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authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2512120000 definitions=main-2601150171 On Thursday 15 January 2026 16:57:59 Central European Standard Time Damien = Ri=C3=A9gel wrote: > CPC main features are reliable transmission and remote's receive window > management. To implement these features, an additional header is needed. > This header is prepended to all Greybus messages. >=20 > Reliable transmission: to make transmission reliable, messages are > sequenced and acknowledged. That constitutes two bytes of the header, > one for the sequence number, one for the acknowledgment number. If a > message is not acked in a timely manner, a retransmission mechanism will > attempt another transmission. That mechanism will be implemented in a > future patch set. >=20 > Remote's receive window: the remote advertises the number of reception > buffers that are available on this cport. The other peer must take care > of not sending more messages than advertised by the remote. This is a > sort of flow control. That accounts for one byte in the header. >=20 > The remaining byte carries some flags. For instance, there is a flag to > indicate if it's a CPC message or a Greybus message. >=20 > Signed-off-by: Damien Ri=C3=A9gel > --- > Changes in v2: > - Minor tweaks to structure documentation >=20 > drivers/greybus/cpc/header.h | 44 ++++++++++++++++++++++++++++++++++++ > 1 file changed, 44 insertions(+) > create mode 100644 drivers/greybus/cpc/header.h >=20 > diff --git a/drivers/greybus/cpc/header.h b/drivers/greybus/cpc/header.h > new file mode 100644 > index 00000000000..af283fec492 > --- /dev/null > +++ b/drivers/greybus/cpc/header.h > @@ -0,0 +1,44 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > +/* > + * Copyright (c) 2025, Silicon Laboratories, Inc. > + */ > + > +#ifndef __CPC_HEADER_H > +#define __CPC_HEADER_H > + > +#include > +#include > + > +#define CPC_HEADER_MAX_RX_WINDOW U8_MAX > + > +/** > + * struct cpc header - Representation of CPC header. > + * @ctrl_flags: contains the type of frame and other control flags. > + * @recv_wnd: number of buffers that the cport can receive without block= ing. > + * @seq: sequence number. > + * @ack: acknowledge number, indicate to the remote the next sequence nu= mber > + * this peer expects to see. > + * > + * Each peer can confirm reception of frames by setting the acknowledgme= nt number to the next frame > + * it expects to see, i.e. setting the ack number to X effectively ackno= wledges frames with sequence > + * number up to X-1. > + * > + * CPC is designed around the concept that each cport has its pool of re= ception buffers. The number > + * of buffers in a pool is advertised to the remote via the @recv_wnd at= tribute. This acts as > + * software flow-control, and a peer shall not send frames to a remote i= f the @recv_wnd is zero. > + * > + * The eighth-bit (0x80) of the control byte indicates if the frame targ= ets CPC or Greybus. If the > + * bit is set, the frame should be interpreted as a CPC control frame. F= or simplicity, control > + * frames have the same encoding as Greybus frames. > + */ > +struct cpc_header { > + __u8 ctrl_flags; > + __u8 recv_wnd; > + __u8 seq; > + __u8 ack; > +} __packed; > + > +#define CPC_HEADER_SIZE (sizeof(struct cpc_header)) > +#define GREYBUS_HEADER_SIZE (sizeof(struct gb_operation_msg_hdr)) Generally, when I see such macros in the code, I suspect some black magic behind. So, I have to check the definitions. I suggest to not introduce these trivial symbols and just use "sizeof(struct ...)" inline in the code. --=20 J=C3=A9r=C3=B4me Pouiller