How embedded Linux isn’t so special, and most of the « embedded requirements » are also requirements in « big iron » too http://kernel-recipes.org

1. Embedded Linux and the mainline kernel
David Woodhouse
Kernel Recipes
September 2012
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2. Where does Linux run?
Embedded control device... phone...
PDA... Internet tablet... router...
media device... netbook... laptop...
desktop... server... supercomputer...
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3. “Embedded”...?
• Control equipment
• Phones
• PDAs
• “Internet Tablets”
• Routers
• Televisions
• VCR / PVR / DVD / Media
• Netbooks (?)
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4. “Embedded”...?
• Headless?
• Handheld?
• Power source?
• Physical size?
• Limited RAM?
• Storage?
• Other...
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5. Embedded needs
• Power management
• Fast startup
• Headless operation
• Uncluttered user interfaces
• Solid state storage
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6. Embedded needs
• Power management
• Fast startup
• Headless operation
• Uncluttered user interfaces
• Solid state storage
Other users need these features too!
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7. Power Management
• Battery life
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8. Power Management
• Battery life
• Cost of power consumption
• Heat output
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9. Tickless operation
• Power savings
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10. Tickless operation
• Power savings
• Scalability for virtualisation
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11. Fast boot
• Hard limits for mobile telephones
• User experience for consumer electronics
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12. Fast boot
• Hard limits for mobile telephones
• User experience for consumer electronics
• Server availability
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13. User interfaces
• Ease of use for consumer equipment
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14. User interfaces
• Ease of use for consumer equipment
• ... and for everyone else:
– OLPC / Sugar
– Netbooks
– Simple desktop environments
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15. Solid state storage
• FLASH storage in “embedded” devices
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16. Solid state storage
• FLASH storage in “embedded” devices
• Solid State Disk
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17. Others...
• Execute in place (XIP)
– From FLASH for embedded systems
– Shared file system data under virtualisation
• DMA API usage
– For cache coherency on embedded systems (ARM, some PPC)
– For IOMMU on larger systems
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18. We are not so
special!
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19. Community impressions
• “Enterprise” Linux
• “Embedded” Linux
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20. Community impressions
• “Enterprise” Linux
• “Embedded” Linux
– Working with old code
– Not working with upstream
– Inclined towards “special” one-off hacks
– Irrelevant to the general case
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21. Community impressions
• “Enterprise” Linux
• “Embedded” Linux
– Working with old code
– Not working with upstream
– Inclined towards “special” one-off hacks
– Irrelevant to the general case
We must prove them wrong!
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22. “Embedded” success stories
• Tickless
• Preemptive kernel
• Power management
• Suspend to RAM
• Solid state storage
• Squashfs
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23. Working with the community
• Find generic points of interest
• Publish early and often
– In git trees
– Separate trees for separate development efforts
– Also send patches for review
• Solicit and respond to feedback
• Work with upstream maintainers
• BE PART OF THE COMMUNITY!
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24. Staying close to upstream
• Advantages
– Easier for product updates and new products
– Easy to use fixes and new features
– External contributions
– Code review and testing
• Costs
– Writing acceptable code can be hard and takes time
– Upstream kernel is a fast-moving target
– Releasing information may be difficult
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25. Tips on contributing code
• Avoid hacking around problems
• Avoid overengineering
• Care about locking
• Coding Style
• Submit patches carefully
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26. Coding Style
• Simple, short functions
• Avoid:
– typedefs
– StudlyCaps
– Hungarian Notation
• Read Documentation/CodingStyle
http://www.linux.or.jp/JF/JFdocs/kernel-docs-
2.6/CodingStyle.html
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27. Submitting Patches
• Read the patch before you send it
--- a/drivers/net/wireless/libertas/assoc.c
+++ b/drivers/net/wireless/libertas/assoc.c
@@ -13,6 +13,7 @@
static const u8 bssid_any[ETH_ALEN] = { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
static const u8 bssid_off[ETH_ALEN] = { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
+extern int libertas_mesh_config(wlan_private * priv, int action,int channel,char
