Re: [PATCH v3 5/6] fbdev: Move framebuffer I/O helpers into <asm/fb.h>

From: Thomas Zimmermann <tzimmermann@suse.de>
To: Arnd Bergmann <arnd@arndb.de>, Helge Deller <deller@gmx.de>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Javier Martinez Canillas <javierm@redhat.com>,
	Daniel Vetter <daniel@ffwll.ch>, Vineet Gupta <vgupta@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	"David S . Miller" <davem@davemloft.net>,
	"James E . J . Bottomley" <James.Bottomley@HansenPartnership.com>,
	Sam Ravnborg <sam@ravnborg.org>
Cc: linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
	Linux-Arch <linux-arch@vger.kernel.org>,
	linux-snps-arc@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-ia64@vger.kernel.org, loongarch@lists.linux.dev,
	linux-m68k@lists.linux-m68k.org, sparclinux@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-parisc@vger.kernel.org
Subject: Re: [PATCH v3 5/6] fbdev: Move framebuffer I/O helpers into <asm/fb.h>
Date: Wed, 3 May 2023 16:55:05 +0200	[thread overview]
Message-ID: <dd921bae-0145-09e2-24b1-f08d89a78eba@suse.de> (raw)
In-Reply-To: <67d6a188-041f-4604-99a3-548c41af0693@app.fastmail.com>

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Hi

Am 02.05.23 um 22:06 schrieb Arnd Bergmann:
> On Tue, May 2, 2023, at 15:02, Thomas Zimmermann wrote:
>> Implement framebuffer I/O helpers, such as fb_read*() and fb_write*(),
>> in the architecture's <asm/fb.h> header file or the generic one.
>>
>> The common case has been the use of regular I/O functions, such as
>> __raw_readb() or memset_io(). A few architectures used plain system-
>> memory reads and writes. Sparc used helpers for its SBus.
>>
>> The architectures that used special cases provide the same code in
>> their __raw_*() I/O helpers. So the patch replaces this code with the
>> __raw_*() functions and moves it to <asm-generic/fb.h> for all
>> architectures.
>>
>> v3:
>> 	* implement all architectures with generic helpers
>> 	* support reordering and native byte order (Geert, Arnd)
> 
> This looks good for the read/write helpers, but I'm a little
> worried about the memset and memcpy functions, since they do
> change behavior on some architectures:
> 
> - on sparc64, fb_mem{set,cpy} uses ASI_PHYS_BYPASS_EC_E (like __raw_readb)
>    while mem{set_,cpy_from,cpy_to} uses ASI_PHYS_BYPASS_EC_E_L (like readb)
>    I don't know the effect of that, but it seems intentional
> 
> - on loongarch and csky, the _io variants avoid unaligned access,
>    while the normal memcpy/memset is probably broken, so your
>    patch is a bugfix
> 
> - on ia64, the _io variants use bytewise access and avoid any longer
>    loads and stores, so your patch probably makes things slower.
> 
> It's probably safe to deal with all the above by either adding
> architecture specific overrides to the current version, or
> by doing the semantic changes before the move to asm/fb.h, but
> one way or the other I'd prefer this to be separate from the
> consolidation patch that should not have any changes in behavior.

I think I'll add architecture overrides that contain the current code, 
even if they contain some force-casting wrt __iomem. If anyone wants to 
fix the issues, they can then address them easily.

Best regards
Thomas

> 
>       Arnd

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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