On Fri, Mar 4, 2016 at 5:20 PM, David Rivshin (Allworx) <drivshin.allworx@xxxxxxxxx> wrote: > On Fri, 4 Mar 2016 22:18:38 +0100 > Thierry Reding <thierry.reding@xxxxxxxxx> wrote: > >> On Fri, Mar 04, 2016 at 11:27:36AM -0500, David Rivshin (Allworx) wrote: >> > On Fri, 4 Mar 2016 16:19:48 +0100 >> > Thierry Reding <thierry.reding@xxxxxxxxx> wrote: >> > >> > > On Fri, Feb 26, 2016 at 08:31:00PM -0500, David Rivshin (Allworx) wrote: >> > > > On Fri, 29 Jan 2016 23:26:50 -0500 >> > > > "David Rivshin (Allworx)" <drivshin.allworx@xxxxxxxxx> wrote: >> > > > >> > > > > From: David Rivshin <drivshin@xxxxxxxxxxx> >> > > > > >> > > > > When using a short PWM period (approaching the min of 2/clk_rate), >> > > > > pwm-omap-dmtimer does not produce accurate results. In the worst case a >> > > > > requested period of 2/clk_rate would result in a real period of 4/clk_rate >> > > > > instead. This is a series includes a fix for that problem, as well as >> > > > > other related improvements, and is based on the current linux-pwm/for-next >> > > > > tip. >> > > > > >> > > > > I have tested on a Sitara AM335x platform, using a scope to verify the >> > > > > output with a variety of periods and duty cycles. This includes a PWM >> > > > > rate up clk_rate/2 with 50% duty cycle (e.g. generating fclk/2) with >> > > > > both 32768Hz and 24MHz fclks. I do not have an OMAP4 board to test with, >> > > > > although appropriate sections in the the reference manuals appear >> > > > > substantially the same, so I believe the changes are equally correct >> > > > > there. >> > > > > >> > > > > Note that the OMAP4 TRMs do effectively state that the maximum PWM >> > > > > rate is clk_rate/4, so at very fast PWM rates the behavior may not be >> > > > > as reliable as I observed with Sitara. Although I suspect that it's >> > > > > the same module and will also work, at least under some circumstances. >> > > > > If anyone with OMAP4 hardware and a scope is so inclined, I would be >> > > > > curious to know the results. >> > > > > >> > > > > David Rivshin (4): >> > > > > pwm: omap-dmtimer: fix inaccurate period/duty_cycle calculation >> > > > > pwm: omap-dmtimer: add sanity checking for load and match values >> > > > > pwm: omap-dmtimer: round load and match values rather than truncate >> > > > > pwm: omap-dmtimer: add dev_dbg() message for effective period and duty >> > > > > cycle >> > > > > >> > > > > drivers/pwm/pwm-omap-dmtimer.c | 71 ++++++++++++++++++++++++++++++++---------- >> > > > > 1 file changed, 55 insertions(+), 16 deletions(-) >> > > > > >> > > > >> > > > Hi Thierry, >> > > > >> > > > Gentle ping. It does not look like you've taken this series, and I >> > > > wanted to make sure you're not waiting on something from me. It would >> > > > be nice to get at least the first patch into 4.5, if possible. >> > > >> > > I've applied patches 1 and 3, and I'm planning on sending out a pull >> > > request for inclusion in v4.5-rc7 later on. >> > >> > Thanks! >> > >> > > Patches 2 and 4 didn't seem ready/critical, so let's finish those up >> > > for v4.6-rc1. >> > >> > I know there was a lot of discussion on 4, but I'm not sure what the >> > concern is on patch 2. Is there something specific you're thinking of? >> >> Patch 2 sounded like some optional sanity checking which we didn't hit >> anyway in the current code. Hence I didn't consider it a fix. > > Hrm, perhaps there is something I should have added/changed in the > patch description to clarify? > > For further background, patch 2 protects against things that are legal > in the PWM API itself, but not possible for this hardware to actually do. > Specifically a very short period, or duty_cycle too close to either 0 or > 100%. Without the checking, the natural result of the normal math results > in setting up the HW in non-sensical ways. > > As long as noone attempts to configure the PWM in those ways, then I'd > agree it's not critical. What made me think it was more important was > when I saw that Adam (and so probably others) used it for a PWM-backlight, > which will naturally try to set 0 and 100% duty cycle. > >> > FYI, I know that Adam Ford is using this driver as the backend for >> > a pwm-backlight control. Without patch 2 this driver will not configure >> > the HW in a legal way at 0 or 100% duty cycle. However, I forget what >> > the practical effect of that is, and Adam seemed to indicate it was OK >> > for his purposes. >> >> Okay, I'll hold back a little longer to give you some time to test. > > FYI, I just went and retested what the effect is without those checks. > If the duty_cycle is set too low, the result is that the output is constant > high. If the duty_cycle is set too high, the result is that the output is > constant low. IOW, the result is the reverse of what the user requested. > > If the period is too low, then you also get one of those two results, > depending on what the duty_cycle is (essentially the duty_cycle is always > too close to 0/100%). > > With patch 2, it will clamp the duty_cycle to the lowest/highest possible > value. So while imperfect, at least isn't the reverse of the requested > behavior. Sorry it took so long to test this, I had some construction at my house so I was without a computer for a few days, then I put it on the market and some showings, so I wasn't allowed to be home much over the weekend. I have finally tested this today. Applying all 4 patches appears have no negative impact on what I'm doing. The perceived brightness appears to work just fine, but I am not using an oscilloscope, so I can't see the exact duty cycle or frequency. Tested-By: Adam Ford <aford173@xxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
