Date:   Thu, 19 Nov 2020 15:54:37 +0000
From:   Barnabás Pőcze <pobrn@...tonmail.com>
To:     Maximilian Luz <luzmaximilian@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Mark Gross <mgross@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Blaž Hrastnik <blaz@...n.io>,
        Dorian Stoll <dorian.stoll@...p.io>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH 1/9] platform/surface: Add Surface Aggregator subsystem

Hi


2020. november 19., csütörtök 0:06 keltezéssel, Maximilian Luz <luzmaximilian@...il.com> írta:

> [...]

> >> +/**
> >> + * ssam_nf_head_destroy() - Deinitialize the given notifier head.
> >> + * @nh: The notifier head to deinitialize.
> >> + */
> >> +static void ssam_nf_head_destroy(struct ssam_nf_head *nh)
> >> +{
> >> +	cleanup_srcu_struct(&nh->srcu);
> >> +}
> >
> > I'm also wondering if there's any reason why these static one-liner functions are
> > not explicitly marked inline.
>
> Isn't inline more of a linkage keyword at this point? I fully expect any
> compiler to inline things like this automatically at the first hint of
> an optimization flag.
>

I also expect the compiler to inline such functions even without the `inline`
specifier. In retrospect I'm not sure why I actually made this comment, possibly
because of my preference, but I believe it's fine either way, so my comment is
moot, sorry.


> [...]

> >> + * error values via ssam_notifier_from_errno() to notifier values.
> >> + *
> >> + * Also note that any callback that could handle an event should return a value
> >> + * with bit %SSAM_NOTIF_HANDLED set, indicating that the event does not go
> >> + * unhandled/ignored. In case no registered callback could handle an event,
> >> + * this function will emit a warning.
> >> + *
> >> + * In case a callback failed, this function will emit an error message.
> >> + */
> >> +static void ssam_nf_call(struct ssam_nf *nf, struct device *dev, u16 rqid,
> >> +			 struct ssam_event *event)
> >> +{
> >> +	struct ssam_nf_head *nf_head;
> >> +	int status, nf_ret;
> >> +
> >> +	if (!ssh_rqid_is_event(rqid)) {
> >> +		dev_warn(dev, "event: unsupported rqid: 0x%04x\n", rqid);
> >
> > A small note, "%#04x" would insert the "0x" prefix.
>
> Oh neat, didn't know that. Should be "%#06x" though, right?
>

Yes, indeed, sorry, it should be "%#06x".


> [...]

> >> +static int ssam_controller_caps_load_from_acpi(
> >> +		acpi_handle handle, struct ssam_controller_caps *caps)
> >> +{
> >> +	u32 d3_closes_handle = false;
> >
> > Assinging a boolean like this to a `u32` looks very odd to me.
>
> The value returned by the corresponding DSM call is an integer, but
> essentially only contains a bool value (zero vs. one). I thought using
> false here explicitly for initialization helps clarify that. That also
> makes it consistent with the "caps->d3_closes_handle = false" line
> below.
>

Although I still find it pretty odd, your explanation makes sense to me.


> >> +int ssam_request_sync_alloc(size_t payload_len, gfp_t flags,
> >> +			    struct ssam_request_sync **rqst,
> >> +			    struct ssam_span *buffer)
> >> +{
> >> +	size_t msglen = SSH_COMMAND_MESSAGE_LENGTH(payload_len);
> >> +
> >> +	*rqst = kzalloc(sizeof(**rqst) + msglen, flags);
> >> +	if (!*rqst)
> >> +		return -ENOMEM;
> >> +
> >> +	buffer->ptr = (u8 *)(*rqst + 1);
> >> +	buffer->len = msglen;
> >> +
> >> +	return 0;
> >> +}
> >
> > I think there is a bit of incosistency: sometimes you use ** pointer + return int,
> > sometimes you return a pointer with potentially embedded errno. I think it would
> > be better if you stuck with one or the other.
>
> My rule of thumb is: If it's one output parameter, use a pointer as
> return value. If it's two or more output parameters (as in this case,
> "buffer" is written to), use two pointer arguments and an int as return.
>
> I noticed that ssam_client_bind doesn't follow that scheme so I'll
> change that.
>

I see, thanks for the explanation, what prompted me to make this comment
was that, as you noted, there were functions with one output parameter that
were not consistent.


> [...]

