From: khalasa@piap•pl (Krzysztof Hałasa)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH 3/7] usb: gadget: pxa25x_udc: use readl/writel for mmio
Date: Tue, 16 Feb 2016 10:26:14 +0100 [thread overview]
Message-ID: <m3d1rxqa4p.fsf@t19.piap.pl> (raw)
In-Reply-To: <4239208.KBB6rfivoa@wuerfel> (Arnd Bergmann's message of "Mon, 15 Feb 2016 17:12:54 +0100")
Arnd Bergmann <arnd@arndb•de> writes:
> The barriers on a spinlock synchronize between CPUs but not an external
> bus, so (on some architectures) a spinlock protecting an MMIO register
> does not guarantee that two CPUs doing
>
> spin_lock();
> __raw_writel(address);
> __raw_writel(data);
> spin_unlock();
>
> would cause pairs of address/data to be seen on the bus.
>
> Of course this is meaningless on ixp4xx, as there is only one CPU.
I still don't get it. If the spinlocks synchronize between CPUs, there
can only be one CPU (or core) doing the pair of raw_writel(), so how
would it be possible to not get the address/data pair written out?
IOW, how is it different from a system with a single CPU?
> On powerpc, we have in_le32/in_be32 for SoC-internal register access,
> while only PCI devices are allowed to be accessed using readl().
Yeah, this seems like a sane solution.
> I would suggest using an ixp4xx specific set of accessors that comes down
> to either readl() or ioread32_be(), depending on whether CONFIG_CPU_BIG_ENDIAN
> is set. That makes it clear that there is a magic bus involved and that it
> works on this platform but not in portable code.
Hmm. This is actually the opposite - while there may be some magic
(swapping) in readl() and friends, there is absolutely no magic in the
__raw_readl() etc. They are essentially equivalent to
*(volatile u32 *)ptr. This is constant and doesn't depend on endianess,
PCI, anything.
--
Krzysztof Halasa
Industrial Research Institute for Automation and Measurements PIAP
Al. Jerozolimskie 202, 02-486 Warsaw, Poland
next prev parent reply other threads:[~2016-02-16 9:26 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-28 16:15 [PATCH 0/7] USB changes for rare warnings Arnd Bergmann
2016-01-28 16:17 ` [PATCH 1/7] usb: gadget: pxa25x_udc: move register definitions from arch Arnd Bergmann
2016-01-28 16:17 ` [PATCH 2/7] usb: gadget: pxa25x_udc cleanup Arnd Bergmann
2016-01-29 10:13 ` Robert Jarzmik
2016-01-28 16:17 ` [PATCH 3/7] usb: gadget: pxa25x_udc: use readl/writel for mmio Arnd Bergmann
2016-01-29 10:17 ` Robert Jarzmik
2016-01-29 16:18 ` Krzysztof Hałasa
2016-01-29 17:06 ` Arnd Bergmann
2016-02-15 7:33 ` Krzysztof Hałasa
2016-02-15 9:33 ` Arnd Bergmann
2016-02-15 13:51 ` Krzysztof Hałasa
2016-02-15 16:12 ` Arnd Bergmann
2016-02-16 9:26 ` Krzysztof Hałasa [this message]
2016-02-16 11:26 ` Arnd Bergmann
2016-02-16 13:24 ` Krzysztof Hałasa
2016-02-16 13:55 ` Arnd Bergmann
2016-02-17 8:36 ` Krzysztof Hałasa
2016-02-17 10:36 ` Arnd Bergmann
2016-02-17 16:14 ` Krzysztof Hałasa
2016-02-20 20:54 ` Robert Jarzmik
2016-01-29 18:03 ` Sergei Shtylyov
2016-01-29 21:02 ` Arnd Bergmann
2016-01-29 9:32 ` [PATCH 1/7] usb: gadget: pxa25x_udc: move register definitions from arch Robert Jarzmik
2016-01-29 10:07 ` Arnd Bergmann
2016-01-29 15:26 ` Robert Jarzmik
2016-01-29 15:55 ` Krzysztof Hałasa
2016-02-17 15:08 ` Felipe Balbi
2016-02-17 16:00 ` Arnd Bergmann
2016-01-28 16:20 ` [PATCH 4/7] usb: fsl: drop USB_FSL_MPH_DR_OF Kconfig symbol Arnd Bergmann
2016-01-28 16:23 ` [PATCH 5/7] usb: isp1301-omap: mark power_up as __maybe_unused Arnd Bergmann
2016-01-28 16:23 ` [PATCH 6/7] usb: musb: use %pad format string from dma_addr_t Arnd Bergmann
2016-01-28 17:50 ` Tony Lindgren
2016-01-28 16:23 ` [PATCH 7/7] usb: musb/ux500: remove duplicate check for dma_is_compatible Arnd Bergmann
2016-02-03 18:12 ` [PATCH 0/7] USB changes for rare warnings Felipe Balbi
2016-02-03 19:15 ` Robert Jarzmik
2016-02-03 20:49 ` Arnd Bergmann
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