linux-kernel - Re: [PATCH v2] sh: Remove IO memcpy and memset from sh code

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Message-ID: <CAMuHMdXfiqM2Weoeq-PRw-p9wqMrBwMBcSWjFF8f_uvmVrRS7A@mail.gmail.com>
Date: Thu, 30 Jan 2025 10:35:13 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc: Julian Vetter <julian@...er-limits.org>, Arnd Bergmann <arnd@...db.de>, 
	Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker <dalias@...c.org>, linux-sh@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sh: Remove IO memcpy and memset from sh code

Hi Adrian,

On Thu, 30 Jan 2025 at 10:31, John Paul Adrian Glaubitz
<glaubitz@...sik.fu-berlin.de> wrote:
> On Thu, 2025-01-30 at 10:13 +0100, Geert Uytterhoeven wrote:
> > On Thu, 30 Jan 2025 at 09:44, John Paul Adrian Glaubitz
> > <glaubitz@...sik.fu-berlin.de> wrote:
> > > On Tue, 2025-01-28 at 11:13 +0100, Julian Vetter wrote:
> > > > Remove IO memcpy and memset from sh specific code and fall back to the
> > > > new implementations from lib/iomem_copy.c. They use word accesses if the
> > > > buffers are aligned and only fall back to byte accesses for potentially
> > > > unaligned parts of a buffer.
> > > >
> > > > Signed-off-by: Julian Vetter <julian@...er-limits.org>
> > > > ---
> > > > Changes for V2:
> > > > - Removed also SH4 specific memcpy_fromio code
> >
> > > I'm not sure that I understand the motivation to remove hand-optimized sh4 assembler
> > > code for memset and drop it in favor of potentially slower generic C code. What is
> > > the reasoning behind this?
> >
> > See Arnd's feedback on v1
> > https://lore.kernel.org/all/ffe019a1-11b4-4ad7-bbe2-8ef3e01ffeb0@app.fastmail.com
>
> I read Arnd's feedback but I don't really know whether GCC produces better code than
> this hand-written assembly. Is there any compelling argument?
>
> I'm just worried we would slow down something as fundamental as memset().

it's not memset(), but memset_io(), i.e. clearing (slow) mapped I/O memory.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds