On Sunday 22 November 2015 00:14:14 Arnd Bergmann wrote:

On Saturday 21 November 2015 22:11:36 M?ns Rullg?rd wrote:

quoted

Arnd Bergmann [off-list ref] writes:

quoted

On Saturday 21 November 2015 20:45:38 M?ns Rullg?rd wrote:

quoted

On 21 November 2015 20:39:58 GMT+00:00, Arnd Bergmann [off-list ref] wrote:

The ARM ARM says anything with virt has idiv, lpae doesn't matter.

Ok, and anything with virt also has lpae by definition. The question is
whether we care about using idiv on cores that do not have lpae, or that
have neither lpae nor virt.

The question is, are there any such cores?  GCC doesn't know of any, but
then it's missing most non-ARM designs.

Exactly. Stephen should be able to find out about the Qualcomm cores,
and http://comments.gmane.org/gmane.linux.ports.arm.kernel/426289 has
some information about the others:
 * Brahma-B15 supports all three.
 * Dove (PJ4) reports idiv only in thumb mode, which I'm tempted to ignore
   for the kernel, as it supports neither lpae nor idiva.
 * Armada 370/XP (PJ4B) reports support for idiva and idivt, but according to
   https://groups.google.com/a/dartlang.org/forum/#!topic/reviews/9wvsJvq0YYY
   that may be a lie.
 * According to the same source, Krait fails to report idiva and idivt,
   but supports both anyway. However, I found reports on the web where
   /proc/cpuinfo correctly contains the flags on the same SoC (APQ8064)
   that was mentioned there, so maybe they were just running an old
   kernel.

This has some more information: 

commit 120ecfafabec382c4feb79ff159ef42a39b6d33b
Author: Stepan Moskovchenko [off-list ref]
Date:   Mon Mar 18 19:44:16 2013 +0100

    ARM: 7678/1: Work around faulty ISAR0 register in some Krait CPUs
    
    Some early versions of the Krait CPU design incorrectly indicate
    that they only support the UDIV and SDIV instructions in Thumb
    mode when they actually support them in ARM and Thumb mode. It
    seems that these CPUs follow the DDI0406B ARM ARM which has two
    possible values for the divide instructions field, instead of the
    DDI0406C document which has three possible values.
    
    Work around this problem by checking the MIDR against Krait CPUs
    with this faulty ISAR0 register and force the hwcaps to indicate
    support in both modes.
    
    [sboyd: Rewrote commit text to reflect real reasoning now that
        we autodetect udiv/sdiv]
    
    Signed-off-by: Stepan Moskovchenko [off-list ref]
    Acked-by: Will Deacon [off-list ref]
    Signed-off-by: Stephen Boyd [off-list ref]
    Signed-off-by: Russell King [off-list ref]

so Krait clearly supports them, and this also explains why some
machines misreport it depending on the CPU version and kernel
release running on it.

Regarding PJ4, it's still unclear whether that has the same
problem and it only reports idivt when it actually supports idiva,
or whether the lack of idiva support on PJ4 is instead the reason
why the ARM ARM was updated to have separate flags.

	Arnd