*str);
static void print_assoc_req(const char * extra, struct assoc_request * assoc_req)
{
@@ -211,9 +212,17 @@ static int assoc_helper_channel(wlan_private *priv,
{
wlan_adapter *adapter = priv->adapter;
int ret = 0;
+ int restart_mesh = 0;
lbs_deb_enter(LBS_DEB_ASSOC);
+ if(adapter->mesh_connect_status == LIBERTAS_CONNECTED)
+ {
+ libertas_mesh_config(priv,0,adapter->curbssparams.channel,NULL);
+ restart_mesh = 1;
+
+ }
+
ret = update_channel(priv);
if (ret < 0) {
lbs_deb_assoc("ASSOC: channel: error getting channel.");
@@ -225,11 +234,13 @@ static int assoc_helper_channel(wlan_private *priv,
lbs_deb_assoc("ASSOC: channel: %d -> %dn",
adapter->curbssparams.channel, assoc_req->channel);
+
ret = libertas_prepare_and_send_command(priv, CMD_802_11_RF_CHANNEL,
CMD_OPT_802_11_RF_CHANNEL_SET,
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28. Submitting Patches
• Read the patch before you send it
--- a/drivers/net/wireless/libertas/assoc.c
+++ b/drivers/net/wireless/libertas/assoc.c
@@ -13,6 +13,7 @@
static const u8 bssid_any[ETH_ALEN] = { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
static const u8 bssid_off[ETH_ALEN] = { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
+extern int libertas_mesh_config(wlan_private * priv, int action,int channel,char
*str);
static void print_assoc_req(const char * extra, struct assoc_request * assoc_req)
{
@@ -211,9 +212,17 @@ static int assoc_helper_channel(wlan_private *priv,
{
wlan_adapter *adapter = priv->adapter;
int ret = 0;
+ int restart_mesh = 0;
lbs_deb_enter(LBS_DEB_ASSOC);
+ if(adapter->mesh_connect_status == LIBERTAS_CONNECTED)
+ {
+ libertas_mesh_config(priv,0,adapter->curbssparams.channel,NULL);
+ restart_mesh = 1;
+
+ }
+
ret = update_channel(priv);
if (ret < 0) {
lbs_deb_assoc("ASSOC: channel: error getting channel.");
@@ -225,11 +234,13 @@ static int assoc_helper_channel(wlan_private *priv,
lbs_deb_assoc("ASSOC: channel: %d -> %dn",
adapter->curbssparams.channel, assoc_req->channel);
+
ret = libertas_prepare_and_send_command(priv, CMD_802_11_RF_CHANNEL,
CMD_OPT_802_11_RF_CHANNEL_SET,
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29. Submitting Patches
• Read the patch before you send it
• Use scripts/checkpatch.pl
$ scripts/checkpatch.pl 0002-MESH-START-STOP_IOCTLS.patch
WARNING: line over 80 characters
#7: FILE: drivers/net/wireless/libertas/assoc.c:16:
+extern int libertas_mesh_config(wlan_private * priv, int action,int channel,char *str);
ERROR: "foo * bar" should be "foo *bar"
#7: FILE: drivers/net/wireless/libertas/assoc.c:16:
+extern int libertas_mesh_config(wlan_private * priv, int action,int channel,char *str);
ERROR: need space after that ',' (ctx:VxV)
#7: FILE: drivers/net/wireless/libertas/assoc.c:16:
+extern int libertas_mesh_config(wlan_private * priv, int action,int channel,char *str);
^
WARNING: externs should be avoided in .c files
#7: FILE: drivers/net/wireless/libertas/assoc.c:16:
+extern int libertas_mesh_config(wlan_private * priv, int action,int channel,char *str);
ERROR: That open brace { should be on the previous line
#19: FILE: drivers/net/wireless/libertas/assoc.c:219:
+ if(adapter->mesh_connect_status == LIBERTAS_CONNECTED)
+ {
ERROR: need a space before the open parenthesis '('
#19: FILE: drivers/net/wireless/libertas/assoc.c:219:
+ if(adapter->mesh_connect_status == LIBERTAS_CONNECTED)
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30. Submitting Patches
• Read the patch before you send it
• Use scripts/checkpatch.pl
• Send patches to yourself first
$ patch -p1 < saved-email.txt
patching file drivers/net/wireless/libertas/assoc.c
Hunk #1 FAILED at 13.
Hunk #2 FAILED at 212.
Hunk #3 FAILED at 234.
patch: **** malformed patch at line 71: adapter->meshid);
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31. Submitting Patches
• Read the patch before you send it
• Use scripts/checkpatch.pl
• Send patches to yourself first
• Read Documentation/SubmittingPatches
or Documentation/ja_JP/SubmittingPatches
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32. Submitting Patches
• Read the patch before you send it
• Use scripts/checkpatch.pl
• Send patches to yourself first
• Read Documentation/SubmittingPatches
• Include appropriate description
commit 4ac9137858e08a19f29feac4e1f4df7c268b0ba5
Author: Jan Blunck <jblunck@suse.de>
Date: Thu Feb 14 19:34:32 2008 -0800
Embed a struct path into struct nameidata instead of nd->{dentry,mnt}
This is the central patch of a cleanup series. In most cases there is no good
reason why someone would want to use a dentry for itself. This series reflects
that fact and embeds a struct path into nameidata.
Together with the other patches of this series
- it enforced the correct order of getting/releasing the reference count on
<dentry,vfsmount> pairs
- it prepares the VFS for stacking support since it is essential to have a
struct path in every place where the stack can be traversed
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33. What's next for “Embedded” Linux?
• Solid state storage
– More work on SSDs
– Flash file system development (UBI, logfs, btrfs)
• Better power management
• More real time development
• What do you need?
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34. Questions?
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