Messages in this thread Patch in this message | | | Subject | Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API | From | Robin Murphy <> | Date | Thu, 12 Dec 2019 21:08:04 +0000 |
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On 2019-12-12 7:10 pm, Dmitry Torokhov wrote: > On Thu, Dec 12, 2019 at 06:15:16PM +0000, Robin Murphy wrote: >> On 12/12/2019 4:59 pm, Marc Gonzalez wrote: >>> On 12/12/2019 15:47, Robin Murphy wrote: >>> >>>> On 12/12/2019 1:53 pm, Marc Gonzalez wrote: >>>> >>>>> On 11/12/2019 23:28, Dmitry Torokhov wrote: >>>>> >>>>>> On Wed, Dec 11, 2019 at 05:17:28PM +0100, Marc Gonzalez wrote: >>>>>> >>>>>>> What is the rationale for the devm_add_action API? >>>>>> >>>>>> For one-off and maybe complex unwind actions in drivers that wish to use >>>>>> devm API (as mixing devm and manual release is verboten). Also is often >>>>>> used when some core subsystem does not provide enough devm APIs. >>>>> >>>>> Thanks for the insight, Dmitry. Thanks to Robin too. >>>>> >>>>> This is what I understand so far: >>>>> >>>>> devm_add_action() is nice because it hides/factorizes the complexity >>>>> of the devres API, but it incurs a small storage overhead of one >>>>> pointer per call, which makes it unfit for frequently used actions, >>>>> such as clk_get. >>>>> >>>>> Is that correct? >>>>> >>>>> My question is: why not design the API without the small overhead? >>>> >>>> Probably because on most architectures, ARCH_KMALLOC_MINALIGN is at >>>> least as big as two pointers anyway, so this "overhead" should mostly be >>>> free in practice. Plus the devres API is almost entirely about being >>>> able to write simple robust code, rather than absolute efficiency - I >>>> mean, struct devres itself is already 5 pointers large at the absolute >>>> minimum ;) >>> >>> (3 pointers: 1 list_head + 1 function pointer) >> >> Ah yes, I failed to mentally preprocess the debug config :) >> >>> I'm confused. The first patch was criticized for potentially adding >>> an extra pointer for every devm_clk_get (e.g. 800 bytes on a 64-bit >>> platform with 100 clocks). >> >> I'm not sure it was a criticism so much as an observation of an aspect that >> deserved consideration (certainly it was on my part, and I read Dmitry's "It >> might still, ..." as implying the same). I'd say by this point it has been >> thoroughly considered, and personally I'm now happy with the conclusion that >> the kind of embedded platforms that will have many dozens of clocks are also >> the kind that will tend to have enough padding to make it moot, and thus the >> code simplification probably is worthwhile overall. > > I wonder if we could actually avoid allocating the data with > ARCH_KMALLOC_MINALIGN in all the cases. It is definitely needed for the > devm_k*alloc() group of functions as they are direct replacement for > k*alloc() APIs that give users aligned memory, but for other data > structures (clocks, regulators, etc, etc) it is not required.
That's a very good point - perhaps something like this (only done properly)?
Robin.
diff --git a/drivers/base/devres.c b/drivers/base/devres.c index 0bbb328bd17f..2382f963abbe 100644 --- a/drivers/base/devres.c +++ b/drivers/base/devres.c @@ -26,14 +26,7 @@ struct devres_node {
struct devres { struct devres_node node; - /* - * Some archs want to perform DMA into kmalloc caches - * and need a guaranteed alignment larger than - * the alignment of a 64-bit integer. - * Thus we use ARCH_KMALLOC_MINALIGN here and get exactly the same - * buffer alignment as if it was allocated by plain kmalloc(). - */ - u8 __aligned(ARCH_KMALLOC_MINALIGN) data[]; + u8 data[]; };
struct devres_group { @@ -810,6 +803,17 @@ static int devm_kmalloc_match(struct device *dev, void *res, void *data) void * devm_kmalloc(struct device *dev, size_t size, gfp_t gfp) { struct devres *dr; + size_t align; + + /* + * Some archs want to perform DMA into kmalloc caches + * and need a guaranteed alignment larger than + * the alignment of a 64-bit integer. + * Thus we use ARCH_KMALLOC_MINALIGN here and get exactly the same + * buffer alignment as if it was allocated by plain kmalloc(). + */ + align = (ARCH_KMALLOC_MINALIGN - sizeof(*dr)) % ARCH_KMALLOC_MINALIGN; + size += align;
/* use raw alloc_dr for kmalloc caller tracing */ dr = alloc_dr(devm_kmalloc_release, size, gfp, dev_to_node(dev)); @@ -822,7 +826,7 @@ void * devm_kmalloc(struct device *dev, size_t size, gfp_t gfp) */ set_node_dbginfo(&dr->node, "devm_kzalloc_release", size); devres_add(dev, dr->data); - return dr->data; + return dr->data + align; } EXPORT_SYMBOL_GPL(devm_kmalloc);
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