RE: [RFT 00/13] iomap: Constify ioreadX() iomem argument
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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales) ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
On Wed, Jan 8, 2020 at 10:15 AM Krzysztof Kozlowski wrote: > > 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. Ok, sounds good. Maybe mention in the changelog then that the 'volatile' in the interface is intentionally left out, and that only users of readl/writel still have it to deal with existing drivers. Arnd ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
On Wed, Jan 08, 2020 at 09:44:36AM +0100, Arnd Bergmann wrote:
> On Wed, Jan 8, 2020 at 9:36 AM Christophe Leroy
> wrote:
> > Le 08/01/2020 à 09:18, Krzysztof Kozlowski a écrit :
> > > On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven
> > > 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
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Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
Le 08/01/2020 à 09:18, Krzysztof Kozlowski a écrit : On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven wrote: Hi Krzysztof, On Wed, Jan 8, 2020 at 9:07 AM Geert Uytterhoeven wrote: On Tue, Jan 7, 2020 at 5:53 PM Krzysztof Kozlowski wrote: The ioread8/16/32() and others have inconsistent interface among the architectures: some taking address as const, some not. It seems there is nothing really stopping all of them to take pointer to const. Shouldn't all of them take const volatile __iomem pointers? It seems the "volatile" is missing from all but the implementations in include/asm-generic/io.h. As my "volatile" comment applies to iowrite*(), too, probably that should be done in a separate patch. Hence with patches 1-5 squashed, and for patches 11-13: Reviewed-by: Geert Uytterhoeven 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." Christophe ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
Hi Geert, Le 08/01/2020 à 09:43, Geert Uytterhoeven a écrit : Hi Christophe, On Wed, Jan 8, 2020 at 9:35 AM Christophe Leroy wrote: Le 08/01/2020 à 09:18, Krzysztof Kozlowski a écrit : On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven wrote: On Wed, Jan 8, 2020 at 9:07 AM Geert Uytterhoeven wrote: On Tue, Jan 7, 2020 at 5:53 PM Krzysztof Kozlowski wrote: The ioread8/16/32() and others have inconsistent interface among the architectures: some taking address as const, some not. It seems there is nothing really stopping all of them to take pointer to const. Shouldn't all of them take const volatile __iomem pointers? It seems the "volatile" is missing from all but the implementations in include/asm-generic/io.h. As my "volatile" comment applies to iowrite*(), too, probably that should be done in a separate patch. Hence with patches 1-5 squashed, and for patches 11-13: Reviewed-by: Geert Uytterhoeven 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." That is exactly the use case here: all above are accessor functions. Why would ioreadX() not need volatile, while readY() does? My point was: it might be necessary for some arches and not for others. And as pointed by Arnd, the volatile is really only necessary for the dereference itself, should the arch use dereferencing. So I guess the best would be to go in the other direction: remove volatile keyword wherever possible instead of adding it where it is not needed. Christophe ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
On Wed, Jan 8, 2020 at 9:36 AM Christophe Leroy wrote:
> Le 08/01/2020 à 09:18, Krzysztof Kozlowski a écrit :
> > On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven
> > 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;
}
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.
Arnd
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Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
Hi Christophe,
On Wed, Jan 8, 2020 at 9:35 AM Christophe Leroy wrote:
> Le 08/01/2020 à 09:18, Krzysztof Kozlowski a écrit :
> > On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven
> > wrote:
> >> On Wed, Jan 8, 2020 at 9:07 AM Geert Uytterhoeven
> >> wrote:
> >>> On Tue, Jan 7, 2020 at 5:53 PM Krzysztof Kozlowski
> >>> wrote:
> The ioread8/16/32() and others have inconsistent interface among the
> architectures: some taking address as const, some not.
>
> It seems there is nothing really stopping all of them to take
> pointer to const.
> >>>
> >>> Shouldn't all of them take const volatile __iomem pointers?
> >>> It seems the "volatile" is missing from all but the implementations in
> >>> include/asm-generic/io.h.
> >>
> >> As my "volatile" comment applies to iowrite*(), too, probably that should
> >> be
> >> done in a separate patch.
> >>
> >> Hence with patches 1-5 squashed, and for patches 11-13:
> >> Reviewed-by: Geert Uytterhoeven
> >
> > 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."
That is exactly the use case here: all above are accessor functions.
Why would ioreadX() not need volatile, while readY() does?
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-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
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Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
On Wed, 8 Jan 2020 at 09:13, Geert Uytterhoeven wrote: > > Hi Krzysztof, > > On Wed, Jan 8, 2020 at 9:07 AM Geert Uytterhoeven > wrote: > > On Tue, Jan 7, 2020 at 5:53 PM Krzysztof Kozlowski wrote: > > > The ioread8/16/32() and others have inconsistent interface among the > > > architectures: some taking address as const, some not. > > > > > > It seems there is nothing really stopping all of them to take > > > pointer to const. > > > > Shouldn't all of them take const volatile __iomem pointers? > > It seems the "volatile" is missing from all but the implementations in > > include/asm-generic/io.h. > > As my "volatile" comment applies to iowrite*(), too, probably that should be > done in a separate patch. > > Hence with patches 1-5 squashed, and for patches 11-13: > Reviewed-by: Geert Uytterhoeven 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() Thanks, Krzysztof ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
On Wed, 8 Jan 2020 at 09:08, Geert Uytterhoeven wrote: > > Hi Krzysztof, > > On Tue, Jan 7, 2020 at 5:53 PM Krzysztof Kozlowski wrote: > > The ioread8/16/32() and others have inconsistent interface among the > > architectures: some taking address as const, some not. > > > > It seems there is nothing really stopping all of them to take > > pointer to const. > > Shouldn't all of them take const volatile __iomem pointers? > It seems the "volatile" is missing from all but the implementations in > include/asm-generic/io.h. It's kind of separate issue although I could squash it to limit redundant changes. > > Patchset was really tested on all affected architectures. I just spot an error in my first message. I wanted to say: "Patchset was NOT really tested on all affected architectures." Obviously. > > Build testing is in progress - I hope auto-builders will point any issues. > > > > > > Todo > > > > Convert also string versions (ioread16_rep() etc) if this aproach looks OK. > > > > > > Merging > > === > > The first 5 patches - iomap, alpha, sh, parisc and powerpc - should > > probably go > > via one tree, or even squashed into one. > > Yes, they should be squashed, cfr. Arnd's comment. > I also wouldn't bother doing the updates in patches 6-10. Indeed, thanks for comments. Best regards, Krzysztof ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
Hi Krzysztof,
On Wed, Jan 8, 2020 at 9:07 AM Geert Uytterhoeven wrote:
> On Tue, Jan 7, 2020 at 5:53 PM Krzysztof Kozlowski wrote:
> > The ioread8/16/32() and others have inconsistent interface among the
> > architectures: some taking address as const, some not.
> >
> > It seems there is nothing really stopping all of them to take
> > pointer to const.
>
> Shouldn't all of them take const volatile __iomem pointers?
> It seems the "volatile" is missing from all but the implementations in
> include/asm-generic/io.h.
As my "volatile" comment applies to iowrite*(), too, probably that should be
done in a separate patch.
Hence with patches 1-5 squashed, and for patches 11-13:
Reviewed-by: Geert Uytterhoeven
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-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
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Re: [RFT 00/13] iomap: Constify ioreadX() iomem argument
Hi Krzysztof,
On Tue, Jan 7, 2020 at 5:53 PM Krzysztof Kozlowski wrote:
> The ioread8/16/32() and others have inconsistent interface among the
> architectures: some taking address as const, some not.
>
> It seems there is nothing really stopping all of them to take
> pointer to const.
Shouldn't all of them take const volatile __iomem pointers?
It seems the "volatile" is missing from all but the implementations in
include/asm-generic/io.h.
> Patchset was really tested on all affected architectures.
> Build testing is in progress - I hope auto-builders will point any issues.
