Re: [RFC v2 4/5] DT bindings documentation for Synopsys UDC platform driver

From: Scott Branden
Date: Thu Jan 19 2017 - 17:37:29 EST



On 17-01-19 01:55 PM, Ray Jui wrote:


On 1/19/2017 12:17 PM, Florian Fainelli wrote:
On 01/19/2017 12:07 PM, Scott Branden wrote:
Hi Florian,

On 17-01-19 11:40 AM, Florian Fainelli wrote:
On 01/19/2017 11:30 AM, Scott Branden wrote:
Hi Rob,

On 17-01-19 09:36 AM, Rob Herring wrote:
On Tue, Jan 17, 2017 at 01:35:07PM +0530, Raviteja Garimella wrote:
This patch adds device tree bindings documentation for Synopsys
USB device controller platform driver.

Bindings describe h/w, not drivers.

Signed-off-by: Raviteja Garimella <raviteja.garimella@xxxxxxxxxxxx>
---
.../devicetree/bindings/usb/snps,dw-ahb-udc.txt | 27
++++++++++++++++++++++
1 file changed, 27 insertions(+)
create mode 100644
Documentation/devicetree/bindings/usb/snps,dw-ahb-udc.txt

diff --git
a/Documentation/devicetree/bindings/usb/snps,dw-ahb-udc.txt
b/Documentation/devicetree/bindings/usb/snps,dw-ahb-udc.txt
new file mode 100644
index 0000000..0c18327
--- /dev/null
+++ b/Documentation/devicetree/bindings/usb/snps,dw-ahb-udc.txt
@@ -0,0 +1,27 @@
+Synopsys USB Device controller.
+
+The device node is used for Synopsys Designware Cores AHB
+Subsystem Device Controller (UDC).
+
+This device node is used by UDCs integrated it Broadcom's
+Northstar2 and Cygnus SoC's.

You need compatible strings for these in addition.

We don't need compatibility strings when an IP block is integrated into
an SoC. Otherwise each time we add the IP block to a new SoC we would
need to update ever linux driver that supports that SoC. That doesn't
make sense?

You probably do need such a thing, here is how the compatible strings
for IP blocks integrated into SoCs could be used:

- provide a compatible strings which describes exactly the integration
of this peripheral into a given SoC, e.g: brcm,udc-ns2, the reason for
that is that you want to be able to capture the specific IP block
integration into a specific SoC and all its quirks

- if the block has its own revision scheme (and it can be relied on),
provide it: brcm,udc-v1.2 and that is probably the most meaningful
compatible string for a client program here

- have a some kind of fallback/catchall compatible string that describes
the block: brcm,udc which may also work just fine, although is not
preferred

Defining compatible strings is meant to avoid making (possibly
incompatible) Device Tree binding changes in the future, and you always
have the liberty as a client program (OS, bootloader) to match only the
compatible strings you care about, from the most specific (which
includes the exact SoC) to the least specific.

The key thing is that, if the full set of compatible strings are present
and available, you can retroactively fix your driver to be more
specific, very much less so your Device Tree blob (although there is
disagreement).

The driver stands alone from the SoC and does not need compatibility
strings per SoC. New SoCs will use the exact same block.

Even if you take the exact same block and put it in a different SoC,
that's still an integration work that 99% of the time goes just fine
because the validation worked great, and the 1% of the time where you
need to capture an integration bug, you are glad this SoC-specific
compatible string exists such that you can work around it in the driver.

That's a very conservative estimate. Based on my experience, it's more
like 50/50, i.e., roughly half of the time we found SoC integration
specific quirks or workaround are needed.

50% is an exaggeration for sure. Maybe a driver you are has that issue but that is not the case with most drivers. We have many IP blocks in the SoC - both internal and externally sourced IP. We integrate SP805 timer driver into many SoCs and never specify a SoC specific compatibility string with it (nor should we).

That being said - if your driver needs to know SoC specifics is should not need to have an SoC specific compatibility string added per driver. Why can your driver just not query that information from the upper level SoC specific info already present in device tree?

Each SoC is already specified in device tree at the upper level already.
Example:
arch/arm/boot/dts/bcm7445.dtsi has this compatibility info already present in its device tree:

compatible = "brcm,bcm7445", "brcm,brcmstb";

If needed, a driver should query this info rather than adding SoC specific compatibility strings to every single device tree entry.

We should only add driver revision numbers as needed, not SoC specific names. That way drivers don't change when the (same revision) of the IP block is added to a new SoCs. And then if a SoC specific workaround is needed the upper level compatibility string can be queried should be utilized. It already exists today and is available for use to all drivers.


One way to solve that is to use SoC specific compatible strings because
that presents itself as a self-contained and standardized way, or you
can have your driver call into a piece of code that reads the SoC
type/revision, but AFAICT this seems to be frowned upon because it
presents some kind of layering violation.


We don't add compatibility strings to any other drivers when we add the
same block to a new SoC.

Ideally we would define new compatible strings for each new SoC we tape
out, yet don't necessarily match them in client programs, but just
define them as a safeguard in case something went wrong at the
integration stage that is discovered after the fact.