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From: David Laight <David.Laight@ACULAB.COM>
To: 'Christophe Leroy' <christophe.leroy@c-s.fr>,
	Geert Uytterhoeven <geert@linux-m68k.org>
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	"David S. Miller" <davem@davemloft.net>
Subject: RE: [RFT 00/13] iomap: Constify ioreadX() iomem argument
Date: Wed, 8 Jan 2020 17:39:42 +0000
Message-ID: <334434313e1642768995acc03a51214f@AcuMS.aculab.com> (raw)
In-Reply-To: <00a43e5c-0708-d49a-9cc4-eb2ce8b4cf99@c-s.fr>

From: Christophe Leroy
> Sent: 08 January 2020 08:49
...
> And as pointed by Arnd, the volatile is really only necessary for the
> dereference itself, should the arch use dereferencing.

I've had trouble with some versions of gcc and reading of 'volatile unsigned char *'.
It tended to follow the memory read with an extra mask with 0xff.
(I suspect that internally the value landed into a temporary 'int' variable.)

I got better code using memory barriers.
So putting an asm barrier for the exact location of the memory read
either side of the read should have the desired effect without adding
extra instructions.
(You might think 'volatile' would mean that - but it doesn't.)

	David

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Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-07 16:52 Krzysztof Kozlowski
2020-01-07 16:52 ` [RFT 01/13] iomap: Constify ioreadX() iomem argument (as in generic implementation) Krzysztof Kozlowski
2020-01-07 16:52 ` [RFT 02/13] alpha: " Krzysztof Kozlowski
2020-01-08  8:10   ` Geert Uytterhoeven
2020-01-08  9:07     ` Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 02/13] sh: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 03/13] alpha: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 03/13] sh: " Krzysztof Kozlowski
2020-01-07 17:08   ` Arnd Bergmann
2020-01-07 16:53 ` [RFT 04/13] parisc: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 05/13] powerpc: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 06/13] arc: " Krzysztof Kozlowski
2020-01-07 17:10   ` Arnd Bergmann
2020-01-07 16:53 ` [RFT 07/13] drm/mgag200: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 08/13] drm/nouveau: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 09/13] media: fsl-viu: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 10/13] net: wireless: ath5k: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 11/13] net: wireless: rtl818x: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 12/13] ntb: intel: " Krzysztof Kozlowski
2020-01-07 16:53 ` [RFT 13/13] virtio: pci: " Krzysztof Kozlowski
2020-01-08  8:07 ` [RFT 00/13] iomap: Constify ioreadX() iomem argument Geert Uytterhoeven
2020-01-08  8:12   ` Geert Uytterhoeven
2020-01-08  8:18     ` Krzysztof Kozlowski
2020-01-08  8:35       ` Christophe Leroy
2020-01-08  8:43         ` Geert Uytterhoeven
2020-01-08  8:48           ` Christophe Leroy
2020-01-08 17:39             ` David Laight [this message]
2020-01-08  8:44         ` Arnd Bergmann
2020-01-08  9:15           ` Krzysztof Kozlowski
2020-01-08 11:55             ` Arnd Bergmann
2020-01-08  8:15   ` Krzysztof Kozlowski

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