Re: [PATCH] stm: class: Add MIPI OST protocol support

[Jinlong Mao]

Hi Alexande,

On 3/3/2023 2:05 AM, Alexander Shishkin wrote:
> Mao Jinlong <quic_jinlmao@xxxxxxxxxxx> writes:
>
> > Add MIPI OST protocol support for stm to format the traces.
> Missing an explanation of what OST is, what it's used for, how it is
> different from the SyS-T and others.
I will updae the explanation in next version.
> > Framework copied from drivers/hwtracing/stm.p-sys-t.c as of
> You mean stm/p_sys-t.c. Also, it's not a framework, it's a driver.

The driver refers to code structure of p_sys-t driver. So, add this 
comments after
internal review.

> > commit d69d5e83110f ("stm class: Add MIPI SyS-T protocol
> > support").
> Why is this significant?
>
> > diff --git a/drivers/hwtracing/stm/p_ost.c b/drivers/hwtracing/stm/p_ost.c
> > new file mode 100644
> > index 000000000000..2ca1a3fda57f
> > --- /dev/null
> > +++ b/drivers/hwtracing/stm/p_ost.c
> > @@ -0,0 +1,95 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Copied from drivers/hwtracing/stm.p-sys-t.c as of commit d69d5e83110f
> > + * ("stm class: Add MIPI SyS-T protocol support").
> Same as in the commit message.
>
> [...]
>
> > +#define OST_TOKEN_STARTSIMPLE		(0x10)
> > +#define OST_VERSION_MIPI1		(0x10 << 8)
> > +#define OST_ENTITY_FTRACE		(0x01 << 16)
> > +#define OST_CONTROL_PROTOCOL		(0x0 << 24)
> These could use an explanation.
I will add the explanation.
> > +#define DATA_HEADER (OST_TOKEN_STARTSIMPLE | OST_VERSION_MIPI1 | \
> > +			OST_ENTITY_FTRACE | OST_CONTROL_PROTOCOL)
> Does this mean that everything is ftrace? Because it's not.
Only ftrace is supported in p_ost now. Other header type will be added 
later.
> > +
> > +#define STM_MAKE_VERSION(ma, mi)	((ma << 8) | mi)
> > +#define STM_HEADER_MAGIC		(0x5953)
> > +
> > +static ssize_t notrace ost_write(struct stm_data *data,
> > +		struct stm_output *output, unsigned int chan,
> > +		const char *buf, size_t count)
> > +{
> > +	unsigned int c = output->channel + chan;
> > +	unsigned int m = output->master;
> > +	const unsigned char nil = 0;
> > +	u32 header = DATA_HEADER;
> > +	u8 trc_hdr[24];
> > +	ssize_t sz;
> > +
> > +	/*
> > +	 * STP framing rules for OST frames:
> > +	 *   * the first packet of the OST frame is marked;
> > +	 *   * the last packet is a FLAG.
> Which in your case is also timestamped.
I will add the comments.
> > +	 */
> > +	/* Message layout: HEADER / DATA / TAIL */
> > +	/* HEADER */
> > +
> > +	sz = data->packet(data, m, c, STP_PACKET_DATA, STP_PACKET_MARKED,
> > +			  4, (u8 *)&header);
> The /* HEADER */ comment applies to the above line, so it should
> probably be directly before it.
Got it.
> > +	if (sz <= 0)
> > +		return sz;
> > +	*(uint16_t *)(trc_hdr) = STM_MAKE_VERSION(0, 3);
> > +	*(uint16_t *)(trc_hdr + 2) = STM_HEADER_MAGIC;
> > +	*(uint32_t *)(trc_hdr + 4) = raw_smp_processor_id();
> > +	*(uint64_t *)(trc_hdr + 8) = sched_clock();
> Why sched_clock()? It should, among other things, be called with
> interrupts disabled, which is not the case here.
I will check. If it is not necessary here, I will remove it.
> > +	*(uint64_t *)(trc_hdr + 16) = task_tgid_nr(get_current());
> Is there a reason why trc_hdr is not a struct?
No particular reason here.
> Thanks,
> --
> Alex