August 2014 - kernelnewbies@kernelnewbies.org

Strategies for accessing driver data from file operations!?
by Daniel Hilst Selli Aug. 25, 2014

Aug. 25, 2014

I was writing an spi driver, and taking a look into spidev.c, I see the the author allocates a linked list to hold driver data instances. From open it iterates over the list comparing two dev_t fields, one from current element on list other from struct inode * parameter, here is the lines: static int spidev_open(struct inode *inode, struct file *filp) { struct spidev_data *spidev; int status = -ENXIO; mutex_lock(&device_list_lock); list_for_each_entry(spidev, &device_list, device_entry) { if (spidev->devt == inode->i_rdev) { status = 0; break; } } ... Now it set the filp->private data to its driver data, and on read and write file operations he knows how to access is driver data, I was looking for a simpler strategy, on my module I would have same devices on distinct spi busses and chipselects, but it wouldn't go greater than 9 devices. I need to access spi_device struct pointer for the right device from read and write file operations. Isn't there any path from struct file *filp to struct device *devp created with device_create? I can't see it, but if is possible I can set device private data from my probe function. Cheers,

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Questions about btrfs helper threads
by nick Aug. 25, 2014

Aug. 25, 2014

After searching through parts of the btrfs code and docs. I am unable to find any information on the helper threads in btrfs and would like to known more about how their implemented and what are the reasons for helper threads in btrfs versus only 1 for journaling in ext3/ext3. Nick

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Which way is faster , when receive a lot of packets?
by lx Aug. 25, 2014

Aug. 25, 2014

hi: I have two ideas to handle the packet. 1.I allocate a memory pool and mmap into userspace, when packet is coming, I copy packet into the memory pool, then, userspace can access it. 2.When packet is coming , I mmap the data of packet into userspace every time, then userspace can access this packet. Which one is better, why?

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serial console in early boot
by John de la Garza Aug. 25, 2014

Aug. 25, 2014

I'm using qemu like this: qemu -nographic -kernel /home/john/dev-linux/arch/x86/boot/bzImage \ -append "console=ttyS0" I noticed that I can do a printk in arch/x86/kernel/head64.c right after the call to clear_bss (line 162 in x86_64_start_kernel). I see the output of the printk on the serial console. This seems to be before any uart/console has been initialized. I'm curious how the printk can make it to the serial console. Anyone know why printk is sending output to the serial console this early in the boot process? I would expect that the serial line wouldn't be ready and that qemu would behave like actual hardware.

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Use of skb->cb
by Santhosh Kumar Aug. 23, 2014

Aug. 23, 2014

My understanding is every layer is free to use the skb->cb field in any way they wish but it will not be preserved by the upper layers in the network stack. I would like to use the skb->cb in the layer2/bridging code. Are there any caveats that one should be aware of ? Santhosh.

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fd type from number
by Loris Degioanni Aug. 23, 2014

Aug. 23, 2014

(resending making sure this is not part of another thread) I'm looking for an efficient way to determine the type of an fd (file, socket...) given its number, from a kernel module. The closest thing I found by looking at the kernel sources is sockfd_lookup(), which works but is limited to telling me if the fd is a socket or not. Is there something else I can look at? Thank you in advance, Loris

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MESA for i915
by Victor Rodriguez Aug. 22, 2014

Aug. 22, 2014

HI here is the problem: I am trying to compile mesa in a x86 system. After installing all the required packages and ussing the following configure command line: ./configure --with-dri-drivers="i915" --with-gallium-drivers="" make show this error: loader.c:495:24: fatal error: radeon_drm.h: No such file or directory #include <radeon_drm.h> Checking the source code: #if !defined(__NOT_HAVE_DRM_H) /* for i915 */ #include <i915_drm.h> /* for radeon */ #include <radeon_drm.h> static int drm_get_pci_id_for_fd(int fd, int *vendor_id, int *chip_id) { drmVersionPtr version; *chip_id = -1; version = drmGetVersion(fd); if (!version) { log_(_LOADER_WARNING, "MESA-LOADER: invalid drm fd\n"); return 0; } if (!version->name) { log_(_LOADER_WARNING, "MESA-LOADER: unable to determine the driver name\n"); drmFreeVersion(version); return 0; } if (strcmp(version->name, "i915") == 0) { struct drm_i915_getparam gp; int ret; *vendor_id = 0x8086; memset(&gp, 0, sizeof(gp)); gp.param = I915_PARAM_CHIPSET_ID; gp.value = chip_id; ret = drmCommandWriteRead(fd, DRM_I915_GETPARAM, &gp, sizeof(gp)); if (ret) { log_(_LOADER_WARNING, "MESA-LOADER: failed to get param for i915\n"); *chip_id = -1; } } else if (strcmp(version->name, "radeon") == 0) { struct drm_radeon_info info; int ret; *vendor_id = 0x1002; memset(&info, 0, sizeof(info)); info.request = RADEON_INFO_DEVICE_ID; info.value = (unsigned long) chip_id; ret = drmCommandWriteRead(fd, DRM_RADEON_INFO, &info, sizeof(info)); if (ret) { DOes any body know how can I specify the define : NOT_HAVE_DRM_H I do not want to install the radeon libraries ( because I don't need them ) :) Regards Victor Rodriguez

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New to Kernel newbie
by manoj kumar Aug. 22, 2014

Aug. 22, 2014

Hi, i am new to kernelnewbie. I would like to work on the VFS/Filesystem sub system. Any guidance towards the same is much appreciated. RegardsManoj

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Re: Need help understanding the logic of __cpuidle_set_driver
by AYAN KUMAR HALDER Aug. 22, 2014

Aug. 22, 2014

>> However, regarding the logic, I have a few doubts - >> >> 1. for each cpu in drv->cpumask, if there is already a driver >> registered, its calling __cpuidle_unset_driver which loops over for >> each cpu in drv->cpumask again. Isn't it unnecessary to do this nested >> calls? > > > It is to rollback the previous changes done in the loop. However, I have two concerns here:- 1. static inline int __cpuidle_set_driver(struct cpuidle_driver *drv) { int cpu; struct cpuidle_driver *tmp = NULL; for_each_cpu(cpu, drv->cpumask) { tmp = __cpuidle_get_cpu_driver(cpu); /* This would prevent nesting of loops */ if ( tmp != NULL ) { if ( tmp == drv ) per_cpu(cpuidle_drivers, cpu) = NULL; return -EBUSY; } per_cpu(cpuidle_drivers, cpu) = drv; } return 0; } Instead of calling __cpuidle_unset_driver() which would loop-over again for all the cpus , is it not that the above function would be more efficient. 2. Secondly, if the cpuidle driver to be registered matches to the one already registered , then why should it be unset instead of leaving it as it is. Any views would be highly appreciated.

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quick help - linux Industrial IO Hardware Triggers
by Sricharan Chalasani Aug. 21, 2014

Aug. 21, 2014

Hi List, Thanks in advance. In the existing IIO drivers (linux-3.16), my understanding is that Hardware Triggers are just used to copy sampled data from the ADC to the Kernel buffer. I think none of the hardware triggers are really configuring ADC to Start the analog to digital conversion. I have a Hardware Counter on my Board. I am planning to configure my Counter overflow interrupt as an IIO Hardware Trigger. In my IIO ADC driver probe function I am planning to start my Counter for the first time. Once the counter overflow interrupted is generated, inside the counter ISR, I am planning to configure ADC to start analog to digital conversion on all the channels (4 of them), one sample on each channel and once the Conversion is completed, I am planning to copy the Sampled data to Kernel buffer associated with the Trigger. I will program the Counter again and wait for interrupt and repeat the steps above. I am planning to continue this approach indefinitely. Does this approach satisfy the principles of IIO Subsystem ? Am I missing anything here in understanding the IIO Subsystem properly ? Thanks in advance, Sricharan, Pune (India).

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