linux-kernel - Re: RFC: arch: shall we have generic readl_be()/writel_be()/... or in_be32()/out

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Date:   Wed, 9 Dec 2020 15:20:24 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Enrico Weigelt, metux IT consult" <info@...ux.net>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: RFC: arch: shall we have generic readl_be()/writel_be()/... or
 in_be32()/out_be32() ?

On Wed, Dec 09, 2020 at 12:08:51PM +0100, Enrico Weigelt, metux IT consult wrote:
> Hello folks,
> 
> 
> while trying to make some more drivers compile-test'able, i've
> discovered some arch specific calls in here, eg.:
> 
> 
> In file included from
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci-spear.c:23:
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci.h:
> In function 'ehci_readl':
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci.h:743:3:
> error: implicit declaration of function 'readl_be'; did you mean
> 'readsb'? [-Werror=implicit-function-declaration]
>   743 |   readl_be(regs) :
>       |   ^~~~~~~~
>       |   readsb
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci.h:
> In function 'ehci_writel':
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci.h:767:3:
> error: implicit declaration of function 'writel_be'; did you mean
> 'writesb'? [-Werror=implicit-function-declaration]
>   767 |   writel_be(val, regs) :
>       |   ^~~~~~~~~
>       |   writesb
> In file included from
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci-hcd.c:97:
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci.h:
> In function 'ehci_readl':
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci.h:743:3:
> error: implicit declaration of function 'readl_be'; did you mean
> 'readsb'? [-Werror=implicit-function-declaration]
>   743 |   readl_be(regs) :
>       |   ^~~~~~~~
>       |   readsb
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci.h:
> In function 'ehci_writel':
> /home/nekrad/src/apu2-dev/pkg/kernel.apu2.git/drivers/usb/host/ehci.h:767:3:
> error: implicit declaration of function 'writel_be'; did you mean
> 'writesb'? [-Werror=implicit-function-declaration]
>   767 |   writel_be(val, regs) :
>       |   ^~~~~~~~~
>       |   writesb
> 
> 
> It seems that only few archs (microblaze, ppc, sparc) define them.
> 
> Also drivers/usb/host/ehci.h defines them, but only for one particular
> arch/subarch.
> 
> IIRC, these funcs are for accessing hw registers that are in BEs, so
> BE cpus can do direct access, while LE cpus need to do a conversion.
> 
> OTOH, we also have in_be32() / out_be32. They seem to do quite the same
> thing, referenced much more often, but also just defined on a few archs.
> 
> 
> I believe we should have generic functions, that all archs implement
> (possibly doing automatic conversion, if necessary), which are used
> by everybody else.
> 
> What's your oppionion on that ?

It certainly seems reasonable.  Another possibility, less stringent, is 
to require that definitions exist on all architectures that can have 
big-endian MMIO (or port-based IO).  For example, any architecture 
which might select CONFIG_EHCI_BIG_ENDIAN_MMIO, as used in ehci.h.

Otherwise we're left in the unfortunate position of having to provide 
definitions for these functions, but _only_ on architectures that don't 
already make their own definitions -- basically an impossible task.

Alan Stern