linux-kernel - Re: [PATCH 02/14] ARM: ARMv7M: Enlarge vector table to 256 entries

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Date:	Fri, 13 Feb 2015 11:00:50 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Maxime Coquelin <mcoquelin.stm32@...il.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jonathan Corbet <corbet@....net>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Russell King <linux@....linux.org.uk>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Walleij <linus.walleij@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Joe Perches <joe@...ches.com>, Antti Palosaari <crope@....fi>,
	Tejun Heo <tj@...nel.org>, Will Deacon <will.deacon@....com>,
	Nikolay Borisov <Nikolay.Borisov@....com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Kees Cook <keescook@...omium.org>,
	Michal Marek <mmarek@...e.cz>,
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	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
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Subject: Re: [PATCH 02/14] ARM: ARMv7M: Enlarge vector table to 256 entries

On Fri, Feb 13, 2015 at 09:42:46AM +0100, Maxime Coquelin wrote:
> Hi Geert,
> 
> 2015-02-12 21:34 GMT+01:00 Geert Uytterhoeven <geert@...ux-m68k.org>:
> > On Thu, Feb 12, 2015 at 6:45 PM, Maxime Coquelin
> > <mcoquelin.stm32@...il.com> wrote:
> >> From Cortex-M4 and M7 reference manuals, the nvic supports up to 240
> >> interrupts. So the number of entries in vectors table is 256.
> >>
> >> This patch adds the missing entries, and change the alignement, so that
> >> vector_table remains naturally aligned.
> >
> > Shouldn't this depend on ARCH_STM32, or some other M4 or M7 specific
> > Kconfig option, to avoid wasting the space on other CPUs?
> 
> Actually, the STM32F429 has 90 interrupts, so it would need 106
> entries in the vector table.
> The maximum of supported interrupts is not only for Cortex-M4 and M7,
> this is also true for Cortex-M3.
> 
> I see two possibilities:
>  1 - We declare the vector table for the maximum supported number of
> IRQs, as this patch does.
>         - Pro: it will be functionnal with all Cortex-M MCUs
>         - Con: Waste of less than 1KB for memory
>  2 - We introduce a config flag that provides the number of interrupts
>         - Pro: No more memory waste
>         - Con: Need to declare a per MCU model config flag.
I'd vote for 2, something like:

	config CPUV7M_NUM_IRQ
		int
		default 90 if STM32F429
		default 38 if EFM32GG
		default 240

then there is a working default and platforms being short on memory can
configure as appropriate. (The only down side is that if we create
multi-platfrom images at some time in the future either all or none of
the supported platforms must provide a value here.)
 
> Then, regarding the natural alignment, is there a way to ensure it
> depending on the value of a config flag?
The exact wording in ARMARMv7-M is:

	The Vector table must be naturally aligned to a power of two
	whose alignment value is greater than or equal
	to (Number of Exceptions supported x 4), with a minimum
	alignment of 128 bytes.

> Or we should keep it at the maximum value possible?
So we need:

	.align x

with x being max(7, ceil(log((CPUV7M_NUM_IRQ + 16) * 4, 2))). So the
alignment needed is between 7 and 10.

If the assembler supports an expression here I'd use that. But before
adding strange hacks to generate the right value there better go for a
static value like:

	/* The vector table must be naturally aligned */
	#if CONFIG_CPUV7M_NUM_IRQ <= 112
	.align 9 /* log2((112 + 16) * 4) */
	#else
	.align 10
	#endif

Further steps would be:

	CONFIG_CPUV7M_NUM_IRQ <= 48 -> .align 8
	CONFIG_CPUV7M_NUM_IRQ <= 16 -> .align 7

Probably it's not worth to add the respective #ifdefs here.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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