correct DTB file
- "board_rev" variable were missing in U-Boot environment
• Fix:
- Disable “Read only” mode for EEPROM chip
- Figure out how to work with it
- Write correct Board ID value to EEPROM
- Set back “Read only” mode

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Sub-task 1: Disable write protection
Schematic

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First glance at PCB

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Disabling write protection

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Sub-task 2: Writing to EEPROM
static void setup_board_eeprom_env(void)
{
printf("### Writing EEPROM...n");
rc = ti_write_eeprom(CONFIG_EEPROM_BUS_ADDRESS,
CONFIG_EEPROM_CHIP_ADDRESS);
if (rc)
printf("### Error: Unable to write EEPROMn");
else
printf("### Writing EEPROM: Success!n");
}
U-Boot code (1)

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static int ti_i2c_write_eeprom(int bus_addr, int dev_addr, u32 size,
uint8_t *ep, unsigned int mem_addr)
{
/* Memory address size is 2 bytes */
return i2c_write(dev_addr, mem_addr, 2, ep, size);
}
U-Boot code (2)

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int ti_write_eeprom(int bus_addr, int dev_addr)
{
const char *name = "AM572PM_";
const char *rev = "A.20";
const char *serial = "07164P550148";
const char *config = "";
struct ti_am_eeprom am_ep;
uint8_t page[64];
int i, j;
int rc;
U-Boot code (3)

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/* Prepare structure for EEPROM */
memset(&am_ep, 0, sizeof(am_ep));
am_ep.header = TI_EEPROM_HEADER_MAGIC;
strncpy(am_ep.name, name, TI_EEPROM_HDR_NAME_LEN);
strncpy(am_ep.version, rev, TI_EEPROM_HDR_REV_LEN);
strncpy(am_ep.serial, serial, TI_EEPROM_HDR_SERIAL_LEN);
strncpy(am_ep.config, config, TI_EEPROM_HDR_CONFIG_LEN);
for (i = 0; i < TI_EEPROM_HDR_NO_OF_MAC_ADDR; ++i)
for (j = 0; j < TI_EEPROM_HDR_ETH_ALEN; ++j)
am_ep.mac_addr[i][j] = 0xff;
U-Boot code (4)

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/* Init I2C */
gpi2c_init();
rc = ti_i2c_eeprom_init(bus_addr, dev_addr);
if (rc)
return rc;
U-Boot code (5)

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/* Write 1st page */
memset(page, 0, 64);
memcpy(page, (uint8_t *)&am_ep, 64);
rc = ti_i2c_write_eeprom(bus_addr, dev_addr, 64, page, 0x00);
mdelay(100); /* t_WR = 5 msec */
if (rc)
return rc;
U-Boot code (6)

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/* Write 2nd page */
memset(page, 0, 64);
memcpy(page, ((uint8_t *)&am_ep) + 64, 14);
rc = ti_i2c_write_eeprom(bus_addr, dev_addr, 64, page, 0x40);
mdelay(100); /* t_WR = 5 msec */
if (rc)
return rc;
return 0;
}
U-Boot code (7)

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Thank you
Sam Protsenko
Software Engineer
semen.protsenko@globallogic.com

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Abbreviations:
● BSP - Board Support Package
● CFI - Common Flash Interface
● DPLL - Digital Phase-Locked Loop
● DTB - Device Tree Binary
● EEPROM - Electrically Erasable Programmable Read-Only Memory
● eMMC - Embedded MultiMedia Card
● FIT - Flattened Image Tree
● GIC - Generic Interrupt Controller
● GPIO - General Purpose Input Output
● GPMC - General Purpose Memory Controller
● INTC - Interrupt Controller
● MTD - Memory Technology Device
● NOR - Not Or
● SoC - System On Chip
● TRM - Technical Reference Manual
● XIP - eXecute In Place

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