Re: [GIT PULL v2 2/7] firmware: tegra: Changes for v6.2-rc1

["Arnd Bergmann" <arnd@arndb.de>]

On Wed, Nov 23, 2022, at 12:33, Thierry Reding wrote:
> On Tue, Nov 22, 2022 at 10:25:50PM +0100, Arnd Bergmann wrote:
>> On Mon, Nov 21, 2022, at 18:12, Thierry Reding wrote:
>> >       firmware: tegra: bpmp: Do not support big-endian
>> 
>> I pulled the branch, but I think this patch is inconsistent with
>> our normal approach: Since all ARMv7 and ARMv8 processors can
>> run with both big-endian and little-endian kernels, we normally
>> try to keep drivers portable between both ways, even though we
>> don't expect anyone to actually want a big-endian kernel any
>> more. Changing portable code to nonportable code doesn't seem
>> helpful here.
>
> The only reason I dropped this is because the driver is in itself
> inconsistent. Parts of it use byte-swapping for 32-bit values and other
> parts don't. I was originally going to fix big-endian support but it
> would've required changes to the BPMP ABI header to avoid sparse
> warnings in lots of places, then these ABI changes would've needed to
> trickle up to the canonical source, etc. All of that didn't seem worth
> the effort if we couldn't even test this in any way. So the easiest fix
> was to stop pretending and drop the partial support.

Right

>> On the other hand, there are already examples of important
>> drivers that are fundamentally incompatible with big-endian
>> mode, notably drivers/efi/, which is required on a lot of
>> machines.
>> 
>> You don't have to revert this patch, but it would be helpful
>> to mark code that is explicitly unportable with a 'depends
>> on !CPU_BIG_ENDIAN' line in Kconfig. If you agree, I can
>> add that.
>
> Yes, feel free to add that.

Added this commit to the soc/drivers branch now:

commit 4ddb1bf1a83783cebdb174b0efaf62f63ad64e0b
Author: Arnd Bergmann <arnd@arndb.de>
Date:   Wed Nov 23 14:21:16 2022 +0100

    tegra: mark BPMP driver as little-endian only
    
    The BPMP firmware driver never worked on big-endian kernels, and
    cannot easily be made portable. Add a dependency to make this clear
    in case anyone ever wants to try a big-endian kernel on this hardware.
    
    Link: https://lore.kernel.org/linux-arm-kernel/Y34FCQ3xTmcjqKRT@orome/
    Signed-off-by: Arnd Bergmann <arnd@arndb.de>

diff --git a/drivers/firmware/tegra/Kconfig b/drivers/firmware/tegra/Kconfig
index 1c8ba1f47c7c..cde1ab8bd9d1 100644
--- a/drivers/firmware/tegra/Kconfig
+++ b/drivers/firmware/tegra/Kconfig
@@ -14,6 +14,7 @@ config TEGRA_IVC
 config TEGRA_BPMP
 	bool "Tegra BPMP driver"
 	depends on ARCH_TEGRA && TEGRA_HSP_MBOX && TEGRA_IVC
+	depends on !CPU_BIG_ENDIAN
 	help
 	  BPMP (Boot and Power Management Processor) is designed to off-loading
 	  the PM functions which include clock/DVFS/thermal/power from the CPU.



>> Do you know of other tegra drivers that only work on
>> little-endian?
>
> I'm not aware of any that explicitly wouldn't work with big endian, but
> it's not something we've ever tested. I know that people have in the
> past done experiments with running emulated Tegra on QEMU in big endian
> mode, but it's probably not something that's very common.

Ok, thanks!

     Arnd

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel