Messages in this thread | | | Date | Thu, 12 Dec 2019 11:10:02 -0800 | From | Dmitry Torokhov <> | Subject | Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API |
| |
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.
Thanks.
-- Dmitry
| |