Re: [PATCH v9 6/9] i3c: master: Add driver for Cadence IP

[Arnd Bergmann]

On Fri, Oct 26, 2018 at 9:57 AM Boris Brezillon
<boris.brezillon@xxxxxxxxxxx> wrote:
> On Fri, 26 Oct 2018 09:43:25 +0200
> Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> > On Thu, Oct 25, 2018 at 6:30 PM Boris Brezillon
> > <boris.brezillon@xxxxxxxxxxx> wrote:
> > > On Thu, 25 Oct 2018 18:13:51 +0200 Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > On Thu, Oct 25, 2018 at 6:07 PM Boris Brezillon <boris.brezillon@xxxxxxxxxxx> wrote:
> > > > > On Thu, 25 Oct 2018 17:30:26 +0200
> > Ok. Is i3c_master_send_ccc_cmd_locked() what implements the public
> > interfaces then, or is this something else?
>
> i3c_master_send_ccc_cmd_locked() calls master->ops->send_ccc_cmd(), so
> it's part of the master controller interface.
>
> >
> > If you place a buffer on the stack, it is not DMA capable, but
> > it is guaranteed to be at least 32-bit word aligned, and should
> > not cause an exception in readsl(), unless it starts with a couple of
> > (not multiple of four)  extra bytes that are not sent to the devices.
> > Is that what happens here?
>
> Here is the report I received from Vitor:
>
> "
>         Hi Boris,
>
>
>         I'm trying this new patch-set version but I get some issues when use
>         readsl() function.
>
>         Basically the system complain about memory alignment.
>

>         > +static int i3c_master_getpid_locked(struct i3c_master_controller *master,
>         > +                                 struct i3c_device_info *info)
>         > +{
>         > +     struct i3c_ccc_getpid getpid;
>
>         at this point the getpid struct it is already unaligned with
>
>         i3c_master_getpid_locked:1129 getpid_add=0x9a249c7a
>
>         > +     struct i3c_ccc_cmd_dest dest = {
>         > +             .addr = info->dyn_addr,
>         > +             .payload.len = sizeof(struct i3c_ccc_getpid),
>         > +             .payload.data = &getpid,
>         > +     };

>         > +}
>         > +
>
>         and them when
>
>         static void dw_i3c_master_read_rx_fifo(struct dw_i3c_master *master,
>                                 u8 *bytes, int nbytes)
>         {
>              readsl(master->regs + RX_TX_DATA_PORT, bytes, nbytes / 4);
>         ...
>         }

Ok, I spent an hour chasing the ARM implementation and finding
no way this could go wrong here. I see that 'struct i3c_ccc_getpid'
may be misaligned on the stack (it normally won't be), and that
the ARM readsl() has a lot of extra code to handle unaligned
output. However, the dump that Vitor reports

>         [ECR   ]: 0x00230400 => Misaligned r/w from 0x9a249c7a
>         [EFA   ]: 0x9a249c7a
>        [BLINK ]: dw_i3c_master_irq_handler+0x200/0x2fc [dw_i3c_master]

Is from an arch/arc kernel that uses asm-generic/io.h, and
that stores the output using a u32 pointer:

static inline void readsl(const volatile void __iomem *addr, void *buffer,
                          unsigned int count)
{
        if (count) {
                u32 *buf = buffer;

                do {
                        u32 x = __raw_readl(addr);
                        *buf++ = x;
                } while (--count);
        }
}

This is apparently not allowed on ARC when 'buffer' is
unaligned. I think what we need here is to use
put_unaligned() instead of the pointer dereference.
For architectures that can do unaligned accesses,
the result is the same, but for ARC it will fix the problem.

> > One way to address this might be to always bounce any
> > messages that are less than a cache line through a
> > (pre-)kmallocated buffer, and require any longer messages
> > to be cache capable. This could also solve the issue with
> > readsl(), but it would be a rather confusing user interface.
> >
> > Another option might be to have separate interfaces for
> > "short" and "long" messages at the API level and have
> > distinct rules for those: short would always be bounced
> > by the i3c code, and long puts restrictions on the buffer
> > location.
>
> Hm, let's keep the API simple. I'll just mandate that all payload bufs
> passed to i3c_master_send_ccc_cmd_locked() be dynamically allocated.

Ok. What about i2c commands sent to the same i3c controller
then? Do we need to copy those to satisfy the requirements
of the i3c layer?

      Arnd