Message-ID: <20191213001628.GF101194@dtor-ws>
Date:   Thu, 12 Dec 2019 16:16:28 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Marc Gonzalez <marc.w.gonzalez@...e.fr>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-clk <linux-clk@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        x86 <x86@...nel.org>
Subject: Re: [PATCH v1] clk: Convert managed get functions to devm_add_action
 API

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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