Ether device API functions and some other hacks

Hi again

I worked on ether device API functions in a last month before module init/exit cases. Mainly I worked on 3 functions and got understanding of some others. I am going to talk about some other functions along with ether device API functions in this blog. So, this is going to be somehow managed blog.

1. eth_zero_addr:

This is a ether device API function which replaces memset to assign zero address. Linux kernel community prefers to using eth_zero_addr function instead of memset to assign the zero address to the given array. The function definition goes like this:

static inline void eth_zero_addr(u8 *addr)
{
memset(addr, 0x00, ETH_ALEN);
}

There were like 10-12 files having such cases. I sent patches for all of them. Most of them are now added in a kernel tree. Here, is a semantic patch which I used to do this change. It is preety simple. Only thing is sometimes value ‘6’ is used as ETH_ALEN. In that case one need to check that it is really a network code and in a memset 6 means ETH_ALEN.

@eth_zero_addr@
expression e;
@@

-memset(e,0x00,\(6\|ETH_ALEN\));
+eth_zero_addr(e);

2. eth_broadcast_addr:
This is also ether device API function and replaces memset. Only difference is this function is used to assign the broadcast address to the given address array. There were only 2-3 such cases. I handled all of them too. Semantic patch I used is as follows:

@eth_broadcast_addr@
identifier e;
@@

-memset(e,\(0xff\|0xFF\|255\),\(6\|ETH_ALEN\));
+eth_broadcast_addr(e);

3. eth_hw_addr_random:

This is an intersting function. It generates random Ethernet address [MAC] to be used by a net device and set addr_assign_type so that state can be read by sysfs and be used by userspace. Now, definition of this function goes like this:

static inline void eth_hw_addr_random(struct net_device *dev)
{
dev->addr_assign_type = NET_ADDR_RANDOM;
eth_random_addr(dev->dev_addr);
}

Basically I found this case from the deprecated function list. There were like 63 uses of random_eth_addr in linux 3.0 and now very less uses are there. Primary reason behind this was here random_eth_addr is changed to eth_random_addrfor the consistency purpose. So, there were many commits proposing that change. But when I looked in to detail then found a large bunch of commits from Danny Kukawka in 2012, replacing random_eth_addr with eth_hw_addr_random. Although in each case one need to check many things at a time because sometimes original call of random_eth_addr/eth_random_addr do not make assignment to NET_ADDR_RANDOM. And in some cases we have seen that merging the call and the assignment is not correct. So, from my side I can just compile test the change. And in each case I need to write about this possibility under —. For the experiment I just sent a patch explaining the situation and it turned out positive. After tesing it maintainer applied the patch. That patch can be found here. I am going to send some more patches in a future.

4. Deprecated macro DEFINE_PCI_DEVICE_TABLE

This macro is actually deprecated and this comment is made in a header file too. I just accidentally came across this while reading some other code. And thought to handle it. Basically point is we can use struct pci_device_id instead of this macro. When I searched for the remaining use of this macro in a linux-next, it turned out that most of them were handled by Julia’s one of other intern in a Paris. Only 3 uses were remaining. I sent patches for them. One is applied in a local branch and others are on their way. This was a simple case too. Semantic patch I used is as follows:

@@
identifier a;
declarer name DEFINE_PCI_DEVICE_TABLE;
initializer i;
@@
– DEFINE_PCI_DEVICE_TABLE(a)
+ const struct pci_device_id a[]
= i;

5. Unnecessary function snd_pcm_lib_preallocate_free_for_all()

I picked this from deprecated functions list of Julia. Here, there is no point of using this function snd_pcm_lib_preallocate_free_for_all() because ALSA core takes care that all preallocated memory is freed when the PCM itself is freed. Only one case was left. I just sent a patch for it yesterday and to my surprise it was applied in a very next minute by Mark. 🙂 One can look at the patch here.

By the way, I came across non-modular version of module init/exit macros. Recently many such patches are added in a kernel tree. It is interesting to understand the reasons behind introducing and using such macro. I guess one separate post would be fine for that. I will post that blog in a 2-3 days. Also, I am working on fixing some usage of devm functions from last 2 weeks. I’ll write a blog about it in a next week too.

Till then signing off. Have a happy weekend.

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