Date:   Sat, 23 Sep 2017 00:00:15 +0000
From:   Brüns, Stefan <Stefan.Bruens@...h-aachen.de>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
CC:     "linux-sunxi@...glegroups.com" <linux-sunxi@...glegroups.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        Vinod Koul <vinod.koul@...el.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Chen-Yu Tsai <wens@...e.org>, Rob Herring <robh+dt@...nel.org>,
        Code Kipper <codekipper@...il.com>,
        Andre Przywara <andre.przywara@....com>
Subject: Re: [PATCH v2 07/10] dmaengine: sun6i: Retrieve channel count/max
 request from devicetree

On Freitag, 22. September 2017 23:30:27 CEST Maxime Ripard wrote:
> On Tue, Sep 19, 2017 at 04:17:59PM +0000, Brüns, Stefan wrote:
> > On Dienstag, 19. September 2017 16:25:08 CEST Maxime Ripard wrote:
> > > On Mon, Sep 18, 2017 at 02:09:43PM +0000, Brüns, Stefan wrote:
> > > > On Montag, 18. September 2017 10:18:24 CEST you wrote:
> > > > > Hi,
> > > > > 
> > > > > On Sun, Sep 17, 2017 at 05:19:53AM +0200, Stefan Brüns wrote:
> > > > > > +	ret = of_property_read_u32(np, "dma-channels",
> > > > > > &sdc->num_pchans);
> > > > > > +	if (ret && !sdc->num_pchans) {
> > > > > > +		dev_err(&pdev->dev, "Can't get dma-channels.\n");
> > > > > > +		return ret;
> > > > > > +	}
> > > > > > +
> > > > > > +	if (sdc->num_pchans > DMA_MAX_CHANNELS) {
> > > > > > +		dev_err(&pdev->dev, "Number of dma-channels out of range.
\n");
> > > > > > +		return -EINVAL;
> > > > > > +	}
> > > > > > +
> > > > > > +	ret = of_property_read_u32(np, "dma-requests",
> > > > > > &sdc->max_request);
> > > > > > +	if (ret && !sdc->max_request) {
> > > > > > +		dev_info(&pdev->dev, "Missing dma-requests, using %u.\n",
> > > > > > +			 DMA_CHAN_MAX_DRQ);
> > > > > > +		sdc->max_request = DMA_CHAN_MAX_DRQ;
> > > > > > +	}
> > > > > > +
> > > > > > +	if (sdc->max_request > DMA_CHAN_MAX_DRQ) {
> > > > > > +		dev_err(&pdev->dev, "Value of dma-requests out of range.\n");
> > > > > > +		return -EINVAL;
> > > > > > +	}
> > > > > 
> > > > > I'm not really convinced about these two checks. They don't catch
> > > > > all
> > > > > errors (the range between the actual number of channels / DRQ and
> > > > > the
> > > > > maximum allowed per the registers), they might increase in the
> > > > > future
> > > > > too, and if we want to make that check actually working, we would
> > > > > have
> > > > > to duplicate the number of requests and channels into the driver.
> > > > 
> > > > 1. If these values increase, we have a new register layout and and
> > > > need a new compatible anyway.
> > > 
> > > And you want to store a new maximum attached to the compatible? Isn't
> > > that exactly the situation you're trying to get away from?
> > 
> > Yes, and no. H3, H5, A64 and R40 have the exact same register layout, but
> > different number of channels and ports. They could share a compatible (if
> > DMA channels were generalized), and we already have several register
> > offsets/ widths (implicitly via the callbacks) attached to the compatible
> > (so these don't need generalization via DT).
> > 
> > Now, we could also move everything that is currently attached to the
> > compatible, i.e. clock gate register offset, burst widths/lengths etc.
> > into
> > the devicetree binding, but that would just be too much.
> > 
> > The idea is to find a middle ground here, using common patterns in the
> > existing SoCs. The register layout has hardly changed, while the number of
> > DMA channels and ports changes all the time. Moving the number of DMA
> > channels and ports to the DT is trivial, and a pattern also found in
> > other DMA controller drivers.
> 
> I'm sorry, but the code is inconsistent here. You basically have two
> variables from one SoC to the other, the number of channels and
> requests.
> 
> In one case (channels), it mandates that the property is provided in
> the device tree, and doesn't default to anything.
> 
> In the other case (requests), the property is optional and it will
> provide a default. All that in 20 lines.

The channel number is a hardware property. Using more channels than the 
hardware provides is a bug. There is no default.

The port/request is just some lax property to limit the resource allocation 
upfront. As long as the bindings of the different IP blocks (SPI, audio, ...) 
provide the correct port numbers, all required information is available.
 
> I guess we already reached that middle ground by providing them
> through the DT, we just have to make sure we remain consistent.
> 
> > *If* the number of dma channels and ports is ever increased,
> > exceeding the current maximum, this would amount to major changes in
> > the driver and maybe even warrant a completely new driver.
> > 
> > > > 2. As long as the the limits are adhered to, no other
> > > > registers/register
> > > > fields are overwritten. As the channel number and port are used to
> > > > calculate memory offsets bounds checking is IMHO a good idea.
> > > 
> > > And this is true for many other resources, starting with the one
> > > defined in reg. We don't error check every register range, clock
> > > index, reset line, interrupt, DMA channel, the memory size, etc. yet
> > > you could make the same argument.
> > > 
> > > The DT has to be right, and we have to trust it. Otherwise we can just
> > > throw it away.
> > 
> > So your argument here basically is - don't do any checks on DT provided
> > values, these are always correct. So, following this argument, not only
> > the
> > range check, but also the of_property_read return values should be
> > ignored, as the DT is correct, thus of_property_read will never return an
> > error.
> No, my argument is don't do a check if you can catch only half of the
> errors, and with no hope of fixing it.
> 
> The functions you mentionned have a 100% error catch rate. This is the
> difference.
> 
> > That clearly does not match the implementation of drivers throughout the
> > various subsystems for DT properties, which is in general - do all the
> > checks that can be done, trust everything you can not verify.
> 
> And my point is that we're falling into the latter here. You cannot
> verify it properly.

Please check the following line:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/
drivers/dma/sun6i-dma.c#n951

Thats far from 100% - the highest allowed port for each SoC differs between RX 
and TX, and port allocation is sparse.

Regards,

Stefan

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