linux-kernel - Re: [PATCH 3/4] MIPS: Reinstate platform `__div64

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Date:   Thu, 22 Apr 2021 11:36:34 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Huacai Chen <chenhuacai@...nel.org>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-arch@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 3/4] MIPS: Reinstate platform `__div64_32' handler

On Tue, Apr 20, 2021 at 04:50:40AM +0200, Maciej W. Rozycki wrote:
> Our current MIPS platform `__div64_32' handler is inactive, because it 
> is incorrectly only enabled for 64-bit configurations, for which generic 
> `do_div' code does not call it anyway.
> 
> The handler is not suitable for being called from there though as it 
> only calculates 32 bits of the quotient under the assumption the 64-bit 
> divident has been suitably reduced.  Code for such reduction used to be 
> there, however it has been incorrectly removed with commit c21004cd5b4c 
> ("MIPS: Rewrite <asm/div64.h> to work with gcc 4.4.0."), which should 
> have only updated an obsoleted constraint for an inline asm involving 
> $hi and $lo register outputs, while possibly wiring the original MIPS 
> variant of the `do_div' macro as `__div64_32' handler for the generic 
> `do_div' implementation
> 
> Correct the handler as follows then:
> 
> - Revert most of the commit referred, however retaining the current 
>   formatting, except for the final two instructions of the inline asm 
>   sequence, which the original commit missed.  Omit the original 64-bit 
>   parts though.
> 
> - Rename the original `do_div' macro to `__div64_32'.  Use the combined 
>   `x' constraint referring to the MD accumulator as a whole, replacing 
>   the original individual `h' and `l' constraints used for $hi and $lo 
>   registers respectively, of which `h' has been obsoleted with GCC 4.4. 
>   Update surrounding code accordingly.
> 
>   We have since removed support for GCC versions before 4.9, so no need 
>   for a special arrangement here; GCC has supported the `x' constraint 
>   since forever anyway, or at least going back to 1991.
> 
> - Rename the `__base' local variable in `__div64_32' to `__radix' to 
>   avoid a conflict with a local variable in `do_div'.
> 
> - Actually enable this code for 32-bit rather than 64-bit configurations
>   by qualifying it with BITS_PER_LONG being 32 instead of 64.  Include 
>   <asm/bitsperlong.h> for this macro rather than <linux/types.h> as we 
>   don't need anything else.
> 
> - Finally include <asm-generic/div64.h> last rather than first.
> 
> This has passed correctness verification with test_div64 and reduced the 
> module's average execution time down to 1.0668s and 0.2629s from 2.1529s 
> and 0.5647s respectively for an R3400 CPU @40MHz and a 5Kc CPU @160MHz.  
> For a reference 64-bit `do_div' code where we have the DDIVU instruction 
> available to do the whole calculation right away averages at 0.0660s for 
> the latter CPU.
> 
This patch results in:

arch/mips/mti-malta/malta-time.c: In function 'plat_time_init':
./arch/mips/include/asm/div64.h:76:3: error: inconsistent operand constraints in an 'asm'

and similar errors when trying to compile malta_qemu_32r6_defconfig.
I tried with gcc 8.3.0, 8.4.0, 9.3.0, and 10.3.0.

Does this need some additional new compile flags ?

Thanks,
Guenter