On 11/07/2024 11:40, Quentin Schulz wrote:
Hi Bryan,
On 7/11/24 12:20 PM, Bryan O'Donoghue wrote:
The ov5675 specification says that the gap between XSHUTDN deassert
and the
first I2C transaction should be a minimum of 8192 XVCLK cycles.
Right now we use a usleep_rage() that gives a sleep time of between about
430 and 860 microseconds.
On the Lenovo X13s we have observed that in about 1/20 cases the current
timing is too tight and we start transacting before the ov5675's reset
cycle completes, leading to I2C bus transaction failures.
The reset racing is sometimes triggered at initial chip probe but, more
usually on a subsequent power-off/power-on cycle e.g.
[ 71.451662] ov5675 24-0010: failed to write reg 0x0103. error = -5
[ 71.451686] ov5675 24-0010: failed to set plls
The current quiescence period we have is too tight, doubling the minimum
appears to fix the issue observed on X13s.
Fixes: 49d9ad719e89 ("media: ov5675: add device-tree support and
support runtime PM")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@xxxxxxxxxx>
---
drivers/media/i2c/ov5675.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/drivers/media/i2c/ov5675.c b/drivers/media/i2c/ov5675.c
index 92bd35133a5d..0498f8f3064d 100644
--- a/drivers/media/i2c/ov5675.c
+++ b/drivers/media/i2c/ov5675.c
@@ -1018,8 +1018,13 @@ static int ov5675_power_on(struct device *dev)
gpiod_set_value_cansleep(ov5675->reset_gpio, 0);
- /* 8192 xvclk cycles prior to the first SCCB transation */
- usleep_range(delay_us, delay_us * 2);
+ /* The spec calls for a minimum delay of 8192 XVCLK cycles prior to
+ * transacting on the I2C bus, which translates to about 430
+ * microseconds at 19.2 MHz.
+ * Testing shows the range 8192 - 16384 cycles to be unreliable.
+ * Grant a more liberal 2x -3x clock cycle grace time.
+ */
+ usleep_range(delay_us * 2, delay_us * 3);
Would it make sense to have power_off have the same logic? We do a
usleep_range of those same values currently, so keeping them in sync
seems to make sense to me.
I have no evidence to suggest there's a problem on the shutdown path,
that's why I left the quiescence period as-is there.
Also, I'm wondering if it isn't an issue with the gpio not being high
right after gpoiod_set_value_cansleep() returns, i.e. the time it
actually takes for the HW to reach the IO level that means "high" for
the camera. And that this increased sleep is just a way to mitigate that?
No, that's not what I found.
I tried changing
usleep_range(2000, 2200);
to
usleep_range(200000, 220000);
but could still elicit the I2C transaction failure. If the time it took
for the GPIO to hit logical 1 were the issue then multiplying the reset
time by 100 would certainly account for that.
// BOD set the chip into reset
gpiod_set_value_cansleep(ov5675->reset_gpio, 1);
// BOD apply power
ret = regulator_bulk_enable(OV5675_NUM_SUPPLIES, ov5675->supplies);
if (ret) {
clk_disable_unprepare(ov5675->xvclk);
return ret;
}
/* Reset pulse should be at least 2ms and reset gpio released
only once
* regulators are stable.
*/
// BOD spec specifies 2 milliseconds here not a count of XVCLKs
usleep_range(2000, 2200);
gpiod_set_value_cansleep(ov5675->reset_gpio, 0);
// BOD spec calls for a _minimum_ of 8192 XVCLK cycles before I2C
/* 8192 xvclk cycles prior to the first SCCB transation */
usleep_range(delay_us, delay_us * 2);
The issue is initiating an I2C transaction too early _after_ reset
completion not the duration of that reset.
As I stated in the cover letter, I tried a longer reset duration, a
higher drive-strength on the GPIO as well as I didn't put in my cover
letter, inverting the logic of the GPIO reset, which unsurprisingly
didn't work.
No matter how long we hold the chip in reset, unless we give more grace
time _subsequent_ to the reset before initiating an I2C transaction, we
will encounter transaction failures.
This is a fairly common and logical fault if you think about it.
XVCLK is providing a clock to the ov5675 core to "do stuff" whatever
that stuff is. Bring up an internal firmware, lock a fundamental PLL -
whatever.
If we start an I2C transaction before the hypothetical internal core has
booted up then - meh no bueno we'll get no transaction response.
That's the error - speaking too soon.
A little like myself in the mornings, cranky before I've had my coffee
and unresponsive.
;)
With this patch we essentially postpone the power_on by another 430ms
making it almost a full second before we can start using the camera.
That's quite a lot I think? We don't have a usecase right now that
requires this to be blazing fast (and we anyway would need at the very
least 430ms), so take this remark as what it is, a remark.
Not a full second, a millisecond.
8/10ths of 1 millisecond instead of 4/10ths of one millisecond.
19.2MHz is 52.083333333333 nanoseconds per clock
52.083333333333 * 8192 => 426666 nanoseconds => 0.426666 milliseconds or
426.6 microseconds
So our post reset quiesence minimum @ 19.2MHz moves from 426.6
microseconds to 853.
The change looks fine to me even though it feels like a band-aid patch.
I mean it's not a second - if you feel very strongly that 426
milliseconds * 2 is wrong, I guess I could add some more complex logic
however I like this simple fix for backporting.
---
bod
