Re: [PATCH 1/2] dt-bindings: memory-controller: Document rev c.1.5 compatible
From: Florian Fainelli
Date: Mon Jan 13 2025 - 12:33:17 EST
On 12/26/2024 10:58 PM, Krzysztof Kozlowski wrote:
On 26/12/2024 19:50, Florian Fainelli wrote:
On 12/21/2024 12:04 PM, Krzysztof Kozlowski wrote:
On 20/12/2024 00:43, Florian Fainelli wrote:
On 12/19/24 00:58, Krzysztof Kozlowski wrote:
On Wed, Dec 18, 2024 at 09:15:08AM -0800, Florian Fainelli wrote:
On 12/18/24 03:37, Krzysztof Kozlowski wrote:
On Tue, Dec 17, 2024 at 11:44:38AM -0800, Florian Fainelli wrote:
Document the revision c.1.5 compatible string that is present on newer
Broadcom STB memory controllers (74165 and onwards).
Signed-off-by: Florian Fainelli <florian.fainelli@xxxxxxxxxxxx>
---
.../bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml | 1 +
1 file changed, 1 insertion(+)
diff --git a/Documentation/devicetree/bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml b/Documentation/devicetree/bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml
index 4b072c879b02..99d79ccd1036 100644
--- a/Documentation/devicetree/bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml
+++ b/Documentation/devicetree/bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml
@@ -29,6 +29,7 @@ properties:
- brcm,brcmstb-memc-ddr-rev-c.1.2
- brcm,brcmstb-memc-ddr-rev-c.1.3
- brcm,brcmstb-memc-ddr-rev-c.1.4
+ - brcm,brcmstb-memc-ddr-rev-c.1.5
You should use v2.1 fallback and drop driver patch. Or explain in
commit briefly why different approach is suitable.
Are you suggesting that we should have fallback compatible strings, such
that we have something like this:
compatible = "brcm,brcmstb-memc-ddr-rev-c.1.5",
"brcm,brcmstb-memc-ddr-rev-c", "brcm,brcmstb-memc-ddr"
and the driver only needs to match on "brcm,brcmstb-memc-ddr-rev-c" and
apply the adequate register offset table?
Almost, fallback should be brcm,brcmstb-memc-ddr-rev-b.2.1 or whatever
was in the driver first or whatever is the oldest known common
interface.
brcm,brcmstb-memc-ddr-rev-c is not a specific compatible.
If so, that is not how the current binding, and therefore DTBs are being
deployed, so that will introduce a breakage until we update all DTBs in the
field...
No. First, I thought about new comaptible so the one you add here. No
breakage, it's new compatible. This saves you these pointless updates of
driver everytime you add new compatible.
Yes, but that is not how the binding has been defined until now, so all
of the DTBs out there have:
compatible = "brcm,brcmstb-memc-ddr-rev-b.2.x", "brcm,brcmstb-memc-ddr"
I don't understand the problem. We talk about new devices here, it does
not matter what existing/old devices have in binding in that matter.
It does matter, because the DTBs that contain this compatible string are
already in the field, we cannot retrofit them overnight with an
additional compatible string in order to provide a fallback. Because
Then they run downstream and it does not matter whether you change here
something or not. And anyway downstream choices do not shape choices
here. We never accept bindings just because downstream already shipped
something.
Fair enough, but it does matter anyway, because as you can see from the
original binding that you accepted [1] we have a precedent for having
compatible strings of the form:
compatible = "brcm,brcmstb-memc-ddr-rev-c.1.x"
with each possible value of X being documented, this is just adding
another way, and surprise, keeps things consistent.
Why did not you reject the original binding back then? Why the change of
heart now?
All of this feels from my side very very arbitrary, and as such it is
difficult to document for other people and build a correct mental model of.
[1]:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/Documentation/devicetree/bindings/memory-controllers/brcm,brcmstb-memc-ddr.yaml?id=fa0321ba51ddff78ebe3c7c945830a85c987e3ed
this is submitted now does not mean it is a new device, this was part of
my backlog to get submitted earlier on.
Well, that's great yet is not an argument in the bindings.
> >>
(where X is in range [1..5])
and there is no fallback defined to "brcm,brcmstb-memc-ddr-rev-b.2.1",
so it is not like we can retrofit that easily by adding one now.
> > Second, you can introduce fallbacks to older compatibles as well -
there
will be no breakage, because you add one more compatible. The old
compatibles (covered by fallback) of course stays in the driver, so
there is no breakage at all. We did it multiple times for several
different bindings in Qualcomm. People were doing exactly the same:
adding compatible for new device to binding and driver, without
considering the compatibility at all.
Except being logically correct choice - using fallbacks - this really
has huge benefits when later upstreaming complete, big SoCs, like we do
for latest Qualcomm SoCs: several changes will be only bindings updates.
Yes, there are advantages to using fallbacks and we (ab)use that
whenever practical.
The driver only uses a very limited subset of registers (for now), the
registers change between minor revisions as well in a way that using a
fallback like "brcm,brcmstb-memc-ddr-rev-b.2.1" is not accurate enough
not practical. In particular for some of the changes that I am thinking
of adding later on, we would need the precise minor version because the
behavior and/or register interface is subtly different that this matters.
Devices work fine now with the same driver data, so they are compatible.
For now they do, but not for other features that will be submitted
later, they will not be, and at that point we will need to know that
this is a rev C.1.5 controller versus say a rev C.1.4 controller.
Which is exactly what compatibility is for.
Are you saying that the compatible strings must now all be updated to be
of the form:
compatible = "brcm,brcmstb-memc-ddr-rev-c.1.5",
"brcm,brcmstb-memc-ddr-rev-c.1.x", "brcm,brcmstb-memc-ddr"
or something different?
--
Florian