linux-kernel - Re: [PATCH v6,4/4] drivers: misc: new driver sram

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Message-ID: <CAK8P3a05Rr7q_OtqqtG5hCPO3cettAJQNpmD0ULec+3PS-QPHA@mail.gmail.com>
Date:   Sat, 18 Apr 2020 21:07:54 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Wang Wenhu <wenhu.wang@...o.com>
Cc:     gregkh <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Scott Wood <oss@...error.net>,
        christophe leroy <christophe.leroy@....fr>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>, kernel@...o.com,
        Randy Dunlap <rdunlap@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v6,4/4] drivers: misc: new driver sram_uapi for user level
 SRAM access

On Sat, Apr 18, 2020 at 6:22 PM Wang Wenhu <wenhu.wang@...o.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 added to the available array and could be configured
> from user level.

Having a generic user level interface seem reasonable, but it would
be helpful to list one or more particular use cases.

> +if SRAM_UAPI
> +
> +config FSL_85XX_SRAM_UAPI
> +       bool "Freescale MPC85xx Cache-SRAM UAPI support"
> +       depends on FSL_SOC_BOOKE && PPC32
> +       select FSL_85XX_CACHE_SRAM
> +       help
> +         This adds the Freescale MPC85xx Cache-SRAM memory allocation and
> +         free interfaces to the available SRAM API array, which finally could
> +         be used from user level to access the Freescale MPC85xx Cache-SRAM
> +         memory.

Why do you need  a hardware specific Kconfig option here, shouldn't
this just use the generic kernel abstraction for the sram?

> +struct sram_api {
> +       u32 type;
> +       long (*sram_alloc)(u32 size, phys_addr_t *phys, u32 align);
> +       void (*sram_free)(void *ptr);
> +};
> +
> +struct sram_uapi {
> +       struct list_head        res_list;
> +       struct sram_api         *sa;
> +};
> +
> +enum SRAM_TYPE {
> +#ifdef FSL_85XX_CACHE_SRAM
> +       SRAM_TYPE_FSL_85XX_CACHE_SRAM,
> +#endif
> +       SRAM_TYPE_MAX,
> +};
> +
> +/* keep the SRAM_TYPE value the same with array index */
> +static struct sram_api srams[] = {
> +#ifdef FSL_85XX_CACHE_SRAM
> +       {
> +               .type           = SRAM_TYPE_FSL_85XX_CACHE_SRAM,
> +               .sram_alloc     = mpc85xx_cache_sram_alloc,
> +               .sram_free      = mpc85xx_cache_sram_free,
> +       },
> +#endif
> +};

If there is a indeed a requirement for hardware specific functions,
I'd say these should be registered from the hardware specific driver
rather than the generic driver having to know about every single
instance.

> +static long sram_uapi_ioctl(struct file *filp, unsigned int cmd,
> +                           unsigned long arg)
> +{
> +       struct sram_uapi *uapi = filp->private_data;
> +       struct sram_resource *res;
> +       struct res_info info;
> +       long ret = -EINVAL;
> +       int size;
> +       u32 type;
> +
> +       if (!uapi)
> +               return ret;
> +
> +       switch (cmd) {
> +       case SRAM_UAPI_IOCTL_SET_SRAM_TYPE:
> +               size = copy_from_user((void *)&type, (const void __user *)arg,
> +                                     sizeof(type));

This could be a simpler get_user().

> +static const struct file_operations sram_uapi_ops = {
> +       .owner = THIS_MODULE,
> +       .open = sram_uapi_open,
> +       .unlocked_ioctl = sram_uapi_ioctl,
> +       .mmap = sram_uapi_mmap,
> +       .release = sram_uapi_release,
> +};

If you have a .unlocked_ioctl callback, there should also be a
.compat_ioctl one. This can normally point to compat_ptr_ioctl().

> +
> +static struct miscdevice sram_uapi_miscdev = {
> +       MISC_DYNAMIC_MINOR,
> +       "sram-uapi",
> +       &sram_uapi_ops,
> +};

The name of the character device should not contain "uapi", that
is kind of implied here.


        Arnd