Re: [PATCH v3 01/10] drivers/char: Add support for USB3 DbC debugger

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.07.2022 05:23, Marek Marczykowski-Górecki wrote:
> +static uint64_t dbc_work_ring_size(const struct dbc_work_ring *ring)
> +{
> +    if ( ring->enq >= ring->deq )
> +        return ring->enq - ring->deq;
> +
> +    return DBC_WORK_RING_CAP - ring->deq + ring->enq;
> +}

Doesn't unsigned int suffice as a return type here?

> +static int64_t dbc_push_work(struct dbc *dbc, struct dbc_work_ring *ring,
> +                             const char *buf, unsigned int len)
> +{
> +    unsigned int i = 0;
> +    unsigned int end, start = ring->enq;
> +
> +    while ( !dbc_work_ring_full(ring) && i < len )
> +    {
> +        ring->buf[ring->enq] = buf[i++];
> +        ring->enq = (ring->enq + 1) & (DBC_WORK_RING_CAP - 1);
> +    }
> +
> +    end = ring->enq;
> +
> +    if ( end > start )
> +        cache_flush(&ring->buf[start], end - start);
> +    else if ( i > 0 )
> +    {
> +        cache_flush(&ring->buf[start], DBC_WORK_RING_CAP - start);
> +        cache_flush(&ring->buf[0], end);
> +    }
> +
> +    return i;
> +}

The function's return type is int64_t but the sole return statement
hands back an unsigned int - what's the deal here?

> +static struct xhci_trb evt_trb[DBC_TRB_RING_CAP];
> +static struct xhci_trb out_trb[DBC_TRB_RING_CAP];
> +static struct xhci_trb in_trb[DBC_TRB_RING_CAP];
> +static struct xhci_erst_segment erst __aligned(64);
> +static struct xhci_dbc_ctx ctx __aligned(64);
> +static uint8_t out_wrk_buf[DBC_WORK_RING_CAP] __aligned(DBC_PAGE_SIZE);

I've been trying to identify the reason for the alignment here,
compared to the other buffers which are no longer page-aligned. I
haven't even been able to locate the place where the address of
this buffer is actually written to hardware; all I could find was
the respective virt_to_maddr(). Could you please point me at that?

Jan