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

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Message-ID: <c715bcf9716ebefa8272146ce48f674c56a04c52.camel@physik.fu-berlin.de>
Date: Thu, 30 Jan 2025 10:52:59 +0100
From: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
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 Geert,

On Thu, 2025-01-30 at 10:35 +0100, Geert Uytterhoeven wrote:
> 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.

Could you review the patch as well, so I have some peace of mind? ;-)

Thanks,
Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
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