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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Mon, 20 Apr 2020 16:34:53 +0200
Message-ID: <CAK8P3a1ErZh6teKEC8srU5E1WmXxtTgQppt1yDoYmO7huw4gqA@mail.gmail.com>
Subject: Re: [PATCH v2,RESEND] misc: new driver sram_uapi for user level SRAM access
To:     Wang Wenhu <wenhu.wang@vivo.com>
Cc:     gregkh <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, kernel@vivo.com,
        Rob Herring <robh@kernel.org>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Scott Wood <oss@buserror.net>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Randy Dunlap <rdunlap@infradead.org>
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On Mon, Apr 20, 2020 at 5:06 AM Wang Wenhu <wenhu.wang@vivo.com> wrote:
>
> A generic User-Kernel interface that allows a misc device created
> by it to support file-operations of ioctl and mmap to access SRAM
> memory from user level. Different kinds of SRAM alloction and free
> APIs could be registered by specific SRAM hardware level driver to
> the available list and then be chosen by users to allocate and map
> SRAM memory from user level.
>
> It is extremely helpful for the user space applications that require
> high performance memory accesses, such as embedded networking devices
> that would process data in user space, and PowerPC e500 is a case.
>
> Signed-off-by: Wang Wenhu <wenhu.wang@vivo.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Christophe Leroy <christophe.leroy@c-s.fr>
> Cc: Scott Wood <oss@buserror.net>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: linuxppc-dev@lists.ozlabs.org
> ---
> Changes since v1: addressed comments from Arnd
>  * Changed the ioctl cmd definitions using _IO micros
>  * Export interfaces for HW-SRAM drivers to register apis to available list
>  * Modified allocation alignment to PAGE_SIZE
>  * Use phys_addr_t as type of SRAM resource size and offset
>  * Support compat_ioctl
>  * Misc device name:sram

Looks much better already.

> Note: From this on, the SRAM_UAPI driver is independent to any hardware
> drivers, so I would only commit the patch itself as v2, while the v1 of
> it was wrapped together with patches for Freescale L2-Cache-SRAM device.
> Then after, I'd create patches for Freescale L2-Cache-SRAM device as
> another series.

What I meant to suggest was actually to tie it more closely to
the code we already have in drivers/misc/sram.c, which already
has some form of abstraction.

> +static int __init sram_uapi_init(void)
> +{
> +       int ret;
> +
> +       INIT_LIST_HEAD(&sram_api_list);
> +       mutex_init(&sram_api_list_lock);
> +
> +       ret = misc_register(&sram_uapi_miscdev);
> +       if (ret)
> +               pr_err("failed to register sram uapi misc device\n");

The mutex and listhead are already initialized, so this can
be a one-line function

    return misc_register(&sram_uapi_miscdev);

> --- /dev/null
> +++ b/include/linux/sram_uapi.h

The ioctl definitions need to be in include/uapi/linux/

> @@ -0,0 +1,28 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __SRAM_UAPI_H
> +#define __SRAM_UAPI_H
> +
> +/* Set SRAM type to be accessed */
> +#define SRAM_UAPI_IOC_SET_SRAM_TYPE    _IOW('S', 0, __u32)
> +
> +/* Allocate resource from SRAM */
> +#define SRAM_UAPI_IOC_ALLOC            _IOWR('S', 1, struct res_info)
> +
> +/* Free allocated resource of SRAM */
> +#define SRAM_UAPI_IOC_FREE             _IOW('S', 2, struct res_info)

struct res_info needs to also be defined here, so user applications can
see the definition, and it has to use __u64, not phys_addr_t, to ensure
the API does not depend on kernel configuraiton.

        Arnd