Re: [PATCH v2 4/5] iommu: Add Broadcom BCM2712 IOMMU driver

[Krzysztof Kozlowski <krzk@kernel.org>]

On 31/07/2026 22:34, Daniel Drake wrote:
> On 30/07/2026 08:00, Krzysztof Kozlowski wrote:
>> On Mon, Jul 27, 2026 at 09:43:59PM +0100, Daniel Drake wrote:
>>> +static int bcm2712_iommu_init_cache(struct bcm2712_iommu *mmu,
>>> +				    struct platform_device *pdev)
>>> +{
>>> +	struct device *dev = &pdev->dev;
>>> +	struct platform_device *cache_pdev;
>>> +	struct device_node *cache_np;
>>> +
>>> +	cache_np = of_parse_phandle(dev->of_node, "brcm,iommu-cache", 0);
>>> +
>>> +	/* Fall back on 'cache' property used in old/downstream firmware */
>>> +	if (!cache_np)
>>> +		cache_np = of_parse_phandle(dev->of_node, "cache", 0);
>>
>> NAK.
>>
>> You cannot have undocumented ABI. I asked you to drop this at v1.
> Is there any approach I should consider here in order to support 
> existing shipped firmware versions that use the 'cache' property, or 
> should I simply break compatibility?

Kernel is being shipped with DTB for Rpi, because firmware is not
embedded into some lower fixed storage, so there is no problem of
compatibility with some out of tree, unsupported DTB.

And no, you cannot use argument of undocumented, unreviewed ABI in
downstream forks as reason to add such ABI. This defeats the very
purpose of having upstream.

With this approach basically why even reviewing anything? Just accept
everything anyone ever shipped downstream, right?

Best regards,
Krzysztof