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

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Date:	Thu, 19 Feb 2015 17:13:32 +0100
From:	Maxime Coquelin <mcoquelin.stm32@...il.com>
To:	Rob Herring <robherring2@...il.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jonathan Corbet <corbet@....net>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
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	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH 02/14] ARM: ARMv7M: Enlarge vector table to 256 entries

Hi Rob,

2015-02-15 23:42 GMT+01:00 Rob Herring <robherring2@...il.com>:
> On Fri, Feb 13, 2015 at 2:42 AM, Maxime Coquelin
> <mcoquelin.stm32@...il.com> 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
>
> The waste depends on the alignment size as well and could be up to
> almost 2KB worst case. It varies depending on the padding. We should
> try to place it so it always aligned and the wasted space is
> minimized.

Sorry, I just notice I didn't replied to all. That was my question:

Do you mean by forcing its location in the arch/arm/kernel/vmlinux.lds.S file?

Regards,
Maxime

>
> Rob
>
>>  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.
>>
>> Then, regarding the natural alignment, is there a way to ensure it
>> depending on the value of a config flag?
>> Or we should keep it at the maximum value possible?
>>
>> Any feedback will be appreciated, especially from Uwe who maintains
>> the efm32 machine.
>>
>> Kind regards,
>> Maxime
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