>
>
> Todo
>
> Convert also string versions (ioread16_rep() etc) if this aproach looks OK.
>
>
> Merging
> ===
> The first 5 patches - iomap, alpha, sh, parisc and powerpc - should probably
> go
> via one tree, or even squashed into one.
Yes, they should be squashed, cfr. Arnd's comment.
I also wouldn't bother doing the updates in patches 6-10.
The rest looks good to me.
Thanks a lot!
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-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
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[RFT 00/13] iomap: Constify ioreadX() iomem argument
Hi, The ioread8/16/32() and others have inconsistent interface among the architectures: some taking address as const, some not. It seems there is nothing really stopping all of them to take pointer to const. Patchset was really tested on all affected architectures. Build testing is in progress - I hope auto-builders will point any issues. Todo Convert also string versions (ioread16_rep() etc) if this aproach looks OK. Merging === The first 5 patches - iomap, alpha, sh, parisc and powerpc - should probably go via one tree, or even squashed into one. All other can go separately after these get merged. Best regards, Krzysztof Krzysztof Kozlowski (13): iomap: Constify ioreadX() iomem argument (as in generic implementation) alpha: Constify ioreadX() iomem argument (as in generic implementation) sh: Constify ioreadX() iomem argument (as in generic implementation) parisc: Constify ioreadX() iomem argument (as in generic implementation) powerpc: Constify ioreadX() iomem argument (as in generic implementation) arc: Constify ioreadX() iomem argument (as in generic implementation) drm/mgag200: Constify ioreadX() iomem argument (as in generic implementation) drm/nouveau: Constify ioreadX() iomem argument (as in generic implementation) media: fsl-viu: Constify ioreadX() iomem argument (as in generic implementation) net: wireless: ath5k: Constify ioreadX() iomem argument (as in generic implementation) net: wireless: rtl818x: Constify ioreadX() iomem argument (as in generic implementation) ntb: intel: Constify ioreadX() iomem argument (as in generic implementation) virtio: pci: Constify ioreadX() iomem argument (as in generic implementation) arch/alpha/include/asm/core_apecs.h| 6 +-- arch/alpha/include/asm/core_cia.h | 6 +-- arch/alpha/include/asm/core_lca.h | 6 +-- arch/alpha/include/asm/core_marvel.h | 4 +- arch/alpha/include/asm/core_mcpcia.h | 6 +-- arch/alpha/include/asm/core_t2.h | 2 +- arch/alpha/include/asm/io.h| 12 +++--- arch/alpha/include/asm/io_trivial.h| 16 arch/alpha/include/asm/jensen.h| 2 +- arch/alpha/include/asm/machvec.h | 6 +-- arch/alpha/kernel/core_marvel.c| 2 +- arch/alpha/kernel/io.c | 6 +-- arch/arc/plat-axs10x/axs10x.c | 4 +- arch/parisc/include/asm/io.h | 4 +- arch/parisc/lib/iomap.c| 48 +++--- arch/powerpc/kernel/iomap.c| 22 +- arch/sh/kernel/iomap.c | 10 ++--- drivers/gpu/drm/mgag200/mgag200_drv.h | 4 +- drivers/gpu/drm/nouveau/nouveau_bo.c | 2 +- drivers/media/platform/fsl-viu.c | 2 +- drivers/net/wireless/ath/ath5k/ahb.c | 10 ++--- .../net/wireless/realtek/rtl818x/rtl8180/rtl8180.h | 6 +-- drivers/ntb/hw/intel/ntb_hw_gen1.c | 2 +- drivers/ntb/hw/intel/ntb_hw_gen3.h | 2 +- drivers/ntb/hw/intel/ntb_hw_intel.h| 2 +- drivers/virtio/virtio_pci_modern.c | 6 +-- include/asm-generic/iomap.h| 22 +- include/linux/io-64-nonatomic-hi-lo.h | 4 +- include/linux/io-64-nonatomic-lo-hi.h | 4 +- lib/iomap.c| 18 30 files changed, 123 insertions(+), 123 deletions(-) -- 2.7.4 ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
