Re: [PATCH v2 07/15] peci: Add peci-aspeed controller driver

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Aug 26, 2021 at 4:55 PM Winiarska, Iwona
<iwona.winiarska@intel.com> wrote:
>
> On Wed, 2021-08-25 at 18:35 -0700, Dan Williams wrote:
> > On Tue, Aug 3, 2021 at 4:35 AM Iwona Winiarska
> > <iwona.winiarska@intel.com> wrote:
> > >
> > > From: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
> > >
> > > ASPEED AST24xx/AST25xx/AST26xx SoCs supports the PECI electrical
> > > interface (a.k.a PECI wire).
> >
> > Maybe a one sentence blurb here and in the Kconfig reminding people
> > why they should care if they have a PECI driver or not?
>
> Ok, I'll expand it a bit.
[..]
> > > +static int aspeed_peci_xfer(struct peci_controller *controller,
> > > +                           u8 addr, struct peci_request *req)
> > > +{
> > > +       struct aspeed_peci *priv = dev_get_drvdata(controller->dev.parent);
> > > +       unsigned long flags, timeout = msecs_to_jiffies(priv-
> > > >cmd_timeout_ms);
> > > +       u32 peci_head;
> > > +       int ret;
> > > +
> > > +       if (req->tx.len > ASPEED_PECI_DATA_BUF_SIZE_MAX ||
> > > +           req->rx.len > ASPEED_PECI_DATA_BUF_SIZE_MAX)
> > > +               return -EINVAL;
> > > +
> > > +       /* Check command sts and bus idle state */
> > > +       ret = aspeed_peci_check_idle(priv);
> > > +       if (ret)
> > > +               return ret; /* -ETIMEDOUT */
> > > +
> > > +       spin_lock_irqsave(&priv->lock, flags);
> > > +       reinit_completion(&priv->xfer_complete);
> > > +
> > > +       peci_head = FIELD_PREP(ASPEED_PECI_TARGET_ADDR_MASK, addr) |
> > > +                   FIELD_PREP(ASPEED_PECI_WR_LEN_MASK, req->tx.len) |
> > > +                   FIELD_PREP(ASPEED_PECI_RD_LEN_MASK, req->rx.len);
> > > +
> > > +       writel(peci_head, priv->base + ASPEED_PECI_RW_LENGTH);
> > > +
> > > +       memcpy_toio(priv->base + ASPEED_PECI_WR_DATA0, req->tx.buf,
> > > min_t(u8, req->tx.len, 16));
> > > +       if (req->tx.len > 16)
> > > +               memcpy_toio(priv->base + ASPEED_PECI_WR_DATA4, req->tx.buf +
> > > 16,
> > > +                           req->tx.len - 16);
> > > +
> > > +       dev_dbg(priv->dev, "HEAD : 0x%08x\n", peci_head);
> > > +       print_hex_dump_bytes("TX : ", DUMP_PREFIX_NONE, req->tx.buf, req-
> > > >tx.len);
> >
> > On CONFIG_DYNAMIC_DEBUG=n builds the kernel will do all the work of
> > reading through this buffer, but skip emitting it. Are you sure you
> > want to pay that overhead for every transaction?
>
> I can remove it or I can add something like:
>
> #if IS_ENABLED(CONFIG_PECI_DEBUG)
> #define peci_debug(fmt, ...) pr_debug(fmt, ##__VA_ARGS__)
> #else
> #define peci_debug(...) do { } while (0)
> #endif

It's the hex dump I'm worried about, not the debug statements as much.

I think the choices are remove the print_hex_dump_bytes(), put it
behind an IS_ENABLED(CONFIG_DYNAMIC_DEBUG) to ensure the overhead is
skipped in the CONFIG_DYNAMIC_DEBUG=n case, or live with the overhead
if this is not a fast path / infrequently used.

>
> (and similar peci_trace with trace_printk for usage in IRQ handlers and such).
>
> What do you think?

In general, no, don't wrap the base level print routines with
driver-specific ones. Also, trace_printk() is only for debug builds.
Note that trace points are built to be even less overhead than
dev_dbg(), so there's no overhead concern with disabled tracepoints,
they literally translate to nops when disabled.

>
> >
> > > +
> > > +       priv->status = 0;
> > > +       writel(ASPEED_PECI_CMD_FIRE, priv->base + ASPEED_PECI_CMD);
> > > +       spin_unlock_irqrestore(&priv->lock, flags);
> > > +
> > > +       ret = wait_for_completion_interruptible_timeout(&priv-
> > > >xfer_complete, timeout);
> >
> > spin_lock_irqsave() says "I don't know if interrupts are disabled
> > already, so I'll save the state, whatever it is, and restore later"
> >
> > wait_for_completion_interruptible_timeout() says "I know I am in a
> > sleepable context where interrupts are enabled"
> >
> > So, one of those is wrong, i.e. should it be spin_{lock,unlock}_irq()?
>
> You're right - I'll fix it.
>
> >
> >
> > > +       if (ret < 0)
> > > +               return ret;
> > > +
> > > +       if (ret == 0) {
> > > +               dev_dbg(priv->dev, "Timeout waiting for a response!\n");
> > > +               return -ETIMEDOUT;
> > > +       }
> > > +
> > > +       spin_lock_irqsave(&priv->lock, flags);
> > > +
> > > +       writel(0, priv->base + ASPEED_PECI_CMD);
> > > +
> > > +       if (priv->status != ASPEED_PECI_INT_CMD_DONE) {
> > > +               spin_unlock_irqrestore(&priv->lock, flags);
> > > +               dev_dbg(priv->dev, "No valid response!\n");
> > > +               return -EIO;
> > > +       }
> > > +
> > > +       spin_unlock_irqrestore(&priv->lock, flags);
> > > +
> > > +       memcpy_fromio(req->rx.buf, priv->base + ASPEED_PECI_RD_DATA0,
> > > min_t(u8, req->rx.len, 16));
> > > +       if (req->rx.len > 16)
> > > +               memcpy_fromio(req->rx.buf + 16, priv->base +
> > > ASPEED_PECI_RD_DATA4,
> > > +                             req->rx.len - 16);
> > > +
> > > +       print_hex_dump_bytes("RX : ", DUMP_PREFIX_NONE, req->rx.buf, req-
> > > >rx.len);
> > > +
> > > +       return 0;
> > > +}
> > > +
> > > +static irqreturn_t aspeed_peci_irq_handler(int irq, void *arg)
> > > +{
> > > +       struct aspeed_peci *priv = arg;
> > > +       u32 status;
> > > +
> > > +       spin_lock(&priv->lock);
> > > +       status = readl(priv->base + ASPEED_PECI_INT_STS);
> > > +       writel(status, priv->base + ASPEED_PECI_INT_STS);
> > > +       priv->status |= (status & ASPEED_PECI_INT_MASK);
> > > +
> > > +       /*
> > > +        * In most cases, interrupt bits will be set one by one but also
> > > note
> > > +        * that multiple interrupt bits could be set at the same time.
> > > +        */
> > > +       if (status & ASPEED_PECI_INT_BUS_TIMEOUT)
> > > +               dev_dbg_ratelimited(priv->dev,
> > > "ASPEED_PECI_INT_BUS_TIMEOUT\n");
> > > +
> > > +       if (status & ASPEED_PECI_INT_BUS_CONTENTION)
> > > +               dev_dbg_ratelimited(priv->dev,
> > > "ASPEED_PECI_INT_BUS_CONTENTION\n");
> > > +
> > > +       if (status & ASPEED_PECI_INT_WR_FCS_BAD)
> > > +               dev_dbg_ratelimited(priv->dev,
> > > "ASPEED_PECI_INT_WR_FCS_BAD\n");
> > > +
> > > +       if (status & ASPEED_PECI_INT_WR_FCS_ABORT)
> > > +               dev_dbg_ratelimited(priv->dev,
> > > "ASPEED_PECI_INT_WR_FCS_ABORT\n");
> >
> > Are you sure these would not be better as tracepoints? If you're
> > debugging an interrupt related failure, the ratelimiting might get in
> > your way when you really need to know when one of these error
> > interrupts fire relative to another event.
>
> Tracepoints are ABI(ish), and using a full blown tracepoint just for IRQ status
> would probably be too much.

Tracepoints become ABI once someone ships tooling that depends on them
being there. These don't look  attractive for a tool, and they don't
look difficult to maintain if the interrupt handler needs to be
reworked. I.e. it would be trivial to keep a dead tracepoint around if
worse came to worse to keep a tool from failing to load.

> I was thinking about something like trace_printk hidden under a
> "CONFIG_PECI_DEBUG" (see above), but perhaps that's something for the future
> improvement?

Again trace_printk() is only for private builds.

>
> >
> > > +
> > > +       /*
> > > +        * All commands should be ended up with a ASPEED_PECI_INT_CMD_DONE
> > > bit
> > > +        * set even in an error case.
> > > +        */
> > > +       if (status & ASPEED_PECI_INT_CMD_DONE)
> > > +               complete(&priv->xfer_complete);
> >
> > Hmm, no need to check if there was a sequencing error, like a command
> > was never submitted?
>
> It's handled by checking if HW is idle in xfer before a command is sent, where
> we just expect a single interrupt per command.

I'm asking how do you determine if this status was spurious, or there
was a sequencing error in the driver?