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Diffstat (limited to 'Documentation/driver-model/devres.txt')
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diff --git a/Documentation/driver-model/devres.txt b/Documentation/driver-model/devres.txt new file mode 100644 index 000000000000..5163b85308f5 --- /dev/null +++ b/Documentation/driver-model/devres.txt @@ -0,0 +1,268 @@ +Devres - Managed Device Resource +================================ + +Tejun Heo <teheo@suse.de> + +First draft 10 January 2007 + + +1. Intro : Huh? Devres? +2. Devres : Devres in a nutshell +3. Devres Group : Group devres'es and release them together +4. Details : Life time rules, calling context, ... +5. Overhead : How much do we have to pay for this? +6. List of managed interfaces : Currently implemented managed interfaces + + + 1. Intro + -------- + +devres came up while trying to convert libata to use iomap. Each +iomapped address should be kept and unmapped on driver detach. For +example, a plain SFF ATA controller (that is, good old PCI IDE) in +native mode makes use of 5 PCI BARs and all of them should be +maintained. + +As with many other device drivers, libata low level drivers have +sufficient bugs in ->remove and ->probe failure path. Well, yes, +that's probably because libata low level driver developers are lazy +bunch, but aren't all low level driver developers? After spending a +day fiddling with braindamaged hardware with no document or +braindamaged document, if it's finally working, well, it's working. + +For one reason or another, low level drivers don't receive as much +attention or testing as core code, and bugs on driver detach or +initilaization failure doesn't happen often enough to be noticeable. +Init failure path is worse because it's much less travelled while +needs to handle multiple entry points. + +So, many low level drivers end up leaking resources on driver detach +and having half broken failure path implementation in ->probe() which +would leak resources or even cause oops when failure occurs. iomap +adds more to this mix. So do msi and msix. + + + 2. Devres + --------- + +devres is basically linked list of arbitrarily sized memory areas +associated with a struct device. Each devres entry is associated with +a release function. A devres can be released in several ways. No +matter what, all devres entries are released on driver detach. On +release, the associated release function is invoked and then the +devres entry is freed. + +Managed interface is created for resources commonly used by device +drivers using devres. For example, coherent DMA memory is acquired +using dma_alloc_coherent(). The managed version is called +dmam_alloc_coherent(). It is identical to dma_alloc_coherent() except +for the DMA memory allocated using it is managed and will be +automatically released on driver detach. Implementation looks like +the following. + + struct dma_devres { + size_t size; + void *vaddr; + dma_addr_t dma_handle; + }; + + static void dmam_coherent_release(struct device *dev, void *res) + { + struct dma_devres *this = res; + + dma_free_coherent(dev, this->size, this->vaddr, this->dma_handle); + } + + dmam_alloc_coherent(dev, size, dma_handle, gfp) + { + struct dma_devres *dr; + void *vaddr; + + dr = devres_alloc(dmam_coherent_release, sizeof(*dr), gfp); + ... + + /* alloc DMA memory as usual */ + vaddr = dma_alloc_coherent(...); + ... + + /* record size, vaddr, dma_handle in dr */ + dr->vaddr = vaddr; + ... + + devres_add(dev, dr); + + return vaddr; + } + +If a driver uses dmam_alloc_coherent(), the area is guaranteed to be +freed whether initialization fails half-way or the device gets +detached. If most resources are acquired using managed interface, a +driver can have much simpler init and exit code. Init path basically +looks like the following. + + my_init_one() + { + struct mydev *d; + + d = devm_kzalloc(dev, sizeof(*d), GFP_KERNEL); + if (!d) + return -ENOMEM; + + d->ring = dmam_alloc_coherent(...); + if (!d->ring) + return -ENOMEM; + + if (check something) + return -EINVAL; + ... + + return register_to_upper_layer(d); + } + +And exit path, + + my_remove_one() + { + unregister_from_upper_layer(d); + shutdown_my_hardware(); + } + +As shown above, low level drivers can be simplified a lot by using +devres. Complexity is shifted from less maintained low level drivers +to better maintained higher layer. Also, as init failure path is +shared with exit path, both can get more testing. + + + 3. Devres group + --------------- + +Devres entries can be grouped using devres group. When a group is +released, all contained normal devres entries and properly nested +groups are released. One usage is to rollback series of acquired +resources on failure. For example, + + if (!devres_open_group(dev, NULL, GFP_KERNEL)) + return -ENOMEM; + + acquire A; + if (failed) + goto err; + + acquire B; + if (failed) + goto err; + ... + + devres_remove_group(dev, NULL); + return 0; + + err: + devres_release_group(dev, NULL); + return err_code; + +As resource acquision failure usually means probe failure, constructs +like above are usually useful in midlayer driver (e.g. libata core +layer) where interface function shouldn't have side effect on failure. +For LLDs, just returning error code suffices in most cases. + +Each group is identified by void *id. It can either be explicitly +specified by @id argument to devres_open_group() or automatically +created by passing NULL as @id as in the above example. In both +cases, devres_open_group() returns the group's id. The returned id +can be passed to other devres functions to select the target group. +If NULL is given to those functions, the latest open group is +selected. + +For example, you can do something like the following. + + int my_midlayer_create_something() + { + if (!devres_open_group(dev, my_midlayer_create_something, GFP_KERNEL)) + return -ENOMEM; + + ... + + devres_close_group(dev, my_midlayer_something); + return 0; + } + + void my_midlayer_destroy_something() + { + devres_release_group(dev, my_midlayer_create_soemthing); + } + + + 4. Details + ---------- + +Lifetime of a devres entry begins on devres allocation and finishes +when it is released or destroyed (removed and freed) - no reference +counting. + +devres core guarantees atomicity to all basic devres operations and +has support for single-instance devres types (atomic +lookup-and-add-if-not-found). Other than that, synchronizing +concurrent accesses to allocated devres data is caller's +responsibility. This is usually non-issue because bus ops and +resource allocations already do the job. + +For an example of single-instance devres type, read pcim_iomap_table() +in lib/iomap.c. + +All devres interface functions can be called without context if the +right gfp mask is given. + + + 5. Overhead + ----------- + +Each devres bookkeeping info is allocated together with requested data +area. With debug option turned off, bookkeeping info occupies 16 +bytes on 32bit machines and 24 bytes on 64bit (three pointers rounded +up to ull alignment). If singly linked list is used, it can be +reduced to two pointers (8 bytes on 32bit, 16 bytes on 64bit). + +Each devres group occupies 8 pointers. It can be reduced to 6 if +singly linked list is used. + +Memory space overhead on ahci controller with two ports is between 300 +and 400 bytes on 32bit machine after naive conversion (we can +certainly invest a bit more effort into libata core layer). + + + 6. List of managed interfaces + ----------------------------- + +IO region + devm_request_region() + devm_request_mem_region() + devm_release_region() + devm_release_mem_region() + +IRQ + devm_request_irq() + devm_free_irq() + +DMA + dmam_alloc_coherent() + dmam_free_coherent() + dmam_alloc_noncoherent() + dmam_free_noncoherent() + dmam_declare_coherent_memory() + dmam_pool_create() + dmam_pool_destroy() + +PCI + pcim_enable_device() : after success, all PCI ops become managed + pcim_pin_device() : keep PCI device enabled after release + +IOMAP + devm_ioport_map() + devm_ioport_unmap() + devm_ioremap() + devm_ioremap_nocache() + devm_iounmap() + pcim_iomap() + pcim_iounmap() + pcim_iomap_table() : array of mapped addresses indexed by BAR + pcim_iomap_regions() : do request_region() and iomap() on multiple BARs |