linux-kernel - Re: [PATCH v2 4/6] PCI: brcmstb: add Broadcom STB PCIe host controller driver

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Message-ID: <20191121154655.GB43905@e119886-lin.cambridge.arm.com>
Date:   Thu, 21 Nov 2019 15:46:55 +0000
From:   Andrew Murray <andrew.murray@....com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     Florian Fainelli <f.fainelli@...il.com>, mbrugger@...e.com,
        maz@...nel.org, phil@...pberrypi.org, linux-kernel@...r.kernel.org,
        jeremy.linton@....com, Eric Anholt <eric@...olt.net>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        bcm-kernel-feedback-list@...adcom.com,
        Stefan Wahren <wahrenst@....net>, james.quinlan@...adcom.com,
        linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 4/6] PCI: brcmstb: add Broadcom STB PCIe host
 controller driver

On Thu, Nov 21, 2019 at 02:26:15PM +0100, Nicolas Saenz Julienne wrote:
> On Thu, 2019-11-21 at 12:03 +0000, Andrew Murray wrote:
> > On Wed, Nov 20, 2019 at 08:53:30PM +0100, Nicolas Saenz Julienne wrote:
> > One purpose of this function is to validate that the information given in the
> > device tree is valid - I've seen other feedback on these lists where the view
> > is taken that 'it's not the job of the kernel to validate the DT'. Subscribing
> > to this view would be a justification for removing this validation -
> > especially
> > given that the bindings you include have only one dma-range (in any case if
> > there are constraints you ought to include them in the binding document).
> > 
> > Though the problem with this point of view is that if the DT is wrong, it may
> > be possible for the driver to work well enough to do some function but with
> > some horrible side effects that are difficult to track down to a bad DT.
> 
> As for the validation, I think in this specific case it's still worthwhile. As
> you might know, there is a bug on the first revision of RPI4's PCIe integration
> which blocks any access higher than 3GB. Further revisions fix this and allow
> full memory addressing.
> 
> I've been working with Phil Elwell (from the RPi foundation) to solve this in a
> way that plays well with upstream and this driver (I'll be able to test the new
> revision before this gets in). The solution is, unsurprisingly, for the
> firmware to edit the DTB passed to the kernel based on the board revision.
> Given that there is some live manipulation of the dma-ranges I'd rather leave
> the validation check.
> 
> If you don't disagree with the above I'll add an extra code comment explaining
> why we feel the need to verify the device-tree contents.

I'll be interested in seeing it.

Thanks,

Andrew Murray

> 
> Regards,
> Nicolas
>