> >> + * Note that the packet completion callback is, in case of success and for a
> >> + * sequenced packet, guaranteed to run on the receiver thread, thus providing
> >> + * a way to reliably identify responses to the packet. The packet completion
> >> + * callback is only run once and it does not indicate that the packet has
> >> + * fully left the system (for this, one should rely on the release method,
> >> + * triggered when the reference count of the packet reaches zero). In case of
> >> + * re-submission (and with somewhat unlikely timing), it may be possible that
> >> + * the packet is being re-transmitted while the completion callback runs.
> >> + * Completion will occur both on success and internal error, as well as when
> >> + * the packet is canceled.
> >
> > If I understand it correctly, it is possible that submission of a packet fails
> > for the first time, but it's scheduled for resubmission, and this retransmission
> > happens at the same time when the complete() callback is called. If that's the
> > case, then the callback is called with an error condition, no? Thus it is possible
> > that a packet is successfully submitted (for the second, third, etc. time), but the
> > complete() callback receives notification about failure? Or am I missing something?
>
> Not sure I can follow you here. What do you mean by "fails for the first time"?
>
> A couple of notes before I try to explain the error cases:
>
>   - Resubmission only happens on timeout or NAK.
>
>   - Only sequenced packets can be resubmitted (unsequenced are not added
>     to the pending set, thus do not have a packet timeout and do not
>     react to NAKs).
>
>   - Completion means the packet gets locked, which prevents it from being
>     resubmitted and retransmitted, and is removed from all collections.
>
>   - Completion cannot be triggered by NAK. In case a NAK is received and
>     all tries have been exceeded, the packet still waits for the timeout
>     (or ACK, whatever happens first) one last time.
>
> The error cases:
>
>   - serdev: Failure to send via serdev results in immediate completion
>     with error on the transmitter thread. Due to a NAK (or a _very_
>     unlikely timeout), the packet can be queued for a second time at that
>     point, but, as completion runs on the transmitter thread in this
>     case, the packet is removed from the queue before the thread goes on
>     to get the next packet.
>
>   - Timeout: On resubmission, the timeout of a packet is disabled. It is
>     (re-)armed directly before starting transmission on the transmitter
>     thread.
>
>     What _could_ now happen is that, for some reason, the transmitter
>     thread waits the full timeout period between arming the timeout and
>     actually sending it (which I'd consider highly unlikely). Note that
>     "actually sending it" includes whatever asynchronous stuff the serdev
>     core does after serdev_device_write_buf() returns.
>
>     In that case, the packet can be transmitted _after_ it has received a
>     timeout. So assuming this is the last try, then the packet can be
>     sent after completion with -ETIMEDOUT.
>
>     I believe this is what you meant?
>

Yes, that's what I meant, thanks for the clarification and broader explanation
about the things at play here.


>     Again, I consider this highly unlikely as this would require either
>     the scheduler to pause our thread for a full timeout period or
>     require the serdev core to basically do nothing for a full timeout
>     period. I'd argue this is a lot less likely than a dropped ACK packet
>     from the EC (which results in the same thing: EC receives data but we
>     think it didn't, causing a timeout error).
>
>     Note that putting the timeout (re-)arming step after the last call to
>     serdev_device_write_buf() doesn't really help either, as that only
>     writes to the transport buffer. If we would want to guarantee that
>     this doesn't happen, we'd have to explicitly wait for the hardware to
>     finish transmitting, which essentially breaks the whole point of
>     buffering.
>
>     Also I prefer to have the timeout cover the transmission part as
>     well. In the very least, this simplifies reasoning about it once I've
>     restricted it to be (re-)set under the pending lock only (as
>     mentioned in the message to Andy).
>
>   - Finally, there is completion by cancellation or shutdown, for which
>     the packet can have been (or currently be) transmitted successfully
>     but completion returns with -ECANCELLED or -ESHUTDOWN, respectively.
>     This is the expected behavior in those cases. Again, the completion
>     procedure takes care of locking the packet and removing all
>     references from the collections.
>
> Also there's one success case, when we receive an ACK after a timeout
> and during the re-transmission. Then the packet is completed with
> success, but might still be transmitted once more. The EC should detect
> that packet as re-submission via its packet ID and ignore it.
>
> >> [...]
> >
> >> +int ssh_ptl_rx_rcvbuf(struct ssh_ptl *ptl, const u8 *buf, size_t n)
> >> +{
> >> +	int used;
> >> +
> >> +	if (test_bit(SSH_PTL_SF_SHUTDOWN_BIT, &ptl->state))
> >> +		return -ESHUTDOWN;
> >> +
> >> +	used = kfifo_in(&ptl->rx.fifo, buf, n);
> >
> > Isn't it possible that `n` is greater than the free space in the fifo? What
> > happens then?
>
> If I've read the documentation of kfifo_in() right, it takes at most n
> elements. If n is greater than the free space, it will only take as much
> elements as it has space. The return value of kfifo_in() is the number
> of elements actually consumed. We simply return that here (as elements =
> bytes) and ultimately let the serdev/tty core take care of handling
> that.
>

Ahh, sorry, you're right.



> >> +	if (used)
> >> +		ssh_ptl_rx_wakeup(ptl);
> >> +
> >> +	return used;
> >> +}


> [...]

> >> +			 ((struct ssh_frame *)sf.ptr)->len);
> >
> > Why isn't `get_unaligned_le16()` used here? (Or simply even `sp.len`.)
>
> This should actually be "sp.len + SSH_MESSAGE_LENGTH(0)", and also
> format spezifier %zu with that as SSH_MESSAGE_LENGTH returns size_t.
>

Yes, indeed, sorry, I missed the `+ SSH_MESSAGE_LENGTH(0)` part.


> >> +		return -EMSGSIZE;
> >> +	}
> >> [...]
> >> +	*frame = (struct ssh_frame *)sf.ptr;
> >
> > This also violates strict aliasing.
>
> Sure, the whole point of this function (and ssh_parse_command below) is
> to get pointers to the data structures in the given buffer, i.e. create
> aliases of the right types to the right places in the buffer. Not sure
> how I'd do that differently, apart from copying.
>
> I think that this would even work properly (in practice) without setting
> -fno-strict-aliasing, as all data at this point is effectively
> read-only. Furthermore, after the call access is restricted to the
> non-aliasing parts respectively (frame and payload), whereas the
> underlying buffer they do alias isn't accessed for the rest of that
> scope (i.e. until the other references don't exist any more).
>
> Anyway, as far as I know the kernel is built with -fno-strict-aliasing.
> Please correct me if I'm wrong or relying on aliasing is frowned upon.
>

I don't believe it could cause issue here, I just wanted to note it.


> [...]

> >> +enum ssam_ssh_tc {
> >> +	/* Known SSH/EC target categories. */
> >> +				// category 0x00 is invalid for EC use
> >> +	SSAM_SSH_TC_SAM = 0x01,	// generic system functionality, real-time clock
> >> +	SSAM_SSH_TC_BAT = 0x02,	// battery/power subsystem
> >> +	SSAM_SSH_TC_TMP = 0x03,	// thermal subsystem
> >> +	SSAM_SSH_TC_PMC = 0x04,
> >> +	SSAM_SSH_TC_FAN = 0x05,
> >> +	SSAM_SSH_TC_PoM = 0x06,
> >> +	SSAM_SSH_TC_DBG = 0x07,
> >> +	SSAM_SSH_TC_KBD = 0x08,	// legacy keyboard (Laptop 1/2)
> >> +	SSAM_SSH_TC_FWU = 0x09,
> >> +	SSAM_SSH_TC_UNI = 0x0a,
> >> +	SSAM_SSH_TC_LPC = 0x0b,
> >> +	SSAM_SSH_TC_TCL = 0x0c,
> >> +	SSAM_SSH_TC_SFL = 0x0d,
> >> +	SSAM_SSH_TC_KIP = 0x0e,
> >> +	SSAM_SSH_TC_EXT = 0x0f,
> >> +	SSAM_SSH_TC_BLD = 0x10,
> >> +	SSAM_SSH_TC_BAS = 0x11,	// detachment system (Surface Book 2/3)
> >> +	SSAM_SSH_TC_SEN = 0x12,
> >> +	SSAM_SSH_TC_SRQ = 0x13,
> >> +	SSAM_SSH_TC_MCU = 0x14,
> >> +	SSAM_SSH_TC_HID = 0x15,	// generic HID input subsystem
> >> +	SSAM_SSH_TC_TCH = 0x16,
> >> +	SSAM_SSH_TC_BKL = 0x17,
> >> +	SSAM_SSH_TC_TAM = 0x18,
> >> +	SSAM_SSH_TC_ACC = 0x19,
> >> +	SSAM_SSH_TC_UFI = 0x1a,
> >> +	SSAM_SSH_TC_USC = 0x1b,
> >> +	SSAM_SSH_TC_PEN = 0x1c,
> >> +	SSAM_SSH_TC_VID = 0x1d,
> >> +	SSAM_SSH_TC_AUD = 0x1e,
> >> +	SSAM_SSH_TC_SMC = 0x1f,
> >> +	SSAM_SSH_TC_KPD = 0x20,
> >> +	SSAM_SSH_TC_REG = 0x21,
> >> +};
> >
> > Is it known what these abbreviations stand for? Maybe I missed them?
>
> The comments state all we really know about these (through observation
> and experimentation). The table itself has been extracted from the
> Windows driver, but the abbreviations and values are all we're getting
> from it.

I see, thanks for the clarification. For some reason, I believed the
"Known SSH/EC target categories" comment means that those are known, as in,
it is known what they are for, etc., not just the abbreviation.


> [...]


Regards,
Barnabás Pőcze

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