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Subject[PATCH 7/13] devres: add Documentation/driver-model/devres.txt
Date
From
Add Documentation/driver-model/drvres.txt.

Signed-off-by: Tejun Heo <htejun@gmail.com>
---
Documentation/driver-model/devres.txt | 268 +++++++++++++++++++++++++++++++++
1 files changed, 268 insertions(+), 0 deletions(-)

diff --git a/Documentation/driver-model/devres.txt b/Documentation/driver-model/devres.txt
new file mode 100644
index 0000000..5163b85
--- /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
--
1.4.4.3

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