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SubjectRe: [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@linaro.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@linaro.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

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