Messages in this thread Patch in this message | | | Date | Thu, 12 Dec 2019 16:16:28 -0800 | From | Dmitry Torokhov <> | Subject | Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API |
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On Thu, Dec 12, 2019 at 09:08:04PM +0000, Robin Murphy wrote: > 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)?
Yes, but it has to be done carefully.
> > 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);
I think it has to be "devres_add(dev, dr->data + align);" here, as match function checks the pointer passed to devm_kfree() with one stored in devres structure.
> - return dr->data; > + return dr->data + align; > } > EXPORT_SYMBOL_GPL(devm_kmalloc);
Thanks.
-- Dmitry
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