Date:   Wed, 27 Sep 2017 11:09:22 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Brüns, Stefan <Stefan.Bruens@...h-aachen.de>
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 Sat, Sep 23, 2017 at 12:00:15AM +0000, Brüns, Stefan wrote:
> 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.

Using an improper request ID or out of bounds will be just as much as
a bug. You will not get your DMA transfer to the proper device you
were trying to, the data will not reach the device or memory, your
driver will not work => a bug.

It will not be for the same reasons, you will not overwrite other
registers, but the end result is just the same: your transfer will not
work.

> > 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.

But until your patches, you *could* fix it and reach that 100%.

And I guess now we could indeed remove it.

Look, this discussion is going nowhere. I told you what the condition
for my Acked-by was already.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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