From: Krzysztof Kozlowski <krzk@kernel•org>
To: Arnd Bergmann <arnd@arndb•de>
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Subject: Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
Date: Wed, 8 Jan 2020 10:15:49 +0100 [thread overview]
Message-ID: <20200108091549.GB10145@pi3> (raw)
In-Reply-To: <CAK8P3a21yPrmp4ik3Ei1BZfeqZNf0wL5NZNF3uXqb4FLRDyUPw@mail.gmail.com>
On Wed, Jan 08, 2020 at 09:44:36AM +0100, Arnd Bergmann wrote:
> On Wed, Jan 8, 2020 at 9:36 AM Christophe Leroy <christophe.leroy@c-s•fr> wrote:
> > Le 08/01/2020 à 09:18, Krzysztof Kozlowski a écrit :
> > > On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven <geert@linux-m68k•org> wrote:
> > > I'll add to this one also changes to ioreadX_rep() and add another
> > > patch for volatile for reads and writes. I guess your review will be
> > > appreciated once more because of ioreadX_rep()
> > >
> >
> > volatile should really only be used where deemed necessary:
> >
> > https://www.kernel.org/doc/html/latest/process/volatile-considered-harmful.html
> >
> > It is said: " ... accessor functions might use volatile on
> > architectures where direct I/O memory access does work. Essentially,
> > each accessor call becomes a little critical section on its own and
> > ensures that the access happens as expected by the programmer."
>
> The I/O accessors are one of the few places in which 'volatile' generally
> makes sense, at least for the implementations that do a plain pointer
> dereference (probably none of the ones in question here).
>
> In case of readl/writel, this is what we do in asm-generic:
>
> static inline u32 __raw_readl(const volatile void __iomem *addr)
> {
> return *(const volatile u32 __force *)addr;
> }
SuperH is another example:
1. ioread8_rep(void __iomem *addr, void *dst, unsigned long count)
calls mmio_insb()
2. static inline void mmio_insb(void __iomem *addr, u8 *dst, int count)
calls __raw_readb()
3. #define __raw_readb(a) (__chk_io_ptr(a), *(volatile u8 __force *)(a))
Even if interface was not marked as volatile, in fact its implementation
was casting to volatile.
> The __force-cast that removes the __iomem here also means that
> the 'volatile' keyword could be dropped from the argument list,
> as it has no real effect any more, but then there are a few drivers
> that mark their iomem pointers as either 'volatile void __iomem*' or
> (worse) 'volatile void *', so we keep it in the argument list to not
> add warnings for those drivers.
>
> It may be time to change these drivers to not use volatile for __iomem
> pointers, but that seems out of scope for what Krzysztof is trying
> to do. Ideally we would be consistent here though, either using volatile
> all the time or never.
Indeed. I guess there are no objections around const so let me send v2
for const only.
Best regards,
Krzysztof
next prev parent reply other threads:[~2020-01-08 9:17 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-07 16:52 [RFT 00/13] iomap: Constify ioreadX() iomem argument 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
2020-01-08 8:44 ` Arnd Bergmann
2020-01-08 9:15 ` Krzysztof Kozlowski [this message]
2020-01-08 11:55 ` Arnd Bergmann
2020-01-08 8:15 ` Krzysztof Kozlowski
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