Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 2 request_firmware() hotplug interface: 3 ------------------------------------ 4 Copyright (C) 2003 Manuel Estrada Sainz 5 6 Why: 7 --- 8 9 Today, the most extended way to use firmware in the Linux kernel is linking 10 it statically in a header file. Which has political and technical issues: 11 12 1) Some firmware is not legal to redistribute. 13 2) The firmware occupies memory permanently, even though it often is just 14 used once. 15 3) Some people, like the Debian crowd, don't consider some firmware free 16 enough and remove entire drivers (e.g.: keyspan). 17 18 High level behavior (mixed): 19 ============================ 20 21 kernel(driver): calls request_firmware(&fw_entry, $FIRMWARE, device) 22 23 userspace: 24 - /sys/class/firmware/xxx/{loading,data} appear. 25 - hotplug gets called with a firmware identifier in $FIRMWARE 26 and the usual hotplug environment. 27 - hotplug: echo 1 > /sys/class/firmware/xxx/loading 28 29 kernel: Discard any previous partial load. 30 31 userspace: 32 - hotplug: cat appropriate_firmware_image > \ 33 /sys/class/firmware/xxx/data 34 35 kernel: grows a buffer in PAGE_SIZE increments to hold the image as it 36 comes in. 37 38 userspace: 39 - hotplug: echo 0 > /sys/class/firmware/xxx/loading 40 41 kernel: request_firmware() returns and the driver has the firmware 42 image in fw_entry->{data,size}. If something went wrong 43 request_firmware() returns non-zero and fw_entry is set to 44 NULL. 45 46 kernel(driver): Driver code calls release_firmware(fw_entry) releasing 47 the firmware image and any related resource. 48 49 High level behavior (driver code): 50 ================================== 51 52 if(request_firmware(&fw_entry, $FIRMWARE, device) == 0) 53 copy_fw_to_device(fw_entry->data, fw_entry->size); 54 release(fw_entry); 55 56 Sample/simple hotplug script: 57 ============================ 58 59 # Both $DEVPATH and $FIRMWARE are already provided in the environment. 60 61 HOTPLUG_FW_DIR=/usr/lib/hotplug/firmware/ 62 63 echo 1 > /sys/$DEVPATH/loading 64 cat $HOTPLUG_FW_DIR/$FIRMWARE > /sysfs/$DEVPATH/data 65 echo 0 > /sys/$DEVPATH/loading 66 67 Random notes: 68 ============ 69 70 - "echo -1 > /sys/class/firmware/xxx/loading" will cancel the load at 71 once and make request_firmware() return with error. 72 73 - firmware_data_read() and firmware_loading_show() are just provided 74 for testing and completeness, they are not called in normal use. 75 76 - There is also /sys/class/firmware/timeout which holds a timeout in 77 seconds for the whole load operation. 78 79 - request_firmware_nowait() is also provided for convenience in 80 non-user contexts. 81 82 83 about in-kernel persistence: 84 --------------------------- 85 Under some circumstances, as explained below, it would be interesting to keep 86 firmware images in non-swappable kernel memory or even in the kernel image 87 (probably within initramfs). 88 89 Note that this functionality has not been implemented. 90 91 - Why OPTIONAL in-kernel persistence may be a good idea sometimes: 92 93 - If the device that needs the firmware is needed to access the 94 filesystem. When upon some error the device has to be reset and the 95 firmware reloaded, it won't be possible to get it from userspace. 96 e.g.: 97 - A diskless client with a network card that needs firmware. 98 - The filesystem is stored in a disk behind an scsi device 99 that needs firmware. 100 - Replacing buggy DSDT/SSDT ACPI tables on boot. 101 Note: this would require the persistent objects to be included 102 within the kernel image, probably within initramfs. 103 104 And the same device can be needed to access the filesystem or not depending 105 on the setup, so I think that the choice on what firmware to make 106 persistent should be left to userspace.