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

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Date:   Fri, 30 Nov 2018 17:19:53 +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 5:03 PM Srinivas Kandagatla
<srinivas.kandagatla@...aro.org> wrote:
> On 30/11/18 15:08, Arnd Bergmann wrote:
> > 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?
> >
>
> Currently the instance which does not need session is used as simple
> memory allocator (rpcmem), TBH, this is the side effect of trying to fit
> in with downstream application infrastructure which uses ion for andriod
> usecases.

That does not sound like enough of a reason then, user space is
easy to change here to just allocate the memory from the device itself.
The only reason that I can see for needing a dmabuf would be if
you have to share a buffer between two instances, and then you
can use either of them.

> >>>> +static void fastrpc_notify_users(struct fastrpc_user *user)
> >>>> +{
> >>>> +       struct fastrpc_invoke_ctx *ctx, *n;will go
> >>>> +
> >>>> +       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?
>
> user is allocated on every open(). Having multiple users means that
> there are more than one compute sessions running on a given dsp.
>
> They reason why all the users are notified here is because the dsp is
> going down, so all the compute sessions associated with it will not see
> any response from dsp, so any pending/waiting compute contexts are
> explicitly notified.

I don't get it yet. What are 'compute sessions'? Do you have
multiple threads running on a single instance at the same time?
I would have expected to only ever see one thread in the
'wait_for_completion()' above, and others possibly waiting
for a chance to get to but not already running.

> >> 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'.
> Yes, I see, I overdone it!
> Other idea, is, may be I can try to combine these into single structure
> something like:
>
> struct fastrpc_invoke_arg {
>         __u64 ptr;
>         __u64 len;
>         __u32 fd;
>         __u32 reserved1
>         __u64 attr;
>         __u64 crc;
> };
>
> struct fastrpc_ioctl_invoke {
>         __u32 handle;
>         __u32 sc;
>         /* The minimum size is scalar_length * 32*/
>         struct fastrpc_invoke_args *args;
> };

That is still two structure, not one ;-)

> >> 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?
> Yes, they are variable length and will be scalar length long.
> Scalar length is derived from sc variable in this structure.

Do you mean we have a variable number 'sc', but each array
always has the same length as the other ones? In that
case: yes, combining them seems sensible.

The other question this raises is: what is 'handle'?
Why is the file descriptor not enough to identify the
instance we want to talk to?

      Arnd