linux-kernel - Re: [RFC PATCH 3/6] char: fastrpc: Add support for context Invoke method

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Date:   Fri, 30 Nov 2018 16:08:03 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        gregkh <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-msm@...r.kernel.org, bkumar@....qualcomm.com,
        thierry.escande@...aro.org
Subject: Re: [RFC PATCH 3/6] char: fastrpc: Add support for context Invoke method

On Fri, Nov 30, 2018 at 4:01 PM Srinivas Kandagatla
<srinivas.kandagatla@...aro.org> wrote:
> Thanks Arnd for the review comments!
> On 30/11/18 13:41, Arnd Bergmann wrote:
> > On Fri, Nov 30, 2018 at 11:48 AM Srinivas Kandagatla
> > <srinivas.kandagatla@...aro.org> wrote:

> >> +static long fastrpc_device_ioctl(struct file *file, unsigned int cmd,
> >> +                                unsigned long arg)
> >> +{
> >> +       struct fastrpc_user *fl = (struct fastrpc_user *)file->private_data;
> >> +       struct fastrpc_channel_ctx *cctx = fl->cctx;
> >> +       char __user *argp = (char __user *)arg;
> >> +       int err;
> >> +
> >> +       if (!fl->sctx) {
> >> +               fl->sctx = fastrpc_session_alloc(cctx, 0);
> >> +               if (!fl->sctx)
> >> +                       return -ENOENT;
> >> +       }
> >
> > Shouldn't that session be allocated during open()?
> >
> Yes, and no, we do not need context in all the cases. In cases like we
> just want to allocate dmabuf.

Can you give an example what that would be good for?

>
> >> +static void fastrpc_notify_users(struct fastrpc_user *user)
> >> +{
> >> +       struct fastrpc_invoke_ctx *ctx, *n;
> >> +
> >> +       spin_lock(&user->lock);
> >> +       list_for_each_entry_safe(ctx, n, &user->pending, node)
> >> +               complete(&ctx->work);
> >> +       spin_unlock(&user->lock);
> >> +}
> >
> > Can you explain here what it means to have multiple 'users'
> > a 'fastrpc_user' structure? Why are they all done at the same time?
> >
> This is the case where users need to be notified if the dsp goes down
> due to crash or shut down!

What is a 'user' then? My point is that it seems to refer to two
different things here. I assume 'fastrpc_user' is whoever
has opened the file descriptor.

> >
> >> +struct fastrpc_ioctl_invoke {
> >> +       uint32_t handle;        /* remote handle */
> >> +       uint32_t sc;            /* scalars describing the data */
> >> +       remote_arg_t *pra;      /* remote arguments list */
> >> +       int *fds;               /* fd list */
> >> +       unsigned int *attrs;    /* attribute list */
> >> +       unsigned int *crc;
> >> +};
> >
> > This seems too complex for an ioctl argument, with
> > multiple levels of pointer indirections. I'd normally
> > try to make each ioctl argument either a scalar, or a
> > structure with only fixed-length members.
> >
> I totally agree with you and many thanks for your expert inputs,
> May be something like below with fixed length members would work?
>
> struct fastrpc_remote_arg {
>         __u64 ptr;      /* buffer ptr */
>         __u64 len;      /* length */
>         __u32 fd;       /* dmabuf fd */
>         __u32 reserved1
>         __u64 reserved2
> };
>
> struct fastrpc_remote_fd {
>         __u64 fd;
>         __u64 reserved1
>         __u64 reserved2
>         __u64 reserved3
> };
>
> struct fastrpc_remote_attr {
>         __u64 attr;
>         __u64 reserved1
>         __u64 reserved2
>         __u64 reserved3
> };
>
> struct fastrpc_remote_crc {
>         __u64 crc;
>         __u64 reserved1
>         __u64 reserved2
>         __u64 reserved3
> };

I don't see a need to add extra served fields for structures
that are already naturally aligned here, e.g. in
fastrpc_remote_arg we need the 'reserved1' but not
the 'reserved2'.

>
> struct fastrpc_ioctl_invoke {
>         __u32 handle;
>         __u32 sc;
>         /* The minimum size is scalar_length * 32 */
>         struct fastrpc_remote_args *rargs;
>         struct fastrpc_remote_fd *fds;
>         struct fastrpc_remote_attr *attrs;
>         struct fastrpc_remote_crc *crc;
> };

Do these really have to be indirect then? Are they all
lists of variable length? How do you know how long?

      Arnd