linux-kernel - Re: [PATCH 3/5] powerpc/ftw: Implement a simple FTW driver

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Message-ID: <e2d57c8c-9667-3b56-8d11-40d5430c9f05@infradead.org>
Date:   Wed, 17 Jan 2018 10:30:37 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        mikey@...ling.org, hbabu@...ibm.com, linuxppc-dev@...abs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] powerpc/ftw: Implement a simple FTW driver

On 01/16/18 18:50, Sukadev Bhattiprolu wrote:
> The Fast Thread Wake-up (FTW) driver provides user space applications an
> interface to the low latency Core-to-Core wakeup functionality in POWER9.
> 
> This mechanism allows a thread on one core to efficiently send a message
> to a "waiting thread" on another core on the same chip, using the Virtual
> Accelrator Switchboard (VAS) subsystem.
> 
> This initial FTW driver implements the ioctl and mmap operations on an
> FTW device node. Using these operations, a pair of application threads
> can establish a "communication channel" and use the COPY, PASTE and WAIT
> instructions to wait/wake up.
> 
> PATCH 5/5 documents the API and includes an example of the usage.
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
> ---
> Changelog[v2]
> 	- [Michael Neuling] Rename from drop "nx" from name "nx-ftw".
> 	- [Michael Neuling] Use a single VAS_FTW_SETUP ioctl to simplify
> 	  interface.
> 	- [Michael Ellerman] To work with paste emulation patch, mark
> 	  PTE dirty in ->mmap() to ensure there is no fault on paste
> 	  (the emulation patch must disable pagefaults when updating
> 	  thread reconfig registers).
> 	- Check return value from set_thread_tidr().
> 	- Move driver drivers/misc/ftw.
> 
> ---
>  drivers/misc/Kconfig      |   1 +
>  drivers/misc/Makefile     |   1 +
>  drivers/misc/ftw/Kconfig  |  16 +++
>  drivers/misc/ftw/Makefile |   4 +
>  drivers/misc/ftw/ftw.c    | 346 ++++++++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 368 insertions(+)
>  create mode 100644 drivers/misc/ftw/Kconfig
>  create mode 100644 drivers/misc/ftw/Makefile
>  create mode 100644 drivers/misc/ftw/ftw.c

> +static long ftw_ioctl(struct file *fp, unsigned int cmd, unsigned long arg)
> +{
> +	switch (cmd) {
> +
> +	case FTW_SETUP:
> +		return ftw_ioc_ftw_setup(fp, arg);
> +
> +	default:
> +		return -EINVAL;
> +	}
> +}

Nit:  some versions of gcc (or maybe clang) complain about a typed function
not always having a return value in code like above, so it is often done as:

> +static long ftw_ioctl(struct file *fp, unsigned int cmd, unsigned long arg)
> +{
> +	switch (cmd) {
> +
> +	case FTW_SETUP:
> +		return ftw_ioc_ftw_setup(fp, arg);
> +
> +	default:
> +		break;
> +	}

	return -EINVAL;
> +}

Do you expect to implement more ioctls?  If not, just change the switch to
an if ().

-- 
~Randy