Hi Doug,
Thank you for reviewing the patch. I will take a stab at a few comments
below. We will address most of the other comments in next version of I2C
patch.
+
+#define I2C_AUTO_SUSPEND_DELAY 250
Why 250 ms? That seems like an eternity. Is it really that expensive
to turn resources off and on? I would sorta just expect clocks and
stuff to get turned off right after a transaction finished unless
another one was pending right behind it...
The response from RPMh to turn on/off shared resources also take quite a
few msecs. The QUP serial bus block sits quite a few shared-NOCs away
from the memory and runtime-PM is used a bandwidth vote/NOC vote for
these NOCs from QUP to memory. Also the RPC between apps and RPMh can
sometimes take longer depending on other tasks running on apps. This 250
msec avoids thrashing of these RPCs between apps and RPMh.
If you plan to keep these NOCs on forever, then your are right:
runtime-PM will be only used to turn on/off local clocks and we won't
even need autosuspend. that's not true on products where this driver is
currently deployed.
+
+static const struct geni_i2c_clk_fld geni_i2c_clk_map[] = {
+ {KHz(100), 7, 10, 11, 26},
+ {KHz(400), 2, 5, 12, 24},
+ {KHz(1000), 1, 3, 9, 18},
So I guess this is all relying on an input serial clock of 19.2MHz?
Maybe document that?
Assuming I'm understanding the math here, is it really OK for your
100kHz and 1MHz mode to be running slightly fast?
19200. / 2 / 24
400.0
19200. / 7 / 26
105.49450549450549
19200. / 1 / 18
1066.6666666666667
It seems like you'd want the fastest clock that you can make that's
_less than_ the spec.
It would also be interesting to know if it's expected that boards
might need to tweak the t_high / t_low depending on their electrical
characteristics. In the past I've had lots of requests from board
makers to tweak things because they've got a long trace, or a stronger
or weaker pull, or ... If so we might later need to add some dts
properties like "i2c-scl-rising-time-ns" and make the math more
dynamic here, unless your hardware somehow automatically adjusts for
this type of thing...
These values are derived by our HW team to comply with the t-high and
t-low specs of I2C. We have confirmed on scope that the frequency of SCL
is indeed less than/equal to the spec. We have not come across slaves
who have needed to tweak these things. We are open to adding these
properties in dts if you have any such slaves not conforming due to
board-layout of other reasons.
+ mode = msg->len > 32 ? GENI_SE_DMA : GENI_SE_FIFO;
DMA is hard and i2c transfers > 32 bytes are rare. Do we really gain
a lot by transferring i2c commands over DMA compared to a FIFO?
Enough to justify the code complexity and the set of bugs that will
show up? I'm sure it will be a controversial assertion given that the
code's already written, but personally I'd be supportive of ripping
DMA mode out to simplify the driver. I'd be curious if anyone else
agrees. To me it seems like premature optimization.
Yes, we have multiple clients (e.g. touch, NFC) using I2C for data
transfers bigger than 32 bytes (some transfers are 100s of bytes). The
fifo size is 32, so we can definitely avoid at least 1 interrupt when
DMA mode is used with data size > 32.
+ geni_se_select_mode(&gi2c->se, mode);
+ writel_relaxed(msg->len, gi2c->se.base + SE_I2C_RX_TRANS_LEN);
+ geni_se_setup_m_cmd(&gi2c->se, I2C_READ, m_param);
+ if (mode == GENI_SE_DMA) {
+ rx_dma = geni_se_rx_dma_prep(&gi2c->se, msg->buf, msg->len);
Randomly I noticed a flag called "I2C_M_DMA_SAFE". Do we need to
check this flag before using msg->buf for DMA? ...or use
i2c_get_dma_safe_msg_buf()?
...btw: the relative lack of people doing this in the kernel is
further evidence of DMA not really being worth it for i2c busses.
I cannot comment about other drivers here using or not using DMA since
they may not be exercised with slaves like NFC?
+ ret = pm_runtime_get_sync(gi2c->se.dev);
+ if (ret < 0) {
+ dev_err(gi2c->se.dev, "error turning SE resources:%d\n", ret);
+ pm_runtime_put_noidle(gi2c->se.dev);
+ /* Set device in suspended since resume failed */
+ pm_runtime_set_suspended(gi2c->se.dev);
+ return ret;
Wow, that's a cluster of arcane calls to handle a call that probably
will never fail (it just enables clocks and sets pinctrl). Sigh.
...but as far as I can tell the whole sequence is right. You
definitely need a "put" after a failed get and it looks like
pm_runtime_set_suspended() has a special exception where it can be
called if you got a runtime error...
We didn't have this in before either. But this condition is somewhat
frequent if I2C transactions are called on cusp of exiting system
suspend. (e.g. PMIC slave getting a wakeup-IRQ and trying to read from
PMIC through I2C to read its status as to what caused that wake-up. At
that time, get_sync doesn't really enable resources (kernel 4.9) since
the runtime-pm ref-count isn't decremented. We run the risk of unclocked
access if these arcane calls aren't present. You can go through
runtime-pm documentation chapter 6 for more details.
+ ret = devm_request_irq(&pdev->dev, gi2c->irq, geni_i2c_irq,
+ IRQF_TRIGGER_HIGH, "i2c_geni", gi2c);
+ if (ret) {
+ dev_err(&pdev->dev, "Request_irq failed:%d: err:%d\n",
+ gi2c->irq, ret);
+ return ret;
+ }
+ disable_irq(gi2c->irq);
Can you explain the goal of the disable_irq() here. Is it actually
needed for something or does it somehow save power? From drivers I've
reviewed in the past this doesn't seem like a common thing to do, so
I'm curious what it's supposed to gain for you. I'd be inclined to
just delete the whole disable/enable of the irq from this driver.
Qualcomm's power team suggests we enable/disable unused IRQs. Otherwise
they can block apps from entering some low-power mode (unless the
interrupt is in some list?) I will confirm again with them and let you know.
+ /* Make sure no transactions are pending */
+ ret = i2c_trylock_bus(&gi2c->adap, I2C_LOCK_SEGMENT);
+ if (!ret) {
+ dev_err(gi2c->se.dev, "late I2C transaction request\n");
+ return -EBUSY;
+ }
Does this happen? How?
Nothing about this code looks special for your hardware. If this is
really needed, why is it not part of the i2c core since there's
nothing specific about your driver here?
There have been some clients that don't implement sys-suspend/resume
callbacks (so i2c adapter has no clue they are done with their
transactions) and this allows us to be flexible when they call I2C
transactions extremely late.
+ if (!pm_runtime_status_suspended(device)) {
+ geni_i2c_runtime_suspend(device);
+ pm_runtime_disable(device);
+ pm_runtime_set_suspended(device);
+ pm_runtime_enable(device);
+ }
Similar question. Why do you need this special case code? Are there
cases where we're all the way at suspend_noirq and yet we still
haven't runtime suspended? Can you please document how we get into
this state?
This is when transaction happens less-than 250 msec of the
system-suspend. PM-runtime has not gotten a chance to auto-suspend us
since timer hasn't expired before system-suspend is attempted. These
calls make sure that we truly turn off driver resources and make
runtime-PM state consistent with the HW state. We can document this.
Thanks
Sagar
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