Message-ID: <20171222144016.GD29536@w540>
Date:   Fri, 22 Dec 2017 15:40:16 +0100
From:   jacopo mondi <jacopo@...ndi.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Jacopo Mondi <jacopo+renesas@...ndi.org>, magnus.damm@...il.com,
        geert@...der.be, mchehab@...nel.org, hverkuil@...all.nl,
        linux-renesas-soc@...r.kernel.org, linux-media@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 03/10] v4l: platform: Add Renesas CEU driver

Hi Laurent,

On Fri, Dec 22, 2017 at 02:03:41PM +0200, Laurent Pinchart wrote:
> Hi Jacopo,
>
> On Thursday, 21 December 2017 18:27:02 EET jacopo mondi wrote:
> > On Mon, Dec 18, 2017 at 05:28:43PM +0200, Laurent Pinchart wrote:
> > > On Monday, 18 December 2017 14:25:12 EET jacopo mondi wrote:
> > >> On Mon, Dec 11, 2017 at 06:15:23PM +0200, Laurent Pinchart wrote:
> > >
> > [snip]
> >
> > >>>> +/**
> > >>>> + * ceu_soft_reset() - Software reset the CEU interface
> > >>>> + */
> > >>>> +static int ceu_soft_reset(struct ceu_device *ceudev)
> > >>>> +{
> > >>>> +	unsigned int reset_done;
> > >>>> +	unsigned int i;
> > >>>> +
> > >>>> +	ceu_write(ceudev, CEU_CAPSR, CEU_CAPSR_CPKIL);
> > >>>> +
> > >>>> +	reset_done = 0;
> > >>>> +	for (i = 0; i < 1000 && !reset_done; i++) {
> > >>>> +		udelay(1);
> > >>>> +		if (!(ceu_read(ceudev, CEU_CSTSR) & CEU_CSTRST_CPTON))
> > >>>> +			reset_done++;
> > >>>> +	}
> > >>>
> > >>> How many iterations does this typically require ? Wouldn't a sleep be
> > >>> better than a delay ? As far as I can tell the ceu_soft_reset()
> > >>> function is only called with interrupts disabled (in interrupt context)
> > >>> from ceu_capture() in an error path, and that code should be reworked
> > >>> to make it possible to sleep if a reset takes too long.
> > >>
> > >> The HW manual does not provide any indication about absolute timings.
> > >> I can empirically try and see, but that would just be a hint.
> > >
> > > That's why I asked how many iterations it typically takes :-) A hint is
> > > enough to start with, preferably on both SH and ARM SoCs.
> >
> > I've seen only 0s when printing out how many cycles it takes to clear
> > both registers. This means 1usec is enough, therefore I would keep using
> > udelay here. Also, I would reduce the attempts to 100 here (or even
> > 10), as if a single one is typically enough, 1000 is definitely an
> > overkill.
>
> I'd go for 10. This being said, please make sure you run tests where the reset
> is performed during capture in the middle of a frame, to see if it changes the
> number of iterations.
>

The only way I can think to do this is to stream_on then immediately
stream_off before we get the frame and thus casue the interface reset.
Any other idea?

[snip]

> > >>>> +
> > >>>> +/**
> > >>>> + * ceu_capture() - Trigger start of a capture sequence
> > >>>> + *
> > >>>> + * Return value doesn't reflect the success/failure to queue the new
> > >>>> buffer,
> > >>>> + * but rather the status of the previous capture.
> > >>>> + */
> > >>>> +static int ceu_capture(struct ceu_device *ceudev)
> > >>>> +{
> > >>>> +	struct v4l2_pix_format_mplane *pix = &ceudev->v4l2_pix;
> > >>>> +	dma_addr_t phys_addr_top;
> > >>>> +	u32 status;
> > >>>> +
> > >>>> +	/* Clean interrupt status and re-enable interrupts */
> > >>>> +	status = ceu_read(ceudev, CEU_CETCR);
> > >>>> +	ceu_write(ceudev, CEU_CEIER,
> > >>>> +		  ceu_read(ceudev, CEU_CEIER) & ~CEU_CEIER_MASK);
> > >>>> +	ceu_write(ceudev, CEU_CETCR, ~status & CEU_CETCR_MAGIC);
> > >>>> +	ceu_write(ceudev, CEU_CEIER, CEU_CEIER_MASK);
> > >>>
> > >>> I wonder why there's a need to disable and reenable interrupts here.
> > >>
> > >> The original driver clearly said "The hardware is -very- picky about
> > >> this sequence" and I got scared and nerver touched this.
> > >
> > > How about experimenting to see how the hardware reacts ?
> >
> > Turns out this was not needed at all, both on RZ and SH4. I captured
> > several images without any issues in both platforms just clearing the
> > interrupt state without disabling interrutps.
>
> I wonder whether it could cause an issue when interrupts are raised by the
> hardware at the same time they are cleared by the driver. That's hard to test
> though.
>
> What happens when an interrupt source is masked by the CEIER register, is it
> still reported by the status CETCR register (obviously without raising an
> interrupt to the CPU), or does it not get flagged at all ?

They get flagged, yes, and right now I'm clearing all of them at the
beginning of IRQ handler writing ~CEU_CETR_ALL_INT to CETCR.

>
> > >> Also, I very much dislike the CEU_CETRC_MAGIC mask, but again the old
> > >> driver said "Acknoledge magical interrupt sources" and I was afraid to
> > >> change it (I can rename it though, to something lioke CEU_CETCR_ALL_INT
> > >> because that's what it is, a mask with all available interrupt source
> > >> enabled).
> > >
> > > I think renaming it is a good idea. Additionally, regardless of whether
> > > there is any hidden interrupt source, the datasheet mentions for all
> > > reserved bits that "The write  value  should always  be 0". They should
> > > read as 0, but masking them would be an additional safeguard.
> >
> > The HW manual is a bit confused (and confusing) on this point.
> > Yes, there is the statement you have cited here, but there's also
> > "to clear only the CPE bit to 0, write H'FFFF FFFE to CETCR" a few
> > lines above, which clearly contradicts the "write 0 to reserved bits"
> > thing.
> >
> > In practice, I'm now writing to 0 only bits to be cleared, and thus
> > writing 1s to everything else, reserved included. I haven't seen any
> > issue both on RZ and SH4 platforms.

And this is the above "wirting ~CEU_CETR_ALL_INT" to CETCR" I
mentioned above.

> >
> > > Also not that on the RZ/A1 platform bit 22 is documented as reserved, so
> > > you might want to compute the mask based on the CEU model.
> >
> > While I can use the .data pointer of 'of_device_id' for OF based
> > devices (RZ) to store the opportune IRQ mask, I'm not sure how to
> > do that for platform devices. Can I assume (platform data == SH) in
> > you opinion?
>
> Yes you can.

Awesome!

Thanks
   j

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