Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API

[Russell King - ARM Linux admin]

On Thu, Dec 12, 2019 at 05:59:04PM +0100, 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)
> 
> 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).
> 
> Let's see. On arm64, ARCH_KMALLOC_MINALIGN is 128.
> 
> So basically, a struct devres looks like this on arm64:
> 
> 	list_head.next
> 	list_head.prev
> 	dr_release_t
> 		.
> 		.
> 		.
> 	104 bytes of padding
> 		.
> 		.
> 		.
> 	data (flexible array)
> 		.
> 		.
> 		.
> 	padding up to 256 bytes
> 
> 
> Basically, on arm64, every struct devres occupies 256 bytes, most of it
> (typically 104 + 112 = 216) wasted as padding.
> 
> Hmmm, given how many devm stuff goes on in a modern platform, there
> might be large savings to be had...
> 
> Assuming 10,000 calls to devres_alloc_node(), we would be wasting ~2 MB
> of RAM. Not sure it's worth trying to save that?
> 
> $ git grep '#define ARCH_DMA_MINALIGN'
> arch/arc/include/asm/cache.h:#define ARCH_DMA_MINALIGN  SMP_CACHE_BYTES
> arch/arm/include/asm/cache.h:#define ARCH_DMA_MINALIGN  L1_CACHE_BYTES
> arch/arm64/include/asm/cache.h:#define ARCH_DMA_MINALIGN        (128)
> arch/c6x/include/asm/cache.h:#define ARCH_DMA_MINALIGN  L1_CACHE_BYTES
> arch/csky/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES
> arch/hexagon/include/asm/cache.h:#define ARCH_DMA_MINALIGN      L1_CACHE_BYTES
> arch/m68k/include/asm/cache.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES
> arch/microblaze/include/asm/page.h:#define ARCH_DMA_MINALIGN    L1_CACHE_BYTES
> arch/mips/include/asm/mach-generic/kmalloc.h:#define ARCH_DMA_MINALIGN  128
> arch/mips/include/asm/mach-ip32/kmalloc.h:#define ARCH_DMA_MINALIGN     32
> arch/mips/include/asm/mach-ip32/kmalloc.h:#define ARCH_DMA_MINALIGN     128
> arch/mips/include/asm/mach-tx49xx/kmalloc.h:#define ARCH_DMA_MINALIGN L1_CACHE_BYTES
> arch/nds32/include/asm/cache.h:#define ARCH_DMA_MINALIGN   L1_CACHE_BYTES
> arch/nios2/include/asm/cache.h:#define ARCH_DMA_MINALIGN        L1_CACHE_BYTES
> arch/parisc/include/asm/cache.h:#define ARCH_DMA_MINALIGN       L1_CACHE_BYTES
> arch/powerpc/include/asm/page_32.h:#define ARCH_DMA_MINALIGN    L1_CACHE_BYTES
> arch/sh/include/asm/page.h:#define ARCH_DMA_MINALIGN    L1_CACHE_BYTES
> arch/unicore32/include/asm/cache.h:#define ARCH_DMA_MINALIGN    L1_CACHE_BYTES
> arch/xtensa/include/asm/cache.h:#define ARCH_DMA_MINALIGN       L1_CACHE_BYTES
> 
> Hmmm, how does arch/x86 do it?

As I understand it, x86 tends to be fully coherent, so has no there
is not much requirement for DMA to be aligned to cachelines.

-- 
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