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linux4sam / at91bootstrap

Second level bootloader for Microchip SoC (aka AT91)
https://www.linux4sam.org/linux4sam/bin/view/Linux4SAM/AT91Bootstrap4
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Why no THUMB mode for SAMA5D3X? #152

Closed lnksz closed 1 year ago

lnksz commented 2 years ago

I came across this configuration, and don't understand, why THUMB is disabled for e.g. SAMA5D3X, even if its datasheet mentions support for the thumb instructions set.

https://github.com/linux4sam/at91bootstrap/blob/2eb1a1efc00fd46db52a3c41b00cf79eaf96fd77/Kconfig#L206-L210

Could you explain? Thank you!

ehristev commented 2 years ago

@noglitch @nirvann @junminlin Do you have any knowledge of why thumb is not available for these MPUs ? Thanks !

lnksz commented 2 years ago

FYI: I forced -mthumb manually into the CPPFLAGS variable, and the compiled binary was ridiculously smaller.

Without any other changes to our code-base.

lnksz commented 2 years ago

Also because of the default of our toolchain (linaro v6.5) being -mthumb we have been compiling and using the bootstrap for the SAMA5D35 board with the thumb instruction set (for at least 2-3 years now). I only noticed the change because of a toolchain update where the default may have been changed.

ehristev commented 2 years ago

@lnksz Since you discovered this and you tested it on your sama5d3 based board, I suggest you create a patch that removes the restriction for sama5d3, such that thumb can be enabled for these boards. Open a Pull Request and I will merge it.

lnksz commented 2 years ago

Gladly. What I could identify as a possible source for this is 177af40cd745eb5ddda83bb282fddb94d8cc770a from 2013 which was aiming to turn off -mthumb-interwork through this.

That flag is still added to the flags through this option: https://github.com/linux4sam/at91bootstrap/blob/97e5fe2193d87cd4a44b15d7ae28eea3b2416ac8/device/device_cpp.mk#L15-L17

So I will test whether with -mthumb-interwork our build still succeeds.

ehristev commented 1 year ago

@lnksz do you have any updates on this ? I would still think it would be useful to have it enabled on sama5d3 SoC

lnksz commented 1 year ago

Hi there @ehristev, thanks for the ping, I will take care of this until the end of this year.

lnksz commented 1 year ago

I verified it, and all still works for us with -mthumb-interwork as well. PR follows shortly.

ehristev commented 1 year ago

Thanks again for figuring this out. Applied your patch.

lnksz commented 1 year ago

Thanks for the patience